Vegetable Crop Updates

Update 8 – June 17 2019

 

Vegetable Insect Update – Russell L. Groves, Professor and Extension Specialist, UW-Madison, Department of Entomology, 608-262-3229 (office), (608) 698-2434 (cell), or e-mail: groves@entomology.wisc.edu.

 

Degree-Day Accumulations – Recent reports from the recent National Climate Assessment of the Midwest (https://nca2018.globalchange.gov/chapter/21/), describe observed changes over the past 30 years.  “Increased rainfall from April to June has been the most impactful climate trend for agriculture in the Midwest, providing a favorable supply of soil moisture while also reducing flexibility for timing of spring planting and increasing soil erosion.  In addition, wet conditions at the end of the growing season have created elevated levels of mold, fungus, and toxins in seeded crops.  The last spring frost has occurred earlier, causing the frost-free season to increase by an average of nine days since 1901.  Daily minimum temperatures have increased in all seasons due to increasing humidity and warming winters have increased the survival and reproduction of existing insect pests and already are enabling a northward range expansion of new insect pests and crop pathogens into the Midwest.  Rising humidity also leads to longer dew periods and high moisture conditions that favor many agricultural pests and pathogens for both growing plants and stored grain”.  The abundant moisture and humidity observed this spring are consistent with these climate observations, and accumulating degree days for insects (and disease development) are slightly ahead of national averages (Figs. 1 and 2), even though the developing spring weather would seem to be lagging.

 

Flea beetles – With the slower pace of degree day accumulations over the past week, damaging populations of flea beetles (e.g. potato flea beetle, crucifer flea beetle, eggplant flea beetle) remain persistent on many crops this season.  Slow crop development also increases the timeframe over which populations of these adult insects can continue to feed and cause damage (Fig. 3).  Because the pest is so mobile, and because so many genera and species are involved, crop rotation alone often has less of an effect as a control strategy.  However, living mulches or polycultures are known to reduce flea beetle damage by limiting the appearance of the crop to mobile adult insects.  Trap cropping, in which more attractive plant species are planted near the main crop, offers some possibilities for flea beetle management.  One widely available trap crop is Chinese mustard (Brassica juncea var. crispifolia), but reseeding of the trap crop may be necessary, especially if the pest destroys the first planting.  This approach may be less effective in protecting crops that are highly preferred by adult flea beetles (e.g. Nappa cabbage).

 

Imported cabbageworm – Imported cabbageworm adults, commonly referred to as the white cabbage butterfly, are white butterflies with black markings on the wing tips.  Adults of this species have emerged from overwintering over the past 3 weeks, laid eggs and the appearance of early larvae is now underway throughout most of southern and central Wisconsin.  Female butterflies have 2 black dots on each fore wing; males, which are smaller, have 1 dot per wing.  Eggs are yellow and conical, laid individually on the leaf surface and occasionally on the stem.  An adult butterfly can lay 300 to 400 eggs in her lifetime.  Larvae appear as velvety green worms up to 1” in length with a faint yellow stripe running down the back.  The caterpillar is commonly found along the veins of leaves and easily blends into the foliage during scouting.  As a species that overwinters in Wisconsin, it is one of the earliest of the caterpillar complex to attack and infest Brassicas.  The imported cabbageworm will feed on all ages of leaves, but prefers the younger leaves.  They feed along the edges of the leaves, leaving only thick veins behind.  The cabbage looper, which has yet to arrive into southern Wisconsin, often feeds between veins on the underside of lower leaves.  A good indicator of the presence of loopers and imported cabbageworms is fresh frass (droppings) on leaves.  Treatment thresholds are well established and based on the percent of infestation by any lepidopteran species.  Economic thresholds (ETs) vary based on the stage of crop development. Cabbage, broccoli and cauliflower in the seedbed stage of development, are particularly susceptible to damage.  Therefore, control measures are warranted when 10% of the plants are affected.  Between transplant and cupping, the ET is raised to 30% from the time plants begin to cup until early heading.  From early heading until harvest, the threshold again drops to 10% to protect market quality of the produce.

 

Striped cucumber beetles – Striped and spotted cucumber beetles can cause damage in vine crops, but the striped cucumber beetle is more common in Wisconsin because it overwinters just outside last year’s fields.  Feeding from larvae and adults cause direct damage to roots, leaves, flowers, and fruits.  Adults can also vector Fusarium wilt and the bacteria, Erwinia tracheiphila.  Infection with E. tracheiphila results in the disease described as bacterial wilt, and young cucumbers and melons (e.g. muskmelon and cantaloupe) are particularly susceptible to bacterial wilt, and damage from this can be severe.  Larvae feed on roots and stems and can stunt or kill seedlings or transplants.  Adults feed on foliage, pollen, petals, and fruit of a variety of plants and can cause moderate to severe defoliation.  Plants in the 1-3 leaf or cotyledon stages are especially vulnerable, and high adult cucumber beetle populations can completely defoliate the plants.  Bacterial wilt is spread through the feces or contaminated mouthparts of beetles.  A distinct wilting of individual lateral leaves is the first symptom of bacterial wilt followed by the entire plant wilting and dying.  The disease causes plant death by plugging the water-conducting vessels.  Serious crop damage can occur if as little as 10% of the beetles are infected.

 

Colorado potato beetle –Adults continue to colonize potato fields along field edges through the past week, but many egg masses are now present in fields with the hatching of initial egg masses.  Forecast temperatures into next week will remain seasonal to slightly below average, but we will manage temperatures sufficient to hatch many of the egg masses present in fields.  Applications of tolfenpyrad (Torac), spinosad (Blackhawk), spinetoram (Radiant), or abamectin (Agri-Mek) should be applied when nearly 50-75% of egg masses have hatched, and a few 2nd instar larvae are present from the earliest hatched egg masses.  This event will likely occur by the end of the coming week, with additional egg masses being deposited this week as well.  Recall, these 1st generation larvicides often require 2-3 subsequent re-applications spaced on a 7-10 day interval to achieve sufficient control of this damaging generation.

http://labs.russell.wisc.edu/vegento/

 

Amanda Gevens, Associate Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, 608-575-3029.  https://wivegdis.plantpath.wisc.edu/

Our disease forecast calculations indicate slowly accumulating risk values for late blight and early blight, but no thresholds indicating need for preventative fungicide applications have been met at this time. Please see the table below for details at each location.  At this time of the growing season, it is early to see symptoms of early blight or other foliar disease.  Seed or soil borne diseases may appear as a poorly emerged, or slow-to-emerge crop.  Virus-like symptoms have been noted on some potatoes in some plantings.

The 2019 list for fungicides registered for late blight control in Wisconsin was attached to this email and will be available under the “Resources” tab of the Wisconsin Vegetable Pathology website.  https://wivegdis.plantpath.wisc.edu/resources/

The late blight reporting website https://usablight.org/2019-map/ is continuing to track locations of confirmed late blight in the U.S.  I routinely track this site and report findings through this newsletter.  To date, reports were confirmed from FL.  No recent detections. Symptoms can include water-soaked foliar lesions that appear ‘greasy’ when moisture is high and dark brown lesions with pale green haloes with white pathogen sporulation (on lower and/or upper leaf surfaces).  Pictures included below.

Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations (Many thanks to Ben Bradford, UW-Madison Entomology; Stephen Jordan, John Hammel, & Samuel Meyer, UW-Madison Plant Pathology).  A P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application.  A DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application.  Red text in table indicates threshold has been met/surpassed.  TBD indicates that data is To Be Determined as time progresses.  Weather data used in these calculations comes from weather stations that are placed in potato fields in each of the four locations.  Data are available in graphical and raw data formats for each weather station at:  https://wivegdis.plantpath.wisc.edu/dsv/

Location Planting Date Emergence Date (50%) Disease Severity Values (DSVs) 6/14/19 Potato Physiological Days (P-Days) 6/14/19
Grand Marsh Early Apr 10 May 20 7 173.55
  Mid May 1 June 1 5 96.35
  Late May 20 June 9 3 37.86
Hancock Early Apr 10 May 22 4 176.79
  Mid Apr 25 May 27 3 140.89
  Late May 15 June 8 1 51.37
Plover Early Apr 22 May 27 2 134.05
  Mid May 1 June 1 2 99.65
  Late May 29 June 13 0 9.43
Antigo

Station

Early May 14 May 29 0 49.7
Station set up on June 5 at airport Mid May 24 June 8 0 42.79
Late Jun 1 TBD

 

 

 

 

 

 

 

 

 

TBD TBD
  • Update 7 - June 9, 2019

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    In this issue:

    • Vegetable & Potato Production Updates
    • Vegetable Insect Updates
    • Potato Disease Forecasting P-Day and DSV Updates

     

    Yi Wang, Assistant Professor & Extension Potato and Vegetable Production Specialist, UW-Madison, Dept. of Horticulture, 608-265-4781, Email: wang52@wisc.edu.

    With the recent warm and long days, potatoes and vegetables are growing fast. Potatoes that were 3’’ tall on May 30th have been up to 7’’ tall on June 6th. For potatoes, most growers have been done or close to be done with hilling and nitrogen application for tuber initiation. In many fields in Central Sands, water table is high enough to be just underneath the seed piece. Therefore sidedressing of nitrogen becomes less desirable due to soil compaction. Instead, a couple times of fertigations have been applied. Wheel rutting will still be a concern but should cause much less compaction than sidedressing. However during the early growing stage, plants are so small that fertigation is not an efficient way to apply nitrogen, and more potential leaching will occur to the side of the hills because there are no roots to uptake the nutrients. Growers have to trade off nitrogen use efficiency for reducing soil compaction.

