Aug 13, 2020
Late blight confirmed in Washington, Wisconsin

Late summer means the threat of late blight for potato growers in the northern U.S.

Thus far, late blight has been confirmed in central Wisconsin and the Columbia Basin, just north of Pasco, Washington.

The Wisconsin case is in Adams County and was confirmed by University of Wisconsin-Madison researchers the week of Aug. 3-7.

“The Phytophthora infestans pathogen causing late blight that was found in Adams County potatoes earlier this week has been typed as US-23,” reported Amanda Gevens, professor and plant pathologist at UW-Madison. “This type is most typically sensitive to phenylamide fungicides, such as metalaxyl and mefenoxam (i.e. Ridomil). As such, treatment with a fungicide like Ridomil should be highly effective at this time in Wisconsin (presuming that this is the primary type of the pathogen present in the state). In our recent analyses of US-23 isolates from Wisconsin, most were still fully sensitive to mefenoxam; less than 10% of isolates showed some tolerance. Many thanks to Dr. Yu Monica Chen in my lab for the quick lab response to this disease situation. We appreciate this information as it aids in best management of such a potentially devastating disease.”

The late blight case near Pasco, Washington, was confirmed by Washington State University Extension specialist Tim Waters in late July. It was found in a similar area as the same strain identified last year, reported the Capital Press. Waters said strain is unique to the area and it’s possible the infection came from volunteer potatoes.

Late blight, which also affects tomatoes, was found earlier this year in Florida, Alabama and North Carolina, according to North Carolina State University’s USABlight tracker. One of the Florida cases, found in St. Johns County in March, and the Alabama case, found in DeKalb County in June, infected potatoes. The other two — a second Florida case and one in North Carolina — were found in tomato fields.


Late blight was confirmed in Pierce County, Wisconsin, near the Minnesota border, on Aug. 20. Gevens said symptoms suggested the infection was “about a week old.”

“It is critical to apply preventative fungicides to susceptible tomato and potato crops in the region surrounding Pierce County,” Gevens said. (Wisconsin Tomato Late Blight Fungicides 2020.)
“For organic producers, copper-containing fungicides provide most effective management for preventative disease control.”

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