Update 7 – July 2, 2023

In this issue:

  • Potato and tomato early blight and late blight disease updates
  • Cucurbit downy mildew updates
  • Striped cucumber beetle, onion thrips, Potato Virus Y
  • UW Langlade County Extension Airport Ag Research Station Field Day – Antigo WI agenda


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/

Early blight of potato/tomato.  Accumulations of P-days (recall these are influenced by heat) ramped up this past week and on average we saw roughly 60 P-days across the state of Wisconsin.  In all locations with the exception of Antigo and Rhinelander, most early and mid-planted potato fields have reached/surpassed threshold and should receive (and continue to receive) preventative fungicide applications for early blight management.  Hotter days generate roughly 10 P-days per day if you are looking ahead to likely accumulations and planned preventative fungicide applications.

Late blight of potato/tomato.  Accumulations of Blitecast DSVs have been low to non-existent.  Since emergence, potatoes in Wisconsin have seen between 1-5 DSVs indicating conditions generally unfavorable for the development of late blight.  Overall, the weather has been very dry, with temperatures a bit too hot to promote the pathogen.  The usablight.org website (https://usablight.org/map/) indicates no reports of late blight in potato or tomato from across the US in 2023.  This website continues to provide a very useful mechanism for tracking this potentially destructive crop disease, but it’s not comprehensive.  Fungicides for management of late blight in tomato and potato crops are provided:  https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin

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 7/1/2023

Potato Physiological Days (P-Days)

through 7/1/2023

Spring Green* Early Apr 3 May 9 1 398
Mid Apr 17 May 12 1 377
Late May 10 May 23 1 308
Arlington* Early Apr 5 May 10 2 395
Mid Apr 20 May 15 2 357
Late May12 May 25 2 297
Grand Marsh Early Apr 5 May 10 2 368
Mid Apr 20 May 15 2 335
Late May 12 May 25 2 283
Hancock Early Apr 10 May 17 5 332
Mid Apr 22 May 19 5 326
Late May 14 May 28 5 277
Plover Early Apr 14 May 19 1 322
Mid Apr 24 May 20 1 317
Late May 19 May 29 1 268
Antigo Early May 1 May 28 3 259
Mid May 15 June 3 3 214
Late June 7 June 23 3 78
Rhinelander* Early May 7 June 1 2 229
Mid May 18 June 5 2 194
Late June 9 June 24 2 71

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

Cucurbit Downy Mildew.  The Cucurbit Downy Mildew forecasting webpage (https://cdm.ipmpipe.org/) is not forecasting the movement of the pathogen, but the group is offering reporting of findings of cucurbit downy mildew from the US. In mid-June, Dr. Mary Hausbeck reported the interception of cucurbit downy mildew spores in an air/spore trap in the Bay County area of Michigan, air samplers in Saginaw and Allegan Counties also resulted in the detection of spores.  Through molecular biological testing, Dr. Hausbeck and her laboratory at Michigan State University characterized the pathogen as the Clade 2 type of the cucurbit downy mildew pathogen which tells us that this type is likely to infect cucumber and melon crops.  While Bay and Saginaw Counties are on the eastern side of MI, Allegan is in the southwestern corner of MI.  In past years, when SW MI had cucurbit downy mildew in production fields, WI did see some movement of the disease into southeastern WI.  For this reason, I am being vigilant in tracking reports of the disease in southern Michigan.  To date, there have been no reports of the disease developing in cucumber fields in MI.  If reports arise, we should be considering preventative treatment of cucumber and melon crops here in southeastern Wisconsin. 

Again, to date, no symptoms of cucurbit downy mildew have been reported here in Wisconsin.  The disease has been confirmed on cucumber in Quebec Canada (last week), NC and NJ; butternut squash and cucumber in SC; and watermelon, acorn/yellow summer squash, and cucumber in Georgia.  These data suggest that there are both strain types of the pathogen active along the east coast.  We should be watchful of all cucurbit crops.  In past recent years, we have predominantly seen the cucumber strain types impacting cucurbits 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/

Striped cucumber beetle – (https://vegento.russell.wisc.edu/pests/cucumber-beetles/).  Striped and spotted cucumber beetles continue to be sporadic in their damage within the state, but can continue to cause significant damage in vine crops through the remainder of the season. Because the striped beetle is more prevalent in Wisconsin, it is often considered more damaging.