    We’ve been having a wet spring until this week (Figure 1). With the rapid plant growth, people need to start their routine irrigation. A soil moisture of 70% to 80% of field capacity is preferred during this growth stage. At tuber initiation, about 1.5’’ per week will be a good irrigation amount.

    Below I listed the potato crop growth updates based on my observations on June 6th between Hancock and Plover. Emergence in almost all fields was greater than 85%. Figure 1 showed that there was a four-day period between April 11th and April 15th when soil temperatures dropped below 35F, and some growers were concerned about frost damage on the seed pieces that were put into ground before the cold period. However, according to what I saw and heard, the cold weather did not cause delayed or failed emergence.

    Planting period Plant height Ground cover
    April 10th – April 20th Up to 10’’ Up to 60%
    April 20th – April 30th Up to 6’’ Around 25%
    May 1st – May 10th Up to 3’’ Around 10%

    Thanks to the nice weather, planting and growing of processing vegetables are going well too. Peas and snap beans that were planted in April are being harvested or will be harvested soon.

     

    Vegetable Insect Update – Russell L. Groves, Professor and Extension Specialist, UW-Madison, Department of Entomology, 608-262-3229 (office), (608) 698-2434 (cell), or e-mail: groves@entomology.wisc.edu.

    Colorado potato beetle – Overwintering Colorado potato beetle (CPB) adults have been emerging over the past few weeks in the Central Sands region, but rates of colonization are still somewhat low.  Adults will continue to colonize fields along field edges close to their overwintering sites and likely near previously planted potato.  Recall, these insects are now emerging over much longer time frames as a result of delayed emergence, or extended diapause.  Pay close attention to these colonizing populations and determine if the at-plant systemic neonicotinoids are failing to control the early instar larvae hatching from the first egg masses.  For those who have used at-plant options, high concentrations of the neonicotinoids should be present in the emerging and rapidly expanding plants which ‘should’ provide good, early season control of newly hatched larvae.  Because adult CPB have not fully emerged, and have only begun to colonize field edges, perimeter foliar applications with the reduced-risk product, novaluron (Rimon®), should likely begin this week.  This material has the unique activity of targeting both eggs and early instar larvae.  Adult female CPB that ingest novaluron-treated foliage will lay eggs that are non-viable.  A second and third application will likely be necessary in 7-10 days following the initial application to provide more complete control of the 1st generation.  Forecast temperatures into next week will remain seasonal to slightly below average.  Applications of tolfenpyrad (Torac), spinosad (Blackhawk), spinetoram (Radiant), or abamectin (Agri-Mek) should be applied when nearly 50-75% of egg masses have hatched, and a few 2nd instar larvae are present from the earliest hatched egg masses.

    Flea beetles – Populations of potato flea beetle, crucifer flea beetle, eggplant flea beetle and corn flea beetle have been observed on many crops this season.  Flea beetles are an early-season pest commonly found on all members of the cole crop group, as well as spinach, beets, potatoes, and eggplant.  There are several different species of flea beetle that pose problems early in the season when they are considered occasional pests. Host plants of many of the flea beetles are easily identified by their common names. For example, the crucifer flea beetle attacks cole crops and mustards while the eggplant flea beetle is commonly associated with eggplant (http://labs.russell.wisc.edu/vegento/pests/flea-beetles/).

    Common Wisconsin flea beetles include the crucifer, eggplant, horseradish, pale-striped, potato, spinach, and striped varieties. All have characteristically large hind legs that give adults the ability to jump. Adult flea beetles range in size from about 1/10 – 1/5 inch. Larvae are delicate and thread-like with white bodies and brown heads.  Adjusting planting dates to avoid damage caused by flea beetles may be useful. Floating row covers can prevent adults from feeding on leaves and laying eggs on the crop. If used, row covers should be set up just before the crop emerges. Water deters adult flea beetles, and any watering should be done in mid-day. Since flea beetles overwinter near fields, planting after adults have emerged or rotating crops can help minimize flea beetle damage. Commercially available nematodes that feed on flea beetle eggs, larvae, and pupae are available.

    A chemical option is only recommended when flea beetle populations exceed threshold levels, particularly early in the season (see Table 2). Most crops can withstand moderate levels of infestation without economic loss. Soil-applied insecticides can provide longer periods of control. Foliar insecticides can provide quick control, but may also disrupt natural enemies of other pests of cole crops.

    Table 2.  Established control thresholds for various crops.

    Crop Threshold
    Beets Treat when beetles cause stand reduction on small plants
    Cole Crops Undetermined
    Eggplant <3” = 2 beetles/plant
    3-6” = 4 beetles/plant
    >6” = 8 beetles/plant
    Horseradish Treat only if beetles are found in high numbers early in the season
    Potato >2 beetles/sweep
    Tomato >2 beetles/plant

    Potato leafhopper – Adult potato leafhopper (PLH) have begun to immigrate into southern and central Wisconsin, arriving over the last couple weeks on weather systems that have brought southerly winds.  Populations of adults have recently increased and sweep net counts at the Arlington Agricultural Experiment Station revealed the presence of nymphs on newly emerged plants following cutting.  These insects have a broad host range attacking alfalfa, snap beans, and potatoes, to name only a few.  They feed with sucking mouthparts similar to mosquitoes and remove plant sap directly from the phloem and cause damage by injection of a salivary toxin that causes cell disruption.  Once populations have been observed, fields should be scouted regularly using standard sweep net sampling.  Recommended treatment thresholds are 1 adult per sweep with a net or 15 nymphs on the undersides of 50 potato leaves.

    The first signs of leafhopper feeding are the leaf veins turning pale and the leaf curling. Continued feeding results in a characteristic triangular yellowing or browning of the leaf tip known as “hopperburn”. As symptoms develop, lesions spread backward and inward from the margin, eventually destroying the entire leaf. Plants become stunted and yellow leaves curl upward. Premature death of the plant may occur in severe infestations. Severe leaf damage and premature plant death is common in potato, whereas leaf discoloration and curling are more characteristic on bean.

    Injury develops most rapidly during hot, dry weather. More damage is attributed to the nymphs than the adults. Leafhopper damage may take weeks before symptoms begin to show and it is typically older leaves that display the “hopperburn” symptomology. Yield loss generally occurs before symptoms are readily seen. Though plants may show little evidence of hopperburn, yield losses can be substantial.

    Vegetable entomology web page: http://labs.russell.wisc.edu/vegento/

     

    Amanda Gevens, Associate Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, 608-575-3029.  https://wivegdis.plantpath.wisc.edu/

    Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations (Many thanks to Ben Bradford, UW-Madison Entomology; Stephen Jordan, John Hammel, & Samuel Meyer, UW-Madison Plant Pathology).  A P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application.  A DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application.  Red text in table indicates threshold has been met/surpassed.  TBD indicates that data is To Be Determined as time progresses.  Weather data used in these calculations comes from weather stations that are placed in potato fields in each of the four locations.  Data are available in graphical and raw data formats for each weather station at:  https://wivegdis.plantpath.wisc.edu/dsv/

    Location Planting Date Emergence Date (50%) Disease Severity Values (DSVs) 6/7/19 Potato Physiological Days (P-Days) 6/7/19
    Grand Marsh Early Apr 10 May 20 4 128.55
      Mid May 1 June 1 2 51.35
      Late May 20 June 9 TBD TBD
    Hancock Early Apr 10 May 22 3 126.72
      Mid Apr 25 May 27 2 90.82
      Late May 15 June 8 TBD TBD
    Plover Early Apr 22 May 27 1 87.08
      Mid May 1 June 1 1 52.68
      Late May 29 TBD TBD TBD
    Antigo Early May 14 May 29 TBD TBD
    Mid May 24 June 8 TBD TBD
    Late Jun 1 TBD TBD TBD
  • Update 6 - June 2, 2019

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    Vegetable Insect Update – Russell L. Groves, Professor and Extension Specialist, UW-Madison, Department of Entomology, 608-262-3229 (office), (608) 698-2434 (cell), or e-mail: groves@entomology.wisc.edu.

     

    Colorado potato beetle – Initial overwintering Colorado potato beetle (CPB) adults continue to emerge in southern Wisconsin at the Arlington Agricultural Experiment Station, and at the Hancock Agricultural Experiment Station in central Wisconsin (http://labs.russell.wisc.edu/vegento/pests/colorado-potato-beetle/).  In the next few weeks, as early planted potato has emerged from hilling, beetles will start to colonize emerging plants and begin to deposit eggs.  When not controlled, CPB can completely defoliate plants resulting in serious yield losses or even plant death.  Beetles prefer to feed on potato, but will also use eggplant and other solanaceous crops and weeds. Both larval and adult life stages commonly cause damage to plants throughout the growing season. CPB is a persistent pest annually, once an infestation occurs beetle populations tend to increase annually.

