Aug 31, 2017
Late blight found in Michigan’s Montcalm County

Late blight was found in potato crops near Edmore, McBride and Vestaburg, Michigan, in Montcalm County Aug. 22, by ABC Consultants scouts. Initial genotyping confirmed isolates as US-23 by GPI isomerase testing, conducted by the Michigan State University (MSU) Potato and Sugar Beet Pathology Laboratory. Mating type and Ridomil sensitivity are underway.

Growers should be aggressively treating all fields on a minimum five-day schedule in Montcalm County. Applications should include a translaminar fungicide in combination with a protectant (chlorothalonil plus mancozeb) and Super Tin. Although a few fungicides have yet to be added, current fungicide rates can be viewed at

Areas in fields that are particularly vulnerable are field margins, especially those close to tree lines, raised cable lines and where water can accumulate such as around pivot tracks and tractor wheel lines. Recommendations for late blight treatment remain the same as in previous reports posted at MSU Extension and include treating with one of the translaminar fungicides listed at the Michigan Late Blight Risk Monitoring website.

Conditions remain conducive for late blight development. See the station comparison page at Todays Late blight Forecast. Forecasts and disease severity value (DSV) accumulations can be checked daily at MSU’s Late blight Forecast page. Continued monitoring for late blight is recommended.

National late blight updates can be found at USA Blight. This resource provides updated information on the detection and characterization of late blight on potato and tomato crops in the U.S. This year to date, late blight has been confirmed on tomato or potato in the states of Connecticut, Florida, Massachusetts, Maine, New York, North Carolina, Pennsylvania, Virginia, Washington and Wisconsin, and Manitoba and Ontario, Canada.

For the reported cases that have been genotyped, the pathogen was either US-23 or US-8 (Washington and Wisconsin). The US-23 clonal lineage has been reported in Michigan and most of the U.S. in recent years and in general is still managed well with phenylamide fungicides. There has also been reports of the US-8 clonal lineage in parts of the U.S. in recent growing seasons. The US-8 is resistant to the phenylamide class of fungicides.

Continue scouting on a weekly basis where late blight has not been reported and treat fields aggressively in areas where late has been reported (see Table 1 for recommendations). Sample symptomatic plants and send them for diagnosis and continued application of residual protectant fungicides. Recommendations for appropriate late blight control where late blight has been confirmed or is suspected should include approaches suggested in Table 1 and include desiccation of infected areas.

The indication of high risk was communicated early in the season to the industry; this included information from the university that winter temperatures may have resulted in a high likelihood that volunteer tubers would have survived the winter; feedback from scouts that survival of volunteer potatoes was profuse.

For more on disease biology and mid-season recommendations visit

Source: Michigan State University Extension


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