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Five Wisconsin Counties Now Confirmed to Have Late Blight
Wisconsin Ag Connection - 07/28/2010

A plant pathologist with the University of Wisconsin-Extension says late blight has been confirmed in five Wisconsin counties. According to Amanda Gevens, the reports of the plant disease was found in mid-July on a potato in Marquette County and tomato in Waukesha County.

"Environmental conditions immediately following these first finds did not seem to favor disease progress--higher than late blight-favorable temps and lower than favorable relative humidity--and additional reports did not immediately follow," Gevens said. "However, last week, conditions favored late blight development and spread resulting in multiple new counties with confirmed reports on tomato and potato."

On July 23 late blight was confirmed on potato in Portage County, and on tomato in Monroe and Kewaunee Counties. Initial studies indicated that the late blight strain from potato will infect tomato and the strain from tomato will infect potato. The late blight pathogen is referred to as a water mold since it thrives under wet conditions.

Symptoms of tomato late blight include leaf lesions beginning as pale green or olive green areas that quickly enlarge to become brown-black, water-soaked, and oily in appearance. Lesions on leaves can also produce pathogen sporulation which looks like white-gray fuzzy growth.

Stems can also exhibit dark brown to black lesions with sporulation. Tomato fruit symptoms begin small, but quickly develop into golden to chocolate brown firm lesions or spots that can appear sunken with distinct rings within them; the pathogen can also sporulate on tomato fruit giving the appearance of white, fuzzy growth.

Gevens says with the presence of the late blight pathogen in the state and disease-favorable weather conditions, it is critical that all growers of tomatoes and potatoes regularly scout their plants for disease symptoms.

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