    CPB overwinter as adults in Wisconsin. Typically overwintering adults will burrow 10-30 cm into the soil, often choosing protected areas near trees or in grassy edges surrounding gardens or fields. Adults emerge in the spring, at about the time potato breaks ground. Overwintered adults often colonize crops first along field edges. Border planted crops are often the first places to begin scouting for beetle infestations.  Larvae hatch from the eggs in 4-9 days depending upon ambient air temperature. The larvae will molt three successive times before pupating. Each immature life stage (stadium) between molts is called an instar, totaling 4 instars. First instar larvae are blackish-brown in color and very small, approximately the size of a pinhead. Once hatched first instar larvae prefer to feed upon newly expanded foliage at the crown of the plant. Because of their small size feeding damage is minimal. Second instar larvae assume a deep crimson coloring, leaf consumption increases two-fold from first instars. Third and fourth instars have bright red abdomens with black head capsules and legs. The last two larval instars consume increasingly more foliage and result in the majority of economic damage to solanaceous crops. After passing through four instars over 2-3 weeks, larvae return to the soil to pupate. Within 7-10 days the second generation of adult beetles emerge.

    Potato plants can tolerate varying levels of defoliation before they will suffer yield losses. The level of tolerance depends on the plant’s growth stage. Flowering plants can tolerate the least defoliation, only 5-10% of total leaf area. Post-flower potato is able to withstand a slightly greater amount of defoliation (up to 10%), but since this is a critical point for tuber formation and bulking growers should limit the amount of feeding done by CPB.

     

    Aster Leafhoppers – Early season (late April) sweeps for Aster leafhoppers (ALHs), the insect vector of Aster Yellows phytoplasma (AYp), revealed low numbers on areas of eastern Oklahoma, and southeastern Kansas (http://labs.russell.wisc.edu/vegento/pests/aster-leafhopper/).  The ALH can be a serious pest of many plants in the upper Midwest because of its ability to spread AYp.  Leafhoppers prefer lettuce, carrots, celery, and small grains for feeding and breeding, while other crops such as potatoes and onions provide a temporary source of food and refuge. Only adults use these temporary sites; immature leafhoppers fail to develop on these plants.  Early season surveys of these insects at multiple sites in the southern US suggested very low populations, especially given the slow accumulation of degree days through the Midwest over the past 6-8 weeks.

    The first adult ALH that appear in early May do not overwinter in Wisconsin. They overwinter as eggs on grain crops in portions of the mid-southern US (eg. KS, OK, AR, TX), and migrate northward as adults each spring on warm, southerly winds. The migrant leafhopper population is important because of the potential for the migrants to already be infected with the AYp pathogen when they arrive. The migrant females lay eggs in the plant tissues of early grain crops, weeds and susceptible carrot, and celery.

    Low numbers of ALH have now been detected in portions of southern Wisconsin, suggesting that few migratory insects have arrived.  Samples taken in late May from the Arlington Agricultural Research Station resulted in an average of 2.5 adult ALH/100 sweeps in emergent wheat crops.  Infectivity levels of the AYp within insects were estimated to be below 1%.  These few infected adults do not pose much early season risk for newly emerged and highly susceptible crops such as lettuce, celery, susceptible carrot varieties, as the computed  the Aster Yellows Index (AYI) values are are far less than the threshold of 25 (http://labs.russell.wisc.edu/vegento/files/2012/05/Aster-yellows-index.pdf). Migrant leafhoppers could continue to arrive in the state over the next 2-3 weeks with southerly winds that precede cold fronts moving westerly across the Midwest and into the Lake States.

     

    Seed Maggots – The emergence and flights of the second generation of seed corn maggot flies is approaching southern Wisconsin (Fig. 1), and with forecast moderate temperatures, would be expected in the coming week to 10 days in southern Wisconsin (http://labs.russell.wisc.edu/vegento/pests/seedcorn-maggot/). This insect has a base developmental temperature of 39oF and the emergence of adult fly populations are expected at accumulated degree days of 360, 1,080 and 1,800 degree days.  The first generation of adult flies (360 DD39) has already passed throughout much of the state, and is still only present in very northern portions of the state. Adult flies will become very active in the coming week to 10 days and begin to lay eggs at the base of susceptible (young) plants, where larvae tunnel into underground portions.

    The first generation of onion maggots (http://labs.russell.wisc.edu/vegento/pests/onion-maggot/), is very active throughout central Wisconsin where degree day totals of 68040 degree days (spring), have been reached using a slightly different base temperature of 40oF.  The first generation peak of egg-laying adults is now present in central portions of the state.  As onions mature, they are less susceptible to onion maggot infestation unless they are damaged by cultivation equipment.  Soil applications of Verimark or Lorsban can be used to control onion maggot in dry bulb onions and the seed treatments of Supresto or FarMore DI500 are available to minimize damage.  The preventative soil insecticide applications are recommended for the control of the first generation larvae if you have previously documented damage from the previous year’s crop which exceeds 5 to 10%.

    Vegetable Entomology web page: http://labs.russell.wisc.edu/vegento/

     

     

    Amanda Gevens, Associate Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, 608-575-3029.  https://wivegdis.plantpath.wisc.edu/

     

    Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations (Many thanks to Ben Bradford, UW-Madison Entomology; Stephen Jordan, John Hammel, & Samuel Meyer, UW-Madison Plant Pathology).  A P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application.  A DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application.  Red text in table indicates threshold has been met/surpassed.  TBD indicates that data is To Be Determined as time progresses.  Weather data used in these calculations comes from weather stations that are placed in potato fields in each of the four locations.  New this year!  Data are available in graphical format for each weather station at:  https://wivegdis.plantpath.wisc.edu/dsv/#grand-marsh

    Location Planting Date Emergence Date (50%) Disease Severity Values (DSVs) 5/31/19 Potato Physiological Days (P-Days) 5/31/19
    Grand Marsh Early Apr 10 May 20 2 77.26
      Mid May 1 June 1 0 6.87
      Late May 20 TBD TBD TBD
    Hancock Early Apr 10 May 22 2 72.83
      Mid Apr 25 May 27 1 36.93
      Late May 15 TBD TBD TBD
    Plover Early Apr 22 May 27 0 34.90
      Mid May 1 June 1 0 7.45
      Late May 29 TBD TBD TBD
    Antigo Early May 14 May 29 1 TBD
    Mid May 24 TBD TBD TBD
      Late Jun 1 TBD TBD TBD

     

    Downy mildew confirmed this week on hop plants from Dane County (UW-Plant Disease Diagnostic Clinic) and on basil plants from several home garden centers in Dane County.  The pathogens are unique to each crop, but similar weather favors their onset and development.

     

    Basil downy mildew is not new to Wisconsin; 2010 was the first year in which the disease was confirmed in our state on basil.  Since that time, we’ve seen the disease in our state with sporadic occurrence.  It is not uncommon to see symptoms on nursery plants for sale.  The pathogen can be seedborne and development of the disease is at high risk when plants are maintained under high humid, and moderate temperatures with limited airflow.   Basil downy mildew made headlines nationally as a new disease in North America as well as Europe.  First reported in FL in 2007, basil downy mildew was later found in field and greenhouse in Canada, Argentina, and in the US states of NC, PA, NJ, NY, MA, NC, KS, and MO in 2008.  Reports continued in 2009 in the US.

    Basil downy mildew is caused by the fungus-like pathogen Peronospora belbahrii and can be transmitted on seed, infected plant parts, and on the wind.  This particular downy mildew can both ornamental and basil varieties grown as herbs.  It is suspected that basil downy mildew has moved geographically on contaminated seed or leaves.  The spores of basil downy mildew are produced on leaf underside prolifically and can be aerially dispersed long distances.

    The management of basil downy mildew includes planting ‘clean’ basil seed, selecting resistant or tolerant varieties, and applying fungicides when environmental conditions favor disease.  More information on Rutgers University’s recent resistant basil varieties can be found here:  https://vegetablegrowersnews.com/news/downy-mildew-resistant-basil-varieties-now-available/

    Minimizing leaf wetness and humidity will aid in downy mildew management as the pathogen is favored by moist conditions.  It is known that sweet basil varieties are more susceptible than other basil species.

    Basil varieties susceptible to downy mildew
    Aroma 2 Italian Large Leaf
    Genovese Magical Micheal
    Genoveser Martina Mariden
    Nufar Opal Purple Variegated
    Queenette Poppy Joe’s
    Superbo
    Basil varieties tolerant to downy mildew
    Amethyst Imp Mrs. Burns Lemon
    Red Rubin Red Leaf
    Sweet Adin Lemon
    Lemon standard Lemon Mrs. Burns
    Lemona Lime
    Basil varieties resistant to downy mildew
    Spice Blue Spice
    Blue Spice Fil Rutgers Obsession, Devotion, Thunderstruck, Passion

    Applying fungicides frequently and starting before first symptoms are considered necessary to control downy mildew effectively.  Few fungicides are currently labeled for this new disease. Actinovate AG and OxiDate are OMRI-listed fungicide labeled for use on herbs and for suppressing foliar diseases including downy mildew.  OxiDate is labeled for use outdoors and in greenhouses.  The Actinovate label does not have a statement prohibiting use in greenhouses.  There are two phosphorous acid fungicides, ProPhyt and K-Phite, that have downy mildew under herbs on the current label.  These fungicides were effective in fungicide efficacy experiments with applications started before or after initial symptoms were found.  Greenhouse use is not prohibited.  Amistar and Quadris are labeled for use on basil but not specifically for downy mildew; they have the same active ingredient, which has been shown to be effective for this downy mildew.   Greenhouse use is not permitted with Amistar and Quadris.  Other fungicides are expected to be labeled for basil downy mildew in the future.

