Update 15 – Aug 27, 2023

In This Issue:

  • Potato and tomato early blight and late blight disease updates
  • Cucurbit downy mildew updates
  • New vegetable disease fact sheets


Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, Email:  gevens@wisc.edu

Early blight of potato/tomato.  Accumulations of P-days this past week were between 47-64 across the state of Wisconsin.  In all locations and all planting dates, potato fields have surpassed the threshold and should continue to receive fungicide applications for early blight management.  Hotter days generate roughly 10 P-days per day if you are looking ahead to likely accumulations and preventative fungicides.

Late blight of potato/tomato.  Accumulations of Blitecast DSVs were low this past week in the 7 sites detailed in our table, below, ranging from 0 to 3.  The usablight.org website (https://usablight.org/map/) indicates new late blight reports in the past week on tomato and potato in Tompkins County.  So far, all characterizations of the late blight pathogen identified in North America this growing season have resulted in the US-23 type.  Fungicides for the management of late blight in tomato and potato crops are provided:  https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin.  A specific list of fungicides for potato late blight in Wisconsin was also offered in a special report shared via email on July 28.  https://vegpath.plantpath.wisc.edu/wp-content/uploads/sites/210/2023/08/2023-Potato-Late-Blight-Fungicides.pdf

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.  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 8/26/2023 Potato Physiological Days (P-Days) through 8/26/2023
Spring Green* Early Apr 3 May 9 12 849
Mid Apr 17 May 12 12 827
Late May 10 May 23 12 758
Arlington* Early Apr 5 May 10 11 861
Mid Apr 20 May 15 11 823
Late May12 May 25 11 763
Grand Marsh Early Apr 5 May 10 11 823
Mid Apr 20 May 15 11 789
Late May 12 May 25 11 737
Hancock Early Apr 10 May 17 12 793
Mid Apr 22 May 19 12 787
Late May 14 May 28 12 738
Plover Early Apr 14 May 19 16 780
Mid Apr 24 May 20 16 775
Late May 19 May 29 16 728
Antigo Early May 1 May 28 13 698
Mid May 15 June 3 13 653
Late June 7 June 23 13 518
Rhinelander* Early May 7 June 1 8 667
Mid May 18 June 5 8 632
Late June 9 June 24 8 509

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.  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. 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 (see current map below showing red counties with new reports from  PA, VA, NJ, and MD). Dr. Mary Hausbeck reported cucumber downy mildew in commercial fields in 11 Michigan Counties as of 8/16/2023To date, there have been no reports of downy mildew here in WI.  We should be considering preventative treatment of cucumber and melon crops here due to the likelihood of the disease resulting from clade 2 downy mildew.  A list of best-performing conventional fungicides is provided below.

  • Elumin + chlorothalonil or mancozeb
  • Omega (Orbus) + chlorothalonil or mancozeb
  • Orondis Opti (Chlorothalonil is part of the premix, additional chlorothalonil is suggested. See label for maximum chlorothalonil rates.)
  • Previcur Flex + chlorothalonil or mancozeb
  • Ranman + chlorothalonil or mancozeb
  • Zampro + chlorothalonil or mancozeb
Cucurbit downy mildew confirmed detections as of 8/27/2023.


New Vegetable Disease Factsheets! In collaboration with an organic vegetable grower consultative panel, MS graduate Ariana Abbrescia (Agroecology MS granted in Spring 2023), Ben Bradford (UW Vegetable Entomology), Russell Groves (UW Vegetable Entomology), and myself, we developed a series of vegetable disease ‘fact sheets’ at  https://vegpath.plantpath.wisc.edu/diseases/.  These pages provide photos and additional information for the identification and management of key vegetable diseases in Wisconsin and the upper Midwest.


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