Late blight confirmed in five Wisconsin counties

Contact: Amanda Gevens, 608-890-3072,

As of July 23, 2010, we have confirmed late blight in five Wisconsin counties said Amanda Gevens, University of Wisconsin-Extension plant pathologist. The first reports of late blight in Wisconsin came on July 14 on potato in Marquette County and tomato in Waukesha County.

Gevens noted, “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. 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 in Geven’s laboratory have 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.

The time from first infection to lesion development and sporulation can be as fast as seven days, depending upon the weather.

With the presence of the late blight pathogen in the state and disease-favorable weather conditions, it is critical that all growers – both home gardeners and commercial producers – of tomatoes and potatoes regularly scout their plants for disease symptoms.

If late blight is suspected, contact your county extension agent, a crop consultant, the plant disease diagnostic clinic at UW-Madison, or Gevens. Additionally, protectant fungicides can manage late blight when applied in advance of infection and when re-applied as the crop grows.  Please see fungicide details at the vegetable pathology website:


Photos of plants with late blight can be downloaded from

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