    To determine when to initiate a fungicide program and also when it is warranted to consider harvesting early to avoid losses to downy mildew, growers should not only routinely check the on-line spreadsheet to determine when downy mildew is occurring on basil nearby, but also regularly inspect their crop for symptoms.  The cucurbit downy mildew forecasting web site (http://cdm.ipmpipe.org) might be useful for predicting when conditions are favorable for basil downy mildew since both pathogens likely have similar requirements for successful wind dispersal long distances (e.g. overcast skies) and subsequent infection (e.g. wet leaves).  Summer is not a time to forget about this disease: unlike most other downy mildew pathogens, e.g. the ones affecting lettuce and cruciferous crops, which stop developing in summer, the basil downy mildew pathogen seems to develop best under moderate to warm temperatures while also tolerating cool temperatures.  Basil crops should be disked under or otherwise destroyed as soon as possible after last harvest, or when abandoned because of disease, to eliminate this source of inoculum.

     

    Hop Downy Mildew Identification and Control in Wisconsin

    Downy mildew caused by Pseudoperonospora humili, is a common disease on hop in Wisconsin.  Once established in a hop yard, the disease can be persistent, overwintering in the below-ground plant parts.  Earliest symptoms include downcurling of often brittle leaves, pale green-yellow foliage, and presence of dark gray-purple pathogen sporulation on leaf undersides.  Early management of hop downy mildew is critical for limiting inoculum for the rest of the production season.  While the pathogen will not likely be eradicated from the yard, sound management enables a healthy production season with less inoculum for late season when cones are forming.  Further, reducing disease and inoculum in this season, limits the amount of pathogen that will remain with your hop plants for future years.  Cool, wet weather is most favorable for downy mildew and may require weekly applications of fungicides for disease management.  Hot, dry weather provides great conditions for limiting disease – often enabling 10-14 day fungicide application intervals for disease management.  For further information on use of fungicides in management of hop downy mildew, please see the document link, below (from 2016 but registrations still current).  Additionally, I provided the registered fungicide list for Wisconsin hop downy mildew control.

    https://ipcm.wisc.edu/download/misc/Considerations-for-disease-control-in-Wisconsin-hop-production-2016.pdf

     

    Accessing the 2019 University of Wisconsin Madison Extension Commercial Vegetable Crop Production Management Guide:  Our production guide is updated every October with release of a new guide in January.  The book can be downloaded for free as a pdf at the link below, or can be purchased online for $12.50.    https://learningstore.uwex.edu/Assets/pdfs/A3422.pdf

  • Update 5 - May 24, 2019

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    Yi Wang, Assistant Professor & Extension Potato and Vegetable Production Specialist, UW-Madison, Dept. of Horticulture, 608-265-4781, Email: wang52@wisc.edu.

    Even with the recent cool and wet spring weather, potatoes that were planted before the third week of April have had at least 20% emergence at this point. Growers in Central Sands have started their first hilling. For those spuds that were put into ground after late April, it might take another several days to emerge. The forecasted weather in the next 10 days looks promising to encourage emergence and plant growth, and the light-textured soil in the Central Sands region has helped the excessive soil water to drain out of the rooting zone, which avoided soil saturation and seed tuber decaying issues. The recent rains have delayed planting of processing vegetables (peas, green beans, sweet corns, carrots, and beets) by less than 5 days so far, but like potatoes, the upcoming week or so will be good to catch up.

    Growers in other parts of the state with heavier soils are running more behind schedule, by between one and even two weeks. Some folks with wet fields and high water tables might end up with some acres unplanted. For potatoes, seed decaying might be an issue, and for processing vegetables, the cool and wet environment is favorable for root rot and downy mildew development, which are all concerning the growers and processors. Processing vegetable planting in Minnesota and Illinois are being substantially delayed due to the unfavorable weather conditions.

    This past Tuesday I got a chance to visit the planting of dark red kidney (DRK) beans in Dunn County, which is the primary production region of the state for this crop. In recent years, DRK beans have migrated to Central Wisconsin at a good rate (e.g. planted acreage doubled from 2018 to 2019) due to the growing overseas market. DRK bean is a 90-day vegetable crop that can fix nitrogen and contribute to soil health / fertility improvement. In the near future our program will work on making production management practices for this crop grown in Central Wisconsin.

     

    Amanda Gevens, Associate Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, 608-575-3029.

    Please have a look at our new UW-Madison Potato & Vegetable Pathology website.  New resources and easier to access weather and disease forecasting data. https://wivegdis.plantpath.wisc.edu/

    Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations (Many thanks to Ben Bradford, UW-Madison Entomology; Stephen Jordan, John Hammel, & Samuel Meyer, UW-Madison Plant Pathology).

    A P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application.  A DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application.  Red text in table indicates threshold has been met/surpassed.  TBD indicates that data is To Be Determined as time progresses.  Weather data used in these calculations comes from weather stations that are placed in potato fields in each of the four locations.  New this year! Data are available in graphical format for each weather station at: https://wivegdis.plantpath.wisc.edu/dsv

     

    Dr. Michelle Marks, UW-Madison Plant Pathology & Soil Science

    Hop Powdery Mildew Background:  Wisconsin hop growers should be keeping an eye out for hop powdery mildew, a fungal disease of hops that until recently has been relatively sporadic within WI, with formal diagnosis and reporting in 2016.  This disease is a significant issue in the Pacific Northwest and is becoming more prevalent in other hop growing regions of the country as the hop industry expands.  Increasing numbers of growers here in Wisconsin are reporting powdery mildew infections in their hop yards, and may wish to consider incorporating fungicides that protect against this disease into their chemical management rotations.

    Hop powdery mildew is caused by the fungal pathogen Podosphaera macularis.  As will be discussed in detail later in this article, the pathogen is capable of overwintering as asexual mycelia in dormant hop buds or in sexually-produced survival structures called chasmothecia.  The pathogen resumes growth again in the spring, releasing spores that can to on to infect healthy plant tissue.  Disease is favored by moderate temperatures (64-70°F) and cloudy, humid weather.

    Symptoms include powdery, white colonies that start small and usually expand over time.  Colonies are typically more numerous on the leaf surface, and may be discrete or cover larger surface areas.  Chlorosis, or yellowing of plant tissue, may be seen surrounding powdery mildew lesions.  Leaves may also develop blister-like symptoms.  Hop cones are particularly susceptible to infection.  If infected early in their development, cones may abort or become necrotic, distorted, and deformed.  Later infections will exhibit the classic fluffy white fungal growth.  Hop plants themselves can generally withstand heavy infections and it is these cone infections that lead to yield losses and decreased marketability.

     

    Graduate Research Assistant Bill Weldon of Cornell University

    Hop Powdery Mildew Pathogen Disease CycleIt is important to understand the way in which the hop powdery mildew pathogen overwinters so that management of initial disease pressure can be optimized.

    Hop powdery mildew has been a major focus of disease management programs of hop growers since at least as far back as the nineteenth century. As the hop industry has continued to grow throughout the Midwest and Eastern US, so too has the prevalence of hop powdery mildew. This disease, caused by a pathogenic fungus called Podosphaera macularis, can survive winter each year and cause disease the following year in one of two ways.

    From the perspective of the fungus, strategy one is centered on asexual reproduction, which is the fluffy, white growth form of the fungus that is most commonly associated with the disease. If in the fall, this asexual growth form can extend down just below the soil line and grow on to hop buds that go dormant over winter, come spring when these buds break ground, they risk emerging covered in powdery mildew growth as well. These early season infected buds are termed flagshoots due to their strikingly white appearance in comparison to normal green, heathy hop tissue. Previous research out of the Pacific Northwest has documented that at most, hop powdery mildew overwintering through flagshoots will result in <2% of hop shoots infected at the start of the season.

    The second overwintering strategy of P. macularis is the formation of chasmothecia, which is a fungal overwintering structure made through sexual reproduction. In comparison to the white, powdery asexual growth, chasmothecia are more like little microscopic black “shells” that house and protect 4-8 sexually produced spores (called “ascospores”) on the inside. Thousands of chasmothecia can form on a single hop leaf, each containing and protecting a handful of ascospores all winter long until they can be released in the spring. It is not known exactly how much better chasmothecia are at overwintering, but the general thinking is much, much better than the 1-2% success rate associated with hop flag shoots. It is also not known what specific weather conditions cause chasmothecia to crack open in the spring and release the infectious ascospores onto newly emerged hop shoots. The latter question is what labs at Cornell, UW-Madison, and NC State are teaming up to address.

    Our goal is to best describe what weather factors drive the release of hop powdery mildew ascospores in the spring. Why? If we understand what drives the release of these spores, we can then monitor for these weather conditions in the spring each year, identify the high-risk infection periods, and take precise control measures in real time.

    How are we going about this? During the winter of 2019 we kept populations of hop powdery mildew chasmothecia in Wisconsin, New York, and North Carolina. These three geographies broadly represent the range of winter severity where hops are grown — with Wisconsin being the coldest, North Carolina being the most temperate, and New York falling somewhere in the middle. Including three different winter types will help make this data applicable to a broad range of hop growing regions.

