Michigan potato late blight forecast – July 22, 2020

No late blight reported. Weather-based risk is medium throughout Michigan.

Michigan late blight forecast
Figure 1. June 17 to July 22, 2020 – Michigan late blight forecast based on accumulation of disease severity values (DSVs) since early May emergence. Yellow pins indicate medium risk areas and green pins indicate low risk.

The Michigan late blight forecast tool is currently calculating 2020 risk predictions. No potato late blight has been reported yet this season. Over the last two weeks, predicted late blight risk has increased to medium across Michigan (Figure 1). Continue preventative management and intensive scouting programs.

This year, late blight has been reported in Dekalb County in Alabama (April 2020), Lee and St. Johns counties in Florida (March and June 2020), and most recently Buncombe County in North Carolina (July 2020). Processed samples have been confirmed to be the US-23 genotype of P. infestans, which is not known to be resistant to mefenoxam. Information and locations for positive detections of potato and tomato late blight can be found at USAblight.org and on the USA Blight Outbreak Map.

Late blight is caused by Phytophthora infestans, which favors 60-80 degree Fahrenheit temperatures, high humidity and frequent rainfall. The Michigan late blight forecast tool calculates disease severity values (DSVs) based on the duration of temperature and relative humidity conditions that are favorable for disease development. Accumulated DSVs (based on early May emergence) are used to determine the local late blight risk level, indicated by the color of the map marker pins:

Low risk

Medium risk

High risk

Late blight detected within 2-5 miles

Late blight detected within 1 mile

This tool was developed by Baker et al. 2002 and weather data used in this model is provided by Michigan State University's Enviroweather. A customizable tool for early or late emergence scenarios is also available.

Please visit the MSU Potato and Sugar Beet Pathology website for more information. Answers to frequently asked questions about this forecast tool are also available.

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