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Serving: WI

Late Blight Confirmed in Five Wisconsin Counties

Late Blight Confirmed in Five Wisconsin Counties
Disease is affecting potatoes and tomatoes in Barron, Adams, Portage, Oneida and Waushara counties

Late blight has been confirmed in five Wisconsin counties, according to Amanda Gevens, University of Wisconsin-Extension plant pathologist at UW-Madison. Late blight on potatoes was found in Barron, Adams, Portage, Oneida and Waushara counties and late blight on tomato was confirmed in Adams County last weekend.

Disease is affecting potatoes and tomatoes in Barron, Adams, Portage, Oneida and Waushara counties.

"We know that the late blight we have in Wisconsin this season can be aggressive on tomato and potato," Gevens said. "While this late blight strain cannot produce persistent overwintering spores in the soil, it can overwinter on infected plant material that is kept alive through winter. This can include late blight infected tomato plants kept warm in a compost pile and late blight infected potato tubers that remain in the soil after harvest or are stored in a warm place."

For this reason, do not compost late blight infected tomatoes or potatoes, do get seed potatoes from a certified clean source, and do control volunteer tomato and potato plants in your 2013 planting.

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 it is critical that all growers – both home gardeners and commercial producers – of tomatoes and potatoes regularly scout their plants for disease symptoms.

If your tomato plants are infected by late blight, methods to destroy or dispose of the plants, include:

1) Pull up plants by the roots, bag, leave in the sun for a few days for plant and pathogen to die, and put out for trash pickup. This method is OK for a few plants.

2) For many infected plants, plants can be cut at the base and allowed to die in place. Once plants are dead, you can go in and remove stakes, strings, and plastic and dead plant material can be incorporated into the soil. Shallow incorporation of debris is recommended to avoid creating a warm, sheltered environment which would keep the plant tissue and pathogen alive for extended periods of time beneath the soil surface.


3) Plants can be flame-killed with a propane or other torch; and

4) Infected plants can be pulled and placed in a small pile covered over with a dark colored plastic tarp and left in the sun. This will create heat in the pile from the sun beating on the plastic tarp and plants will die within a few days. The winter will provide an excellent freeze kill for exposed infected plants

Once late blight has been identified in a region, it is critical that tomato plants be protected. Although there are several fungicides registered for control of tomato late, there are considerations to be made for your specific production system (ie:  organic, conventional, home garden).

For more information about tomato late blight symptoms and management please visit

The Wisconsin Department of Agriculture, Trade, and Consumer Protection (DATCP) also tracks the occurrence of late blight in Wisconsin and has policy in place to support control of this disease. For further information, please visit

If late blight is suspected, contact your county extension agent, a crop consultant, or send infected leaves in a slightly inflated ziplock bag with no paper towel to Amanda Gevens at 1630 Linden Dr, Room 689, Plant Pathology Dept., University of Wisconsin, Madison, WI 53706.

Source: University of Wisconsin Extension

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