    At weekly intervals starting in January, we collect a subset of chasmothecia at each location and test for their ability to release ascospores. By repeatedly doing this each week over the course of the spring, we developed a maturation curve for chasmothecia spanning from January to May in each of the three locations. This is then compared side by side to recorded weather data such as daylength, daily rainfall, temperature, and relative humidity to elicit the most important conditions associated with when ascospores are released, and when the ascospore supply completely runs out. In this way, we are working towards having a predictive model for the hop grower community to use for targeted disease management of early season hop powdery mildew infection due to chasmothecia.

     

    Amanda Gevens, UW-Madison Plant Pathology

    Status of hop powdery mildew control in WI: We have observed an increase in incidence and severity of hop powdery mildew over the past few years.  In response, growers are incorporating fungicides with specific activity against powdery mildew in prevention of the disease.  While their disease names are similar, the pathogens causing powdery mildew and downy mildew are substantially different.  Powdery mildew is caused by a true fungus and downy mildew is caused by an oomycete or water mold pathogen.  The fungicides relied upon for downy mildew control have little to no activity against the true fungal pathogen that causes powdery mildew.  Below, I provided a listing of registered fungicides for powdery mildew control in hop in Wisconsin.  This listing is also available in the “Hop” Chapter of our 2019 A3422 Commercial Vegetable Production Guide for Wisconsin.  Additionally, I listed the hop downy mildew fungicides for the purpose of comparing which selections are suitable for each mildew disease, and which are suitable for addressing both diseases.

    Our UW-Madison Plant Pathology Plant Disease Diagnostic Clinic offers a suite of hop disease tests.  The PDDC website is:  https://pddc.wisc.edu/

    We are very interested in suspect or certain hop powdery mildew samples for further characterization.  Please contact me at gevens@wisc.edu for shipping information.  Thank you!

     

    Accessing the 2019 University of Wisconsin Madison Extension Commercial Vegetable Crop Production Management Guide:  Our production guide is updated every October with release of a new guide in January.  The book can be downloaded for free as a pdf at the link below, or can be purchased online for $12.50. https://learningstore.uwex.edu/Assets/pdfs/A3422.pdf

  • Update 4 - May 19, 2019

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    Jed Colquhoun, Professor & Extension Specialist, UW-Madison, Dept. of Horticulture, Email: colquhoun@wisc.edu

    Sonalan HFP Special Local Need 24(c) label approved for Wisconsin potatoes:  With much appreciation to Gowan Company and Wisconsin DATCP, a Special Local Need 24(c) label has been approved for Sonalan HFP herbicide on Wisconsin potatoes through December 31, 2023.  Sonalan HFP is less soluble than several other potato herbicides such as S-metolachlor and can assist in the control of weeds such as nightshades and wild oat.  The active ingredient is ethalfluralin.  It controls weeds prior to emergence only and should not be used on soils with >10% organic matter, such as muck.  The use rate varies by soil type.

    The herbicide has been labeled for use in potatoes in several western states and we’ve included it in our research on silt loam and coarse-textured soils for years.  In this research we have not observed crop injury across years, soil types or varieties, nor have we noted any significant negative affect on tuber yield or size distribution.  Our best weed management has been where we’ve combined Sonalan HFP with another potato herbicide (such as metribuzin or linuron, if varieties, soil type and depth to groundwater allow) to broaden the control spectrum in our typical hill-spray operation to potatoes prior to emergence.

    Given the timing relative to this season’s planting and pre-emergent herbicide application, and the fact that this is a new use in Wisconsin, it’s always better to try a new herbicide program in a small area first so that you can compare it to your standard program in terms of crop safety and weed management.

    The 24(c) label is available on the Wisconsin DATCP special pesticide registrations web site: https://datcp.wi.gov/Documents/SpecialUses.pdf.

     

    Amanda Gevens, Associate Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, 608-575-3029.

    Understanding and managing white mold in potato and vegetable crops (previously printed in Badger Common Tater May 2019):  In recent years, growers have seen an increase in white mold in multiple crops in Wisconsin.  A recent report from Dr. Damon Smith, UW-Madison Field Crops Extension Pathologist summarized the history of the disease in Wisconsin, as follows. The disease was initially detected in soybean in central Illinois in 1948 and escalated into a chronic problem by the 1970s in several North Central states, especially where soybeans were grown in rotation with other susceptible crops.  By the early 1990s, the occurrence of white mold became widespread throughout the region, and had progressed from a sporadic disease to an annual threat to soybean production.  In my observation of vegetable crops over the past decade in Wisconsin, we have seen an increase in incidence and severity of white mold in several crops including snap bean, potato, and tomato.  In some years, we see the disease show up on pumpkins, cabbage, and carrots as well, with significant yield- and quality- limiting effects.

    White mold is a plant disease caused by the soilborne fungus, Sclerotinia sclerotiorum.  The disease can cause losses in yield and quality in a broad range of crop species due to damage on many above ground plant parts including foliage, crowns, fruits, and pods (Figure 1).  The pathogen can infect many crops (over 350 plant species!) including snap beans, cabbage, carrot, pea, pepper, potato, pumpkin, soybean, and tomato.  Additionally, sunflower, fruit crops and weeds including dandelion and wild clover can be susceptible.  Because the host range is so broad, inoculum builds up in the soil over time.  The soilborne survival structures, or sclerotia, can survive for up to 8 years in the soil and can germinate under the plant canopy, when near the soil surface, to form apothecia (mushroom-like structures that produce airborne spores) (Figure 1).  The airborne spores typically infect plant flowers and create diseased stems and pods. In other crops, the sclerotia can germinate directly and infect plants at the soil line, or can directly infect fruit.

    White mold symptoms on potato first appear at 2 to 3 weeks after row closure as water-soaked lesions on lower stems, or any plant part that is in contact with the soil.  Potato cultivars vary in susceptibility.  After water-soaking, the pathogen typically produces white cottony growth (mycelia) that quickly spreads to other foliar parts, lower in the canopy, if conditions are wet for several hours.  As disease progresses, the stems can girdle and foliage will wilt.  If conditions become dry, lesions will also dry out, and become tan to bleached white in appearance.  Sclerotia, or the pathogen’s long term storage structures that can remain in soil for many years, form on or outside of infected tissue.  The sclerotia are irregularly-shaped and can greatly range in size depending upon the plant part they form on or within; masses change in color from off white to black as they mature.  White mold symptoms have not been observed on below-ground plant parts.

    Management requires an integrated approach including a 3+ year non-host crop rotation, irrigation management, and chemical and/or biological control to reduce disease risk.   Sclerotia typically die within 3-5 years if they’re not deeply buried.  While deep burial limits inoculum germination in the short-term, if the field is deeply tilled in future, the pathogen will resurface and remains viable longer.  Direct-drilling or spinning grass seed, wheat, or cover crops into standing bean residues can keep sclerotia at or near the soil surface. Alternatively, sclerotia could be left on the soil surface by leaving residues in place without planting a winter or cover crop.  Non-hosts of the pathogen include sweet and field corn, wheat, small grain cover crops, and annual and perennial grass seed crops.

    White mold is favored by a dense, closed plant canopy. Keeping plant surfaces dry reduces disease.  Apothecia form and release spores when soils are continuously moist. Successful infection requires a continuous leaf wetness period of 16 to 48 hours at 54 to 74°F. Wider row spacing and row orientation with the prevailing winds can help keep plants and the soil surface drier. Avoid over-fertilization with nitrogen as that increases canopy size.  No commercially available snap bean cultivars have complete resistance.  Potato, tomato, and several other vegetable cultivars vary in their resistance to the white mold pathogen, but in general, earlier maturing cultivars tend to exhibit the most disease.


    Figure 1.  Far left and center photos show tan to orange-colored apothecia which can be seen at the soil surface under conditions of high soil moisture.  Photos courtesy of Dr. Jaime Willbur, Extension Plant Pathologist, Michigan State University.  Photo on far right shows typical symptoms and signs of white mold on snap bean pods.  Black structures are sclerotia forming within plant tissue.

    We conducted two years of research on the effectiveness of fungicides on white mold of potato at the UW Hancock Agricultural Research Station, however, our disease incidence and severity remained very low and we could not discern white mold differences among treatments.  The programs designed to target white mold and early blight with treatments including tank mixes of a base protectant of chlorothalonil plus either boscalid (Endura, FRAC 7), fluopyram + pyrimethanil (Luna Tranquility, FRAC 7 + 9), or pydiflumetofen or adepidyn + fludioxonil (Miravis Prime, FRAC 7 + 12) in early and late July (specifically July 9 and 23 in 2018) were highly effective against early blight as seen in other trial locations.

    In a 2014 report, Dr. Jeff Miller of Miller Research LLC in Rupert Idaho determined that sequential applications of base protectants alone (chlorothalonil and mancozeb) significantly reduced early blight, but increased white mold compared to the non-treated control plots.  However, when he included fungicides such as boscalid (Endura) or fluopyram + pyrimethanil (Luna Tranquility) to a program with a base protectant, there was significant reduction of both early blight and white mold.  Initiation of fungicide programs was at row closure, with subsequent sprays at either 10 or 14 day intervals depending upon the cultivars, resulting in a total of 4 applications.

