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Farm News: Late Blight Confirmed In WI Potato Field

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Posted: 07.16.2010

It's about two weeks earlier then last year - and now all gardeners and commercial vegetable growers need to pay attention. Late blight has been confirmed in a Marquette county potato field!

Amanda Gevens, University of Wisconsin-Extension plant pathologist, confirmed the tests on Wednesday and said the isolated section of the field probably was actually infected 5-7 days earlier..

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 7 days, depending upon the weather.

Geven says its unusual to find the late blight on potatoes first.  She says there's also more testing that needs to go on. “We do not yet know if this strain of late blight is the same as the one active last year. We will continue to investigate the potential of this year’s late blight and provide updates and appropriate recommendations to growers in the state.”

With the presence of the late blight pathogen in the state and disease-favorable weather conditions, it is critical that all growers – 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:  http://www.plantpath.wisc.edu/wivegdis/.

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