Vegetable Crop Update – Sep 7, 2025

In This Issue:

  • Disease forecasting updates for potato early blight and late blight
  • Cucurbit downy mildew updates
  • Black dot and silver scurf of potato

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


Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations will be posted at our website and available in the weekly newsletters.  Thanks to Ben Bradford, UW-Madison Entomology for supporting this effort and providing a summary reference table:  https://agweather.cals.wisc.edu/thermal-models/potato. 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.  Data from the modeling source: https://agweather.cals.wisc.edu/vdifn are used to generate these risk values in the table below.  I’ve estimated early, mid-, and late planting dates by region based on communications with stakeholders.  These are intended to help in determining optimum times for preventative fungicide applications to limit early and late blight in Wisconsin.

 

 

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

through 9/6/2025

Potato Physiological Days (P-Days)

through 9/6/2025

Spring Green Early Apr 5 May 10 64 937
Mid Apr 18 May 14 64 910
Late May 12 May 26 61 853
Arlington Early Apr 5 May 10 46 936
Mid Apr 20 May 15 46 899
Late May 10 May 24 43 863
Grand Marsh Early Apr 7 May 11 67 907
Mid Apr 17 May 14 67 887
Late May 12 May 27 67 833
Hancock Early Apr 10 May 15 65 872
Mid Apr 22 May 21 65 845
Late May 14 June 2 65 791
Plover Early Apr 14 May 18 49 840
Mid Apr 24 May 22 49 835
Late May 19 June 7 49 743
Antigo Early May 1 May 24 56 794
Mid May 15 June 1 56 757
Late June 1 June 15 51 670
Rhinelander Early May 7 May 25 38 767
Mid May 18 June 8 38 689
Late June 2 June 16 34 649

Late blight of potato/tomato. I’m aware of no new reports of late blight in the US this past week. Findings thus far in potato and tomato have been confirmed as US-23 Phytophthora infestans (still sensitive to mefenoxam/metalaxyl (ie: Ridomil) in western NY and Ontario Canada. Here in Wisconsin, we saw limited accumulations of 0-1 DSVs across WI this past week. All plantings of potatoes in Wisconsin have surpassed the Blitecast threshold of 18 DSVs and should receive preventative fungicides for the management of late blight. Please find a fungicide listing for Wisconsin potato late blight management: https://vegpath.plantpath.wisc.edu/documents/potato-late-blight-fungicides/

Early blight of potato. Accumulations of P-Days were 26-42 over the past week, with P-Day 300 thresholds met for preventative fungicide treatment in potatoes across all of Wisconsin. Some fields are moving through the harvest process at this point. Remaining fields are showing a big transition in overall foliar quality this past week. In the final day/weeks of the crop it becomes difficult to discern advanced early blight from other diseases including potato early dying, brown spot, and/or blackleg/aerial stem blight. https://vegpath.plantpath.wisc.edu/diseases/potato-early-blight/. For Wisconsin-specific fungicide information, please refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available here: https://cropsandsoils.extension.wisc.edu/articles/2025-commercial-vegetable-production-in-wisconsin-a3422/

For custom values, please explore the UW Vegetable Disease and Insect Forecasting Network tool for P-Days and DSVs across the state (https://agweather.cals.wisc.edu/vdifn). This tool utilizes NOAA weather data. 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 2025 Commercial Veg. Production in WI Extension Document A3422: https://cropsandsoils.extension.wisc.edu/articles/2025-commercial-vegetable-production-in-wisconsin-a3422/

Cucurbit Downy Mildew: No downy mildew was seen on cucurbits this past week at HARS, and none reported through our UW Plant Disease Diagnostic Clinic. There were no new reports of downy mildew on cucurbits through the Cucurbit Downy Mildew ipm PIPE website: https://cdm.ipmpipe.org/.

Additional reading:

 


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