    Several fungicides are registered for white mold control in vegetable crops in Wisconsin.  In the following paragraph, I’ve included a non-comprehensive list of fungicides that have activity on white mold of potato given the likely readership of this trade magazine.  While several of the fungicides are, or include, a FRAC 7 fungicide (SDHI or Succinate dehydro-genase inhibitors), they fall into 4 unique subgroups of SDHIs and can result in disease control differences.

    boscalid FRAC 7 (Endura), fluazinam FRAC 29 (Omega 500F), fluopyram FRAC 7 (Velum Prime), fluopyram + pyrimethanil FRAC 7 + 9 (Luna Tranquility), iprodione FRAC 2 (Rovral 4 FLowable), metconazole FRAC 3 (Quash), penthiopyrad FRAC 7 (Vertisan), picoxystrobin FRAC 11 (Aproach), pydiflumetofen + fludioxonil FRAC 7 + 12 (Miravis Prime), pyraclostrobin FRAC 11(ie: Headline), pyraclostrobin + fluxapyroxad FRAC 11 + 7 (Priaxor Xemium), and thiophanate methyl FRAC 1 (ie: Topsin M WSB).  My program continues to pursue field research trials to evaluate best white mold disease management plans in potato and snap bean crops.  Our goal is to define fungicide selection and application timing for best management of white mold, along with early blight in Wisconsin fields.

     

    Accessing the 2019 University of Wisconsin Madison Extension Commercial Vegetable Crop Production Management Guide:  Our production guide is updated every October with release of a new guide in January.  The book can be downloaded for free as a pdf at the link below, or can be purchased online for $12.50.    https://learningstore.uwex.edu/Assets/pdfs/A3422.pdf

  • Update 3 - May 12, 2019

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    Vegetable crop production updates: There were several good days for planting in the last two weeks, with about half inch rain on other days. Folks south of Plainfield are mostly done with planting at this point, and those to the north have about 30% left. Seed potato planting in the Antigo area has not been started yet due to rainy weather. However with the upcoming forecasted conditions, planting should go pretty smoothly and those that have been put into ground should sprout fast. In general, potato planting in Wisconsin is only delayed by a few days from optimal.

    Tillage is a common practice for Wisconsin potato and vegetable growers. On light-textured sandy soils, it is not advisable to make the tilth too fine, because after a heavy rainfall or irrigation there might be a high chance of severe erosion of the ridges. Planting depth and method of soil preparation can affect soil temperature and moisture conditions around the seed tuber/pieces, both of which can affect emergence. Planting depth should be adjusted according to the soil conditions. Soil in the deeper layers dries out more slowly than surface soil, and therefore planting should be deep enough to reach the drier layers. In cool or moist conditions, shallower planting is recommended. The soil should not be cultivated deeper than necessary. Soil moisture can be lost with each cultivation, and therefore the number of operations should be kept to a minimum and performed shortly before planting.

    The plant actually depends on the seed tuber for energy and nutrients for quite a while after being planted. Research has shown that the longer the seed tuber remains intact, the more productive the plant will be. Ideally, the seed tuber should remain sound until the plant has a chance to utilize all the nutrients and energy stored within it. The seed tubers provide a large portion of the plants’ nutrient and energy up until the time the plants grow to be about 10 to 12 inches tall. At that time they have enough leaf area and root system to support future growth. Starting planting after soil temperature reaches 45F and climbs, using whole seed or leaving the cut seed to wound heal long enough (at least 5 days), can all help prevent seed decaying and enhance vigorous emergence.

     

    Disease Forecasting: What are potato Blitecast DSVs and P-days?: Locations of in-field weather stations/disease forecasts will include:  Antigo, Plover, Hancock, and Grand Marsh. Thank you to Jim Okray in Plover and Jim Mortenson in Grand Marsh for hosting two of our weather stations.  The Hancock station is located at the UW Hancock ARS.  We will place the Antigo station at the Langlade County airport research area later this month.  Many thanks to Ben Bradford (Groves Lab) for support in establishing our new website and tools for disease forecasting.

    Blitecast (late blight forecasting):  Computation of 18 disease severity values (DSVs) relies on maximum and minimum temperatures each day, the duration of relative humidity periods above 90% and the maximum/minimum temperatures during the relative humidity periods above 90%.   For a given day, up to 4 DSVs can accumulate.  We start the severity value calculations at approximately 50% crop emergence.  When we reach a total of 18 severity values, we issue a warning which indicates that environmental conditions have been met which favor late blight.  At 18 DSVs, the recommendation for preventive applications of effective late blight fungicides is made.  An additional alert is issued when the first symptoms of late blight appear anywhere in the state.  The determination of late blight management recommendations is made by taking into consideration DSVs, projected weather forecast, and presence/risk of inoculum.  This information is published in our newsletter and will be disseminated in various other outlets as the season progresses.

    The Potato P-Day accumulator (early blight prediction tool) is based on potato physiological development and accumulated weather conditions to generate early blight recommendations.  Once we reach 300 P-Days, calculated from 50% crop emergence onward, our spray recommendations take both the P-Day and severity value totals into account to generate 5 day, 7 day or 10 day spray interval recommendations.  The interval is variable depending upon prevailing weather conditions and the presence of disease in the area.  Typically, P-Day 300 is reached in early July and when potato rows are just beginning to touch (row closure).

    As in past years, we will continue to provide Blitecast information via this newsletter and through the vegetable pathology website:   https://wivegdis.plantpath.wisc.edu/.  We will have in-potato-field weather stations in Grand Marsh, Hancock, Plover, and Antigo as in past years, with access to the station data (with DSV and PDay values at:  https://wivegdis.plantpath.wisc.edu/dsv/).  New in recent years, is the Vegetable Disease and Insect Forecasting Network (VDIFNet) site which provides information on DSVs from NOAA weather data across the state of WI, as well as insect phenological data (Dr. Russell Groves, UW-Madison Entomology).  The link to the VDIFNet site is:  https://agweather.cals.wisc.edu/vdifn/maps.

     

    Accessing the 2019 University of Wisconsin Madison Extension Commercial Vegetable Crop Production Management Guide: Our production guide is updated every October with release of a new guide in January.  The book can be downloaded for free as a pdf at the link below, or can be purchased online for $12.50. https://learningstore.uwex.edu/Assets/pdfs/A3422.pdf

  • Update 2 - April 28, 2019

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    Yi Wang, Assistant Professor & Extension Potato and Vegetable Production Specialist, UW-Madison, Dept. of Horticulture, 608-265-4781, Email: wang52@wisc.edu.

    Potato and vegetable planting – crop updates:  Potato and vegetable planting has been well underway in Wisconsin this week, with Mother Nature being cooperative most of the time. The forecasted couple of inches of snow on April 27th should melt pretty fast under the current warm soil temperature and will likely not affect the progress of planting too much. However, weather next week looks to be wet and cloudy, which might slow things down especially in fields with raised water table and low elevations.

    The Potato Seed Calculation is a good tool published by Dr. Andy Robinson on the NDSU potato extension website: https://www.ag.ndsu.edu/potatoextension/tables-for-potato-seed-and-plant-population. Formulas are available in the Excel tables to calculate the amount of potato seed needed to plant at different row widths, within-row spacing, and by seed piece size.

    Potato tuber black heart:  In the past several weeks, Amanda and I received some questions about a dark black discoloration issue in or around the center of tubers (picture below).

    In the field, black heart is more commonly seen in waterlogged soils particularly in low areas. Saturated or nearly saturated soils can promote anaerobic conditions and reducing gas exchange within the tuber. Long exposure to high field temperatures (>90°F) before harvest can also lead to black heart. During storage, it can occur when proper ventilation is not supplied to the tubers. Warmer storage temperatures can increase tuber respiration rates and oxygen demands, which will facilitate black heart development especially when ventilation is not enough. Tubers used as seeds with black heart issues will have lower vigor and may not support emergence.

    Therefore, management practices to avoid or reduce black heart include:

    • Appropriate irrigation scheduling to minimize soil moisture accumulation, especially in the low areas of the field
    • Maintenance of sufficient ventilation and proper temperature during storage
    • Avoid deep pile and closed bins
    • Minimize the amount of dirt and debris in storage that might prevent good airflow
    • Proper distribution of air ducts in the storage facility
    • Avoid poorly aerated trucks during transportation

     

    Amanda Gevens, Associate Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, 608-575-3029

    Potato early season disease considerations: Wet and cool soils delay germination and emergence.  Such conditions also promote activity of plant pathogens, such as Rhizoctonia solani, a potentially seed-, soil-, or debris-borne fungal pathogen which causes stem or stolon cankers resulting in reduced stands, stunted plants, and/or reduction in tuber number, size, or quality.  Later in the season, Rhizoctonia can also cause black scurf on tubers.  Cultural management approaches such as planting when soil temperatures are more consistently above 46°F, planting into well-drained soils, avoiding planting too deep, and avoiding hilling prior to adequate emergence can limit early season stem and stolon canker.

    Several other seed-, soil-, and/or debris-borne diseases can also impact the potato crop, including Fusarium seed piece decay caused by the fungus Fusarium sambucinum, Silver scurf caused by the fungus Helminthosporium solani, and Late blight caused by the oomycete Phytophthora infestans.  While optimum temperatures for promoting each of these diseases vary, all require high soil moisture levels.

