Michigan potato late blight forecast – Aug. 27, 2020

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

Michigan late blight forecast
Figure 1. Aug. 27, 2020 – Michigan late blight forecast based on accumulation of disease severity values (DSVs) since early May emergence. Yellow pins indicate medium risk areas.

The Michigan late blight forecast tool is currently calculating 2020 risk predictions. In Michigan, no potato late blight has been reported yet this season. Predicted late blight risk has remained medium across the state (Figure 1).

This year, late blight has been recently reported in Adams and Pierce counties in Wisconsin (August 2020) and Bledsoe County in Tennessee (August 2020). Other detections were reported in Alabama, Florida and North Carolina. Processed samples have been confirmed to be the US-23 genotype of P. infestans, which is typically sensitive to phenylamide fungicides such as metalaxyl and mefenoxam (i.e. Ridomil). Please refer to the 2020 Midwest Vegetable Production Guide for Commercial Growers for appropriate treatment information (potato late blight information begins on Page 220). 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.

A reminder that MSU Plant & Pest Diagnostics is still not accepting walk-in consultations. They are open for all samples that are dropped off and mailed to the lab, and it is recommended to use USPS mail to ship samples. Clients are strongly encouraged to email digital images prior to bringing or sending physical samples. Images can be sent to pestid@msu.edu.

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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