Late Blight Confirmed in Southern Michigan

Late blight has been confirmed on commercial potatoes in Monroe County. There are predicted high risk levels throughout southern Michigan.

Published online: Aug 20, 2018 Articles, Fungicide Jaime Willbur
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Source: Michigan State University Extension

Potato late blight has been reported in Michigan’s Monroe County. Late blight was confirmed by Michigan State University Diagnostic Services, and the causal organism identified as Phytophthora infestans. The genotype is believed to be US-23. Initial testing is being conducted by a third party laboratory and will be confirmed by researchers at the University of Wisconsin – Madison; this genotype is generally still sensitive to mefenoxam/metalaxyl. Risk-based fungicide recommendations are available for Michigan through the MSU Potato Disease website.

Rob Schafer, Mid-Michigan Agronomy, reported that affected potatoes were located at the edge of the field with lesions occurring in the upper parts of the canopy. Lesions were brown and water-soaked (Photo 1) with sporulation, noticeable as gray, fuzzy growth on leaf undersides (Photo 2).

potato late blight sample

Potato late blight sporulation on reverse side of affected leaf. Photo by Jaime Willbur, MSU.

This report corresponds with the predicted high late blight risk in southern areas of the state. Michigan State University Extension strongly recommends intensive monitoring of potato crops for late blight detection. Samples should be submitted to MSU Diagnostic Services and reported to MSU Potato Disease or your local MSU Extension expert (find an expert or call 888-678-3464).