    Fusarium, as a dry rotting pathogen which requires wounds for entry, can affect quality of seed potatoes in storage and lead to further disease concerns when potatoes are moved and warmed for planting.  As a seed piece decay pathogen, Fusarium can affect seed immediately after cutting and through to sprouting.  If initial and subsequent sprouts continue to be affected by Fusarium, the seed piece loses vigor and stand is reduced.

    The Silver scurf pathogen is favored by warmer conditions and is recognized as a weak soil-borne and a stronger seed-borne pathogen.  Typically, symptoms are not evident on tubers at harvest, but develop over time in storage.  The longer the tubers remain in the ground after vine kill, the greater the risk for development silver scurf. Blemishes on tubers are restricted to the periderm.  However, damage to the periderm causes increased water loss and shrink.  The pathogen is not known to cause above ground plant symptoms.

    Fungicide seed treatments have a place in an integrated pest management (IPM) plan which includes cultural practices such as i) planting certified potato, ii) proper handling and sanitation of storage/cutting/curing facilities prior to planting, iii) cultivar resistance, iv) biological control, and v) chemical control.  In combination, IPM practices minimize economic losses to disease, minimize environmental effects, limit risk of pesticide residues in the food supply, limit development of fungicide-resistant pathogen strains, and limit development of pathogen strains which may overcome host disease resistance.

    Seed cutting and planting events provide opportunities for application of fungicides to reduce negative effects of diseases such as Rhizoctonia, Fusarium, silver scurf, and late blight.  While this article specifically addresses seed treatments in potato disease control, several potato fungicides are registered for in-furrow application and are also effective in managing seed- and soil-borne diseases.  While seed-applied fungicides can enhance disease control and crop success, be mindful that some of the fungicides are contact only (ie: mancozeb and fludioxonil) and are active by limiting direct infection to the protected seed piece.  Systemic fungicides (ie: flutolanil and cymoxanil) are xylem mobilized, moving the fungicide upward and outward (acropetally) for protection beyond the point of contact.  Generally, seed-applied fungicides provide, at most, 10-14 days of disease protection.  However, some active ingredients can protect seedlings considerably longer when applied at the highest labeled rate.

    Typically, seed treatments are applied right after cutting with either a liquid or powder formulation.  Taking care to avoid clumping or thick coating of the treatment is important as you can cut off oxygen to the seed piece and limit suberization (and promote soft rot).  Good suberization of cut seed pieces is a critical component of potato disease management and should include a 3-4 day, 50-55°F, 90-95% relative humidity period with cut seed piled no deeper than approximately 6 ft to maximize airflow throughout the pile.

    Seed treatments in potato have received increased interest and use in recent years due to improvements in active ingredients available, and the return on the investment of early season disease control.  As there are no true rescue treatments for underground diseases post-planting, seed treatments provide a most effective use pattern with added benefits of relative ease of application, small volumes of fungicide necessary, no spray drift, and no waste or negative impact on non-target sites.

    Several fungicides with effective control of multiple diseases are available with registration for application to seed pieces prior to planting.  Always read and follow the pesticide label prior to use.

     

    In special consideration of late blight control in potato, key components of management include:

    • Destroy all potato cull piles (May 20 deadline by DATCP)
    • Manage potato volunteers in all fields –volunteers pose great risk for late blight introduction
    • Acquire disease free seed from a reputable certified source –infected seed poses great risk for introduction
    • If there is a risk of disease associated with seed, use seed treatment or in-furrow application of effective late blight controlling fungicides (seed treatment is best)
    • Apply only proven effective fungicides for control of late blight when disease forecast tool indicates environmental risk and stay on a fungicide spray program (DSVs reach 18)
      1. For conventional systems, a current list of registered late blight-specific materials can be found in the Commercial Vegetable Production in Wisconsin A3422 publication (further information below)
      2. For organic systems, copper-containing fungicides have been long-standing effective materials for preventing late blight in susceptible crops. Some newer organic fungicides are also available with promising late blight control (ie: Zonix, EF400).
    • Scout regularly and thoroughly for disease in all potato fields
    • Re-apply effective fungicides for disease control on a 7 day schedule (recommendation adjusts to a 5 day schedule when late blight is in the area and weather favors disease; recommendation adjusts to a 10 day schedule when late blight is not found in area and weather is hot and very dry)
    • If late blight is identified in a field, have a mitigation plan in place for specific site. Depending on days to vine kill, environmental conditions, and extent of infection – plan may vary from complete crop destruction to early vine kill with continued maintenance fungicide sprays.  Mitigation plan should limit disease spread within field and from field-to-field.

    We will continue to provide Blitecast information via this newsletter and through the vegetable pathology website:   https://wivegdis.plantpath.wisc.edu/.  We will have in-potato-field weather stations in Grand Marsh, Hancock, Plover, and Antigo as in past years, with access to the station data (with DSV and PDay values at:  https://wivegdis.plantpath.wisc.edu/dsv/#hancock).  New in recent years, is the Vegetable Disease and Insect Forecasting Network (VDIFNet) site which provides information on DSVs from NOAA weather data across the state of WI, as well as insect phenological data (Dr. Russell Groves, UW-Madison Entomology).  The link to the VDIFNet site is:  https://agweather.cals.wisc.edu/vdifn/maps.

     

    Accessing the 2019 University of Wisconsin Madison Extension Commercial Vegetable Crop Production Management Guide:  Our production guide is updated every October with release of a new guide in January.  The book can be downloaded for free as a pdf at the link below, or can be purchased online for $12.50.    https://learningstore.uwex.edu/Assets/pdfs/A3422.pdf

  • Update 1 - April 3, 2019

    View as PDF

     

    Get ready for volunteer potatoes – Jed Colquhoun, Professor of Horticulture and Extension Weed Specialist, UW-Madison, colquhoun@wisc.edu

    Several Wisconsin potato fields unfortunately couldn’t be harvested last fall given wet weather that led right into freezing temperatures.  While it might seem that the record cold in late January would eliminate the risk of potato volunteers this spring, keep in mind that snow is a great insulator.  Volunteer potatoes are most common when winter soil temperatures at the tuber depth remain above 28 F in moist soils or 25 F in dry soils. Minimum soil temperatures at a 2-inch depth in Hancock ranged from 20 to 26 F in the last two weeks of January, but deeper tubers and in areas where the ground was blanketed by deep snow likely didn’t reach those lows.  With that in mind and given the number of tubers still in the ground at various depths we expect some reports of volunteers over coming weeks.

    Volunteer potatoes can not only significantly reduce rotational crop yield, but more importantly can serve as untreated hosts for pests that threaten this year’s nearby potato crop, such as late blight and Colorado potato beetles.

    Volunteer potato control in Wisconsin rotation crops can be very difficult given the lack of effective herbicides in specialty crops.  Three management goals should be kept in mind when dealing with volunteer potatoes:

    1. Minimize untreated host foliage and tuber formation for potato pests;
    2. Reduce competition with the current rotational crop; and
    3. Minimize viable daughter tuber formation that will impact subsequent rotational crops.

    Successful management will require an integrated approach given the large carbohydrate reserves in the tuber and the ability of potatoes to re-sprout after treatment.  If the rotational crop has not yet been planted and there is flexibility in moving crops among fields, the choice of rotational crop is by far the most important decision where volunteer potatoes could be severe.  Volunteer potato control in less competitive specialty crops with limited herbicide options, such as carrot or onion, is extremely difficult.  Volunteer potato control is probably most feasible in sweet and field corn.  Registered herbicides containing the active ingredient mesotrione, such as Callisto, are very effective in not only reducing potato foliage but also daughter tuber production.  Postemergence applications of mesotrione are most effective – applications prior to potato emergence will not be effective at all.  In our volunteer potato control studies over the years Callisto was more effective and consistent than other corn herbicides in reducing daughter tuber formation.  Keep in mind that the rotational restrictions for mesotrione can be lengthy for some crops, and some premix herbicides that contain mesotrione also contain atrazine – thus use may not be appropriate for your particular location and crop rotation.

    Several other herbicides have been observed in research and practice to reduce volunteer potato growth and tuber weight when growing in a number of crops, such as fluroxypyr (example trade name: Starane), atrazine (several trade names; check prohibited zones), imazamox (example trade name: Raptor), dicamba (several trade names), and glyphosate (Roundup and several other trade names).  Contact herbicides such as oxyfluorfen (example trade name: Goal) can also be moderately effective, particularly if repeated applications are allowed on the crop.  Check the labels for registered crops and use rates for these options.  In many cases, the effectiveness of these herbicides in reducing daughter tuber formation is improved when followed by cultivation after the herbicide has been adequately translocated throughout the plant.

    In the processing row crops that won’t be planted until later in the spring, such as snap bean and sweet corn, volunteer potato can be reduced by allowing the potato to sprout and removing the plants with tillage or glyphosate (if appropriate and labeled for the crop to be planted) prior to rotational crop planting.  Re-sprouting potatoes can be removed with cultivation and/or post-emergence herbicides during the crop season.

     

    (UPDATED) Potatoes left in the field – fertility and other considerations – Ken Schroeder, Associate Professor Agriculture Agent, University of Wisconsin-Madison Extension, ken.schroeder@wisc.edu & Matt Ruark, Associate Professor & Extension Soil Scientist, University of Wisconsin-Madison, mdruark@wisc.edu

    For fields where potatoes were unable to be harvested, the unharvested tubers represent a potential source of nutrients for the next crop. However, there are knowns and unknowns in knowing if the nutrients in the potatoes will become available to the subsequent crop.

