University of Wisconsin - Extension

Madison, Wis. – Late blight, a potentially devastating plant disease, has been found in Wisconsin. Gardeners and commercial growers are urged to scout their tomato and potato plants for signs and symptoms, according to both university plant pathologists and state agriculture officials.

Late blight, caused by the pathogen Phytophthora infestans, was identified on a tomato plant that had been submitted to the University of Wisconsin-Madison's Plant Disease Diagnostic Clinic. The sample came from a Dane County home garden. Since then, two additional cases have been identified in southern Wisconsin, one a home garden and one a commercial vegetable farm.

Late blight was the biological cause of the Irish potato famine from 1845-1852. The disease can affect potatoes, tomatoes and eggplant. Late blight is spread by spores carried by rain, wind, people, machinery and wildlife. Under the right conditions, late blight will spread rapidly through a field, killing plants in one to two weeks. If the disease isn’t controlled, neighboring gardens or commercial vegetable fields can be infected and the entire crop can be at risk. Once the fruit or tuber is infected, it will rapidly decay so storage is not possible.

"Gardeners and growers need to know what late blight looks like and take proper steps to protect your plants from the disease," said Amanda Gevens, UW-Madison plant pathologist.

Late blight lesions can occur on leaves, stems or fruit. Lesions usually begin pale green in color, eventually turning brown to black and appearing somewhat greasy. Lesions can grow quite large and destroy entire leaves and stems. Under humid conditions, the edges of the lesion will show white spore-producing structures, particularly on the underside of leaves. On the tomato fruit, brown lesions appear on the top and sides of fruit and can eventually cover the entire fruit. A picture of late blight symptoms can be found at www.plantpath.wisc.edu/wivegdis.

There are similar looking diseases such as early blight and Septoria leaf blight. Pictures and descriptions of these symptoms are available at the Department of Agriculture, Trade and Consumer Protection web site at http://www.datcp.state.wi.us/arm/environment/insects/late-blight/LateBlight.pdf.

Growers and gardeners should check tomato and potato plants closely for symptoms at least twice weekly. If you suspect that you have late blight, contact your local county Extension office (check the government pages of your phone book).

If you have late blight, destroy infected plants to prevent further spread of the disease. Growers with small numbers of plants should pull and bag plants for removal. Large plantings should be tilled down thoroughly. Do not compost infected material.

If you don't have late blight, continue to check your plants at least twice weekly. Fungicide recommendations are available from UW Extension, at http://www.plantpath.wisc.edu/wivegdis/.

The Wisconsin Department