Update 19 – Oct 1, 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

Please note that this will be the last newsletter issued for our 2023 growing season production updates. We will periodically share a newsletter with information and resources. Have a safe harvest season!

Late blight of potato/tomato. Late blight was not identified in tomato or potato plantings in Wisconsin during the 2023 field production season. This is the second year with this status. Certainly, the dry and hot weather limits conditions favorable to late blight, but we can’t underestimate good prevention of disease through appropriately timed fungicides. All locations are at or have surpassed the threshold of DSV 18 over this past week. Prevention of late blight should still be considered even this late in the season since airborne sporangia can move into fields (despite senescing foliage) and make their way down to tubers in the soil to create tuber infection. The usablight.org website (https://usablight.org/map/) indicated limit reports of late blight over this past 2023 growing season. 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.

Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations. Many thanks to Ben Bradford, UW-Entomology; Stephen Jordan, UW-Plant Pathology; and our grower collaborators for supporting this effort. 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: 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 9/30/2023 Potato Physiological Days (P-Days) through 9/30/2023
Spring Green* Early Apr 3 May 9 25 1138
Mid Apr 17 May 12 25 1117
Late May 10 May 23 25 1048
Arlington* Early Apr 5 May 10 20 1149
Mid Apr 20 May 15 20 1111
Late May12 May 25 20 1051
Grand Marsh Early Apr 5 May 10 25 1091
Mid Apr 20 May 15 25 1057
Late May 12 May 25 25 1005
Hancock Early Apr 10 May 17 31 1057
Mid Apr 22 May 19 31 1051
Late May 14 May 28 31 1012
Plover Early Apr 14 May 19 35 1043
Mid Apr 24 May 20 35 1040
Late May 19 May 29 35 989
Antigo Early May 1 May 28 40 927
Mid May 15 June 3 40 890
Late June 7 June 23 40 747
Rhinelander* Early May 7 June 1 18 897
Mid May 18 June 5 18 862
Late June 9 June 24 18 739

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 is offering reporting of findings of cucurbit downy mildew from the US (map below shows new reports this past week in MD, KY, and SC). To date, there have been no reports of cucurbit downy mildew here in WI. Depending upon the status of the crop, preventative treatment of cucumber and melon crops (clade 2 downy mildew) may have some justification.


Read this newsletter as a PDF