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: