Update 4 – June 11, 2023

In this issue:

  • Potato production updates
  • Potato and vegetable disease model updates
  • Onion Botrytis
  • Colorado potato beetle, potato leafhopper, and seedcorn maggot updates

 

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


We have had warm and dry weather over the past two weeks. Daily max soil temperatures exceeded 80°F last week, but it mostly fell below 80°F this week. There were several days with daily min soil temperatures above 60°F, causing abnormal emergence and tuber bulking issues, but looking at the forecasting, it should be cool to support tuber growth in the next 10 days or so. However, we only had one rainfall event in the past two weeks, making it necessary to irrigate at least every other day with 0.4 – 0.5’’.

Soil moisture in the Midwest will be moderately or substantially below average in general, as shown in the graph above. Overall, with the warm temperature, the long dry spell, and frequent irrigation, soil moisture fluctuates from dry to moist and back to dry on a daily basis. Many folks have said that common scab could be a concern this season.

Common scab could be the most severe when tubers develop under warm and dry soil conditions with a soil pH above 5.2. Soil moisture during tuber initiation has a big impact on common scab development. In particular, tubers are the most susceptible to common scab infection during the first three to four weeks after tuber initiation, when compared to six to eight weeks after tuber initiation. Earlier infection can cause deeper scab lesions on the tuber. There is not much information available about tuber susceptibility to this disease during late season tuber bulking. For cultural practices, maintaining soil moisture levels near field capacity during the first four weeks following tuber initiation will likely inhibit infection. However, irrigation may not always be the most practical method, especially on sandy soils. Furthermore, other diseases like Pythium leak and pink rot, could be worsened by excessive irrigation.

For crop progression, folks in the southern part of the state have reported creamer-size tubers and flowering on some early-planted varieties (planted on April 10th). Plants in the Central Sands have achieved about 20 – 30% canopy closure on average, including our research plots at Hancock. We are seeing pea-size tubers on some early varieties. However, some folks have reported variable crop emergence that was rarely observed over the years. Some plants emerged two weeks ago but are still pretty small in size, likely due to the heat. Some young plants were burned off and started to branch again. Others are showing rhizoc-like symptoms on stems in the top one inch. It is too early to predict yield yet, but for sure this will be a year with lots of uncertainties.

Potato emergence as of Jun 10, 2023 (planted April 10)

For our snap beans and kidney beans that were planted on June 1st, we have seen pretty good emergence rate, as shown below.

Kidney bean emergence, June 10, 2023

 

Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, Email:  gevens@wisc.edu, Lab Website: https://vegpath.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, UW-Madison Plant Pathology; and our grower collaborator weather station hosts for supporting this disease management effort again in 2023.  A Potato Physiological Day or P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application.  A Disease Severity Value or DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application.  Red text in table indicates threshold has been met or surpassed.  TBD indicates that data are To Be Determined as time progresses.  Weather data used in these calculations is from weather stations that are placed in potato fields in each of the four locations, as available.  Data from an alternative modeling source: https://agweather.cals.wisc.edu/vdifn will be used to supplement as needed for missing data points and for additional locations (indicated with *).  Data are available in graphical and raw formats for multiple locations at:  https://vegpath.plantpath.wisc.edu/dsv/.

 

 

Planting Date 50% Emergence Date Disease Severity Values (DSVs)

through 6/10/2023

Potato Physiological Days (P-Days)

through 6/10/2023

Spring Green* Early Apr 3 May 9 0 232
Mid Apr 17 May 12 0 210
Late May 10 May 23 0 141
Arlington* Early Apr 5 May 10 0 226
Mid Apr 20 May 15 0 188
Late May12 May 25 0 128
Grand Marsh Early Apr 5 May 10 0 206
Mid Apr 20 May 15 0 173
Late May 12 May 25 0 121
Hancock Early Apr 10 May 17 0 165
Mid Apr 22 May 19 0 160
Late May 14 May 28 0 110
Plover Early Apr 14 May 19 0 153
Mid Apr 24 May 20 0 150
Late May 19 May 29 0 101
Antigo Early May 1 May 28 0 101
Mid May 15 June 3 0 56
Late June 7 TBD TBD TBD
Rhinelander* Early May 7 June 1 0 73
Mid May 18 June 5 0 38
Late June 9 TBD TBD TBD