    We do have a good estimate of the nutrient content of the tubers (Table 1). Good yielding potatoes contain over 200 lb/ac of N and K. The nutrient content of P and S is not substantial, so only cutting back on N and K should be considered.

     

    Table 1. Estimates of nutrient content of tubers from a 500 cwt/ac yielding Russet Burbank (adapted from Horneck and Rosen, 2008).

    Nutrient Nutrient content of tubers (lb/ac)
    Nitrogen 210
    Phosphorus 29
    Potassium 240
    Sulfur 22

    The big unknown is how much of the N and K will be released during the growing season (and at a time available for subsequent crop uptake). Here we provide considerations for taking nutrient credits from tubers left in the ground, working under the assumption that the fields will be tilled in the spring and that a high nutrient demand crop (corn or sweet corn) will be grown).

    Assess the state of tuber decomposition. Are the tubers decomposed or observable at the time of corn planting? If they aren’t observable (already decomposed), then not likely there will be much N left in the soil. Would recommend taking spring soil samples to guide P and K fertilizer applications. If they are a mushy mess (i.e. currently decomposing) (Figure 1) then it is likely that some of the nutrients could become available for the next crop. The C:N ratio of potato tubers have been measured to be 26 to 28 (Amber Moore, personal communication). For most soils, this would suggest there wouldn’t be much in terms of available N. C:N ratios around 30 typically result in no net mineralization or immobilization (i.e. more N or less N). However, on sandy soils, mineralization can happen quickly, and it may be more likely that the tubers can provide N to the subsequent crop. Based on this information and as long as the potatoes are starting to decompose at the time of corn planting, we would expect there would be some N available. In this scenario, we would recommend taking no more than 1/3 of the estimated nutrient content as nutrient credits. It’s important to note that there is data to suggest a specific credit.

    An interesting comparison is with previous research trials assessing if a radish cover crop will supply N to the subsequent corn crop. On silt loam soils, radish was planted after winter wheat, and died during the winter. The radish turned to mush quite readily in the spring and little observable root biomass existed at time of corn planting. In these scenarios, the N contained in the radish biomass (which ranged between 20 and 100 lb/ac of N) was not available to the corn crop. And thus, we concluded there would be no N credit associated with a radish cover crop. These results would suggest that caution should be applied when attempting to credit biomass from root crops.

    If the tubers are desiccated in place (i.e. dried and hard, but not decomposed), then need to observe the decomposition of the tubers over time. Would recommend taking no more than a ¼ of the nutrient content as nutrient credits. In this case, it is unknown if the tubers are going to function as a slow-release fertilizer or not.

    Soil nitrate testing would not be recommended on sandy soils. Both PPNT and PSNT soil tests are only calibrated and recommended for medium and fine-textures (loamy) soils.

    Consider planting spring cover crops for nutrient scavenging. Sandy soils have high potential for nitrate leaching and wind erosion. If these unharvested fields will be planted later in spring to early summer, consider putting out a cover crop to take up the available nitrate. Look to cover crop varieties that are good N scavengers and have rapid emergence in cool soils making them effective with short growing windows. Cool-season annual grasses will be best suited for these conditions.

    Variety N scavenger Germination Temperature oF Days to Germination Growth Rate
    Annual Ryegrass Excellent 40 14 Rapid
    Oats Very good 38 5-8 Very rapid
    Barley Very good 38 6-8 Very rapid
    Mustards Very good 40 5-7 Very rapid
    Rapeseed Very good 41 4-10 Very rapid

     

    Spring-seeded cover crops. Previous research at the Hancock Agricultural Experiment Station has shown that spring seeded oat cover crops can be a valuable cover crop if followed by a later planted sweet corn crop. But the study resulted in two extremely different springs growing conditions. In 2014 the oat biomass never really took off and less then ¼ ton of dry matter biomass was produced (left picture in Figure 3), but in 2015 almost 3 tons of dry matter biomass were produced. So basically, who knows what will happen, but if oat won’t grow, then likely nothing will. The large amount of biomass did result in requiring 25 more lb-N/ac to maximize sweet corn yield, but did not affect the maximum yield achieved. If the cover crop starts to get above 1 ton/ac of dry matter biomass, then we would suggesting terminating at that time, regardless of when the subsequent crop will be planted.

    Observations from the Field. On Tuesday March 26, soil on top of the hills was frozen down 3 to 4 inches, on Wednesday all frost was out of the hills.  Tubers in the hill appeared to have been frozen and cells had burst leaving tubers very watery and at various stages of breakdown ranging from mushy to rubbery consistency and white in color to brownish-grey inside.  Skins were mostly shriveled and loose from the interior potato.

    Take Home Thoughts.

    • Scout fields early to assess condition of tubers
    • Develop a plan for volunteer potato management
    • Test your soil and consider potential nutrient credits
    • Consider use of cover crops for nutrient scavenging and wind erosion control
    • Take notes and observe differences in crop growth and development in fields with both harvested and unharvested areas

     

    Determining volunteer risk for preventive disease management – Amanda Gevens, Associate Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, 608-575-3029

    Potato volunteers can be a challenge to manage when considering appropriate cultivation and chemical approaches.  In any year, some potato tubers can remain buried in the soil and unharvested if they are too small, or if they are intermingled with stones.  As previously stated in this newsletter, Fall of 2018 brought on too much water at harvest, followed abruptly by freezing conditions making harvest unattainable for many Wisconsin production acres.  Mild winter conditions can then support the viability of the tubers producing an unwanted seed source in the subsequent year.  Volunteers pose a threat as weeds as well as a source of pathogens, such as Phytophthora infestans, causal agent of late blight, and potato virus Y or PVY of potato.  Given the late season environmental conditions in 2018, some fields were necessarily left unharvested posing added risk for volunteer emergence this spring.

    Studies at Michigan State University led by Dr. Willie Kirk showed that tubers of most cultivars appear to breakdown after exposure to 27ºF for about one day.  Several years back, Kirk’s research group (including Phillip Wharton, Kathleen Baker, and Lee Duynslager of MSU Enviroweather) developed a model which predicts the likelihood of tuber survival over the winter based on soil temperatures at 2 and 4 inches between November 1 and March 31st.  If the tubers were exposed to temperatures below 27°F for more than 120 hours between November 1st through March 31st at 4 and 2 inch depths then the risk of tuber survival is considered low (indicated in figure below and on map pin by green color).  If tubers were exposed to temperatures below 27°F for less than 120 hours at 4 inch depth and greater than 120 hours at 2 inch depth then there is moderate risk of tuber survival (indicated by yellow color).  If tubers were exposed to temperatures below 27°F for less than 120 hours at 4 inch depth and less than 120 hours at 2 inch depth then there was a high risk of tuber survival (indicated by orange color).

    In a visit to the site, you can see that Hancock (Waushara Co.) has low risk for potato tuber survival/volunteer emergence (green) and Arlington (Columbia Co.) has moderate risk for volunteer emergence (orange).  The map is provided below, along with a table of the average soil temps at 2 and 4 inch depths from Nov 2018 to Mar 2019.

    In addition to infected/pathogen reservoir volunteers, potential sources of late blight in any season include, introduced infected tomato plants, cull piles, infected seed, and mismanaged compost piles containing late blight-infected material.  The WI DATCP requires the destruction of cull piles by May 20 of each year.  A few field samples have come to my lab from Florida over the past recent weeks, from a foliar potato sample, we confirmed late blight of the US-23 clonal lineage.

    We will continue to provide Blitecast information via this newsletter and through the vegetable pathology website:   https://wivegdis.plantpath.wisc.edu/.  We will have in-potato-field weather stations in Grand Marsh, Hancock, Plover, and Antigo as in past years, with access to the station data (with DSV and PDay values at:  https://wivegdis.plantpath.wisc.edu/dsv/#hancock).  New in recent years, is the Vegetable Disease and Insect Forecasting Network (VDIFNet) site which provides information on DSVs from NOAA weather data across the state of WI, as well as insect phenological data (Dr. Russell Groves, UW-Madison Entomology).  The link to the VDIFNet site is:  https://agweather.cals.wisc.edu/vdifn/maps.

     

    Accessing the 2019 University of Wisconsin Madison Extension Commercial Vegetable Crop Production Management Guide:  Our production guide is updated every October with release of a new guide in January.  The book can be downloaded for free as a pdf at the link below, or can be purchased online for $12.50.    https://learningstore.uwex.edu/Assets/pdfs/A3422.pdf

Calendar of events

  • July 18, 2019 – UW-Hancock Agricultural Research Station Field Day, Hancock, WI
  • July 25, 2019 – UWEX Langlade County Airport Research Station Field Day, Antigo, WI
  • August 2, 2019 – UW-Lelah Starks Elite Foundation Seed Potato Farm Field Day, Rhinelander, WI
  • December 3-5, 2019 – Midwest Food Producers Association Annual Convention/Processing Crops Conference, Wisconsin Dells, WI
  • January 26-28, 2020 – Wisconsin Fresh Fruit & Vegetable Growers Conference, Wisconsin Dells, WI
  • February 4-6, 2020 – UWEX & WPVGA Grower Education Conference, Stevens Point, WI