Update 17 – Sep 10, 2023

In This Issue:

  • Potato and tomato early blight and late blight disease updates
  • Cucurbit downy mildew updates


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 46-56 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 depending upon the time-to-harvest of the field

Late blight of potato/tomato. Accumulations of Blitecast DSVs were extremely low with a range of 0-1 this past week in the 7 sites detailed in our table, below. In my past 14 years with UW-Madison, we have not seen such a low accumulation of DSVs during the production season! If we make it through the next few weeks, we’ll have a second year with no reported late blight on tomato and potato in Wisconsin! The usablight.org website (https://usablight.org/map/) indicates no new late blight reports in the past week. 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 9/9/2023 Potato Physiological Days (P-Days) through 9/9/2023
Spring Green* Early Apr 3 May 9 12 968
Mid Apr 17 May 12 12 946
Late May 10 May 23 12 877
Arlington* Early Apr 5 May 10 11 978
Mid Apr 20 May 15 11 940
Late May12 May 25 11 880
Grand Marsh Early Apr 5 May 10 13 933
Mid Apr 20 May 15 13 899
Late May 12 May 25 13 847
Hancock Early Apr 10 May 17 13 903
Mid Apr 22 May 19 13 898
Late May 14 May 28 13 848
Plover Early Apr 14 May 19 16 890
Mid Apr 24 May 20 16 885
Late May 19 May 29 16 836
Antigo Early May 1 May 28 14 796
Mid May 15 June 3 14 752
Late June 7 June 23 14 621
Rhinelander* Early May 7 June 1 9 763
Mid May 18 June 5 9 728
Late June 9 June 24 9 605

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 just a single red county with a new report from PA). To 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. https://vegpath.plantpath.wisc.edu/2023/08/28/update-15-aug-27-2023/


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