Michigan potato late blight update – July 20, 2023

Late blight was reported in Ontario, Canada on July 18, 2023.

Potato leaves with lesions and discoloration.
Photo 1. Potato late blight example. Typical water-soaked lesions on leaves. Light green border present around lesions. Faint gray-white, fuzzy growth typically observed at margins of lesions on the underside of leaves and is more apparent in humid conditions. Photo by Jamie Willbur, MSU.

Potato late blight was reported in Simcoe County Ontario, Canada, on July 18, 2023. Genotype testing is in progress by North Carolina State University. Michigan State University Extension will provide updates as more information becomes available. For typical foliar symptoms, see Photo 1. No other reports of late blight have been received at this time. A map of current late blight observations can be monitored at USABlight.

In Michigan, no late blight has been reported and current weather conditions indicate low to medium risk throughout the state (Photo 2). However, following potato late blight incidences in Montcalm County last year and abundant volunteer pressure noted in areas this season, careful monitoring is recommended. If left unmanaged, volunteers can serve as possible inoculum sources, especially for late blight.

Map of Michigan showing late blight risk.
Photo 2. Current late blight risk in Michigan calculated by the University of Wisconsin – Madison Vegetable Disease & Insect Forecasting Network. Risk calculated using NOAA weather data and colors indicate risk levels as follows: green = very low, light green = low, yellow = medium, orange = high, red = very high.

Please contact MSU Potato and Sugar Beet Pathology at willbur1@msu.edu or 517-355-3863, or MSU Plant & Pest Diagnostics at pestid@msu.edu or 517-355-4536 for disease diagnosis and management support. Visit our websites for more information.

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