Michigan potato late blight detected on July 23, 2024

Jaime Willbur<willbur1@msu.edu>, Michigan State University, Department of Plant, Soil and Microbial Sciences - July 24, 2024

Late blight has been confirmed in St. Joseph County, Michigan.

3 different pictures showing the dark brown lesions on potato leaves. — Figure 1. Examples of potato late blight on whole plant (top) with typical water-soaked lesions on forward (bottom left) and reverse (bottom right) sides of leaves. Light green border sometimes present around lesions. Faint gray-white, fuzzy growth observed at margins of lesions on the underside of leaves, apparent in humid conditions. Photo by Jaime Willbur, MSU.

Potato late blight has been confirmed in a commercial field in St. Joseph County, Michigan. In a report from Amanda Gevens, PhD, University of Wisconsin-Madison, “…late blight on potato was confirmed [July 23, 2024] in St. Joseph County, Michigan…on the southwestern edge, bordering Indiana. The genotype of the Phytophthora infestans pathogen was US-23. The disease was noted in a very small section of a commercial field (about 5 square feet). Recall that US-23 can typically still be managed with phenylamide fungicides including mefenoxam and metalaxyl. Thank you to Rob Schafer of Mid-Michigan Agronomy for quickly working on this diagnostic and sharing the information so that producers in the region can take enhanced action to manage this disease.” Preventative foliar programs and diligent crop monitoring are strongly recommended.

Current late blight risk is low to medium in Michigan, however some areas have experienced favorable conditions in the past several weeks due to more frequent and heavy rain events. Late blight forecasting for the Upper Midwest is available through the University of Wisconsin-Madison Vegetable Disease & Insect Forecasting Network.

20240724 Potato late blight VDIFN.PNG — Figure 2. July 24, 2024, late blight risk as reported by the UW-Madison Vegetable Disease & Insect Forecasting Network: https://agweather.cals.wisc.edu/vdifn?model=late-blight. Green = very low, light green = low, and yellow = medium.

Since the early July reports of potato and tomato late blight (US-23 genotype) in Kent and Elgin counties in Ontario, Canada, no other regional detections have been documented. Current year information is available at USABlight. No Phytophthora infestans DNA has been detected from experimental spore samplers maintained by the Michigan State University (MSU) Potato and Sugar Beet Pathology program, currently deployed in research and commercial fields in Montcalm County.

Please contact the MSU Potato and Sugar Beet Pathology at willbur1@msu.edu or 517-355-4754 or MSU Plant & Pest Diagnostics at pestid@msu.edu or 517-355-4536 to report potential late blight detections or visit our websites for more information.

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).