In addition to the potato field weather stations, we have the UW Vegetable Disease and Insect Forecasting Network tool to explore P-Days and DSVs across the state (https://agweather.cals.wisc.edu/vdifn).  This tool utilizes NOAA weather data (stations are not situated within potato fields).  In using this tool, be sure to enter your model selections and parameters, then hit the blue submit button at the bottom of the parameter boxes.  Once thresholds are met for risk of early blight and/or late blight, fungicides are recommended for optimum disease control.  Fungicide details can be found in the 2023 Commercial Vegetable Production in Wisconsin Guide, Extension Document A3422, linked here: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin

 

Vegetable Insect Update – Russell L. Groves, Professor and Department Chair, UW-Madison, Department of Entomology, 608-262-3229 (office), (608) 698-2434 (cell), e-mail rgroves@wisc.edu. Vegetable Entomology Webpage: https://vegento.russell.wisc.edu/


Colorado potato beetle (CPB) – (https://vegento.russell.wisc.edu/pests/colorado-potato-beetle/). Check for CPB early larvae now as many potato plants are reaching canopy closure in central and southern portions of the state. Emerging adults have colonized many fields in southern and central Wisconsin this past week. In northern Wisconsin, early detection of adult colonization and infestation is especially critical in deploying and perimeter sprays. In these northern fields, focus early season scouting on border rows that are adjacent to last year’s potato. These have the greatest probability for early infestation by adult CPB and egg masses.

Over the past 20 years, chemical control options have progressed away from very broad-spectrum insecticides to more reduced-risk and biological insecticides. These compounds are designed to control only specific stages of insect development (e.g, eggs, early larvae), so it is critical to follow the development stages of insects within fields to understand the best application times. Remember that insect development is directly related to temperature: cool weather slows growth, warm weather accelerates development. Using a degree-day based, temperature dependent system incorporating daily high and low temperatures instead of calendar dates will help to anticipate pest outbreaks. Begin tracking temperatures when you find the first egg mass (or use an online degree-day calculator like the Vegetable Disease and Insect Forecasting Network). The VDIFN site maintains a running total of degree days to chart insect development and provides a visual map illustrating risk. The number of degree days needed for each stage of Colorado potato beetle development is provided (see inset). First generation emergence (and subsequent risk) is illustrated across central and southern Wisconsin. (Source: https://agweather.cals.wisc.edu/vdifn).

 

Potato leafhopper (PLH).  In Wisconsin and much of the upper Midwest, the potato leafhopper has remained very low. These (very low) populations are likely the result of a blocking high pressure system referred to as an ‘Omega block’. For southern Wisconsin, agricultural producers were under the high pressure portion of the block for as many as 3 weeks. This Omega block has significantly limited the migration of many insects that often move into the state in later spring. This pattern results in a north-to-south jet stream that causes weather patterns to move slowly, which is where the term “block” results. Weather patterns are clogged up, and this prevents weather systems from progressing at a steady pace from west to east like normal. Blocking patterns typically come and go, but the U.S. has been stuck in a rut with various forms of blocking since the start of May. All of them include an exaggerated north-to-south jet stream alignment that causes weather.

If the pattern subsides, leafhoppers and other migratory insects could move into the state in the coming weeks. Recall that leafhopper feeding can result in stunted plants, brown leaves and reduced plant vigor. Both adults and nymphs feed by inserting their mouth parts into the plant’s vascular tissue and extracting sap. Damage results when the insect injects saliva containing toxic substances and creates physical damage during feeding, plugging the vascular tissue and permanently reducing the plant’s photosynthetic efficiency. Snap beans and potatoes should be scouted regularly for PLH activity. Leafhoppers tend to migrate into other crops in early summer after alfalfa is cut. This is a key time to watch for early migrants in vegetable plantings. With snap beans, the greatest amount of injury caused by PLH occurs during the seedling stage.

 

Seedcorn maggot ‘Round #2’ across Wisconsin and upper Midwest – (https://vegento.russell.wisc.edu/pests/seedcorn-maggot/). The second generation of the seedcorn maggot has entered southern Wisconsin and is progressing northward. Recall that seedcorn maggots (SCM) have a large host range including numerous common vegetable crops. In high numbers and when peak SCM are predicted, egg laying and larval development can decimate entire crop stands if left untreated. Larvae will feed in the cotyledons and below-ground hypocotyl (stem) tissue of seedling plants, resulting in a variety of damage symptoms. Feeding damage in germinating seeds will often kill seedlings before they emerge. Poor germination or poor stands of susceptible crops may indicate a SCM problem.

Peak flight activity for 2nd generation of seedcorn maggot in the upper Midwest and across southern Wisconsin. The risk of infestation from the developing 2nd generation is illustrated by the warm colors (reds, oranges, yellows) on the map. (Source: https://agweather.cals.wisc.edu/vdifn).

 


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