Feeding from adults causes direct damage to leaves, flowers, and fruits, and adults can transmit the bacteria, Erwinia tracheiphila. Cucumbers and melons are particularly susceptible to bacterial wilt, and damage from this can be severe. To limit damage from bacterial wilt, it is critical to first diagnose the disease, and then respond appropriately which includes vine removal.

Several chemical insecticides are available when beetles exceed thresholds. However, chemical control will be limited if beetle populations are already high. Systemic neonicotinoid insecticides should be used with caution. Contact insecticides (including botanicals) should be applied to seedlings before transplanting and continued on a regular basis to keep numbers low. Cucumber leaves are sensitive and can be burned by chemical sprays. Spraying in the afternoon or evening is preferable to avoid killing beneficial insects and pollinators.

Onion thrips – (https://vegento.russell.wisc.edu/pests/onion-thrips/) Onion thrips (and thrips damage more generally) is increasing rapidly throughout many drought-affected regions of Wisconsin. Serious damage is generally limited to onions, but significant damage can also be expected in cabbage, cucumber succulent bean and other susceptible flowering crops. Feeding damage causes whitish blotches and dry, yellow areas on leaves, decreased pollen set, and, under heavy infestations, brown leaf tips and distorted leaves. Feeding by both adults and larvae can cause silvery streaking on leaves which becomes necrotic. Immature thrips prefer to feed on the youngest leaves and are most easily counted on the undersurface of leaves.

Since thrips prefer tight spaces, cabbage varieties with extremely dense heads are most susceptible to damage. Thrips are often found several layers deep within developing cabbage heads. Heavy thrips buildup may cause the cabbage head to become distorted. Red onions are particularly susceptible, while Spanish onions tend to be somewhat resistant. Cultivars with leaves tightly held to stem are more susceptible to thrips damage, while cultivars with more open growth, circular leaf structure, and glossy foliage suffer less damage.

Although some regions of the state received needed rainfall over the weekend, many areas are still lacking sufficient moisture and thrips populations can be expected to build with forecast warm temperatures.

Insecticide active ingredients that are part of management guidelines for onion thrips include spirotetramat (Movento), abamectin (Agri-Mek), spinetoram (Radiant), cyantraniliprole (Exirel, Minecto Pro), methomyl (Lannate) and lambda-cyhalothrin (Warrior). Many of these active ingredients are also available as generics and pre-mixes, so be cautious when developing your annual program of control and do not use similar modes of action (MoA) over successive generations. It is often recommended to use the same active ingredient as a series of two, successive applications (spaced 7-10 days apart) followed by a switch to a new MoA.

Potato virus Y (PVY) – (https://vegento.russell.wisc.edu/pests/plant-pathogens/).  Potato Virus Y is a potyvirus that primarily infects plants in the Solenacea family. Aphid flight models have been developed and are available at the Wisconsin Vegetable Disease and Insect Forecasting Network (VDIFN). A screen shot from VDIFN (July 2, 2023) illustrates the risk of aphid activity across a range of colors (high to low, red to green). To access this daily map, simply visit VDIFN, and select ‘Insect’ in the upper left tab, followed by ‘Potato’ in the crop tab, and finally ‘Aphid PVY Vectors’ in the insect tab – then click the blue ‘Submit’ button. Once at the correct map, you are able to ‘zoom in’ on the image and click on any cell to obtain location specific estimates of the accumulated FDD and the associated risk. The risk of PVY transmission by aphid vectors begins around 1967 FDD, peaks around 2473 FDD, and ends around 3228 FDD. Today’s PVY risk map illustrates that peak risk for transmission is just entering southern Wisconsin, but will progress across the state in mid to late July.

Foliar applications of paraffinic oils have previously been shown to modify the feeding behaviors of non-potato colonizing, migrating aphids alighting onto the potato canopy as they move through the local landscape. Specifically, these investigations have revealed that aphids are discouraged from probing on leaves that possess residues of compounds containing (> 95%) of paraffinic oils, resulting in limited inoculation attempts. A portion of our applied research program has investigated the value of these paraffinic oils in limiting non-persistent PVY transmission, by (1) determining the periods of greatest risk for aphid movement and transmission, coupled with (2) experiments to evaluate the timing and coverage of these different oil-containing compounds.


Update 7 – July 2, 2023