Michigan potato late blight forecast – July 8, 2020

Jaime Willbur<willbur1@msu.edu> and Lee Duynslager<duynslag@msu.edu>, Michigan State University, Department of Plant, Soil and Microbial Sciences -

No late blight reported. Weather-based risk low to moderate throughout Michigan.

Michigan late blight forecast
Figure 1. July 8, 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 in Michigan. Due to the sustained warmer than normal weather we have been experiencing, late blight risk remains low to moderate throughout the state (Figure 1). 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.

This year, potato late blight has been reported in Dekalb County in Alabama (June 8, 2020) and St. Johns County in Florida (April 2, 2020). These detections were confirmed to be the US-23 genotype of P. infestans, which is not known to be resistant to mefenoxam.

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
(Green)

Medium risk
(Yellow)

High risk
(Orange)

Late blight detected within 2-5 miles
(Purple/Pink)

Late blight detected within 1 mile
(Red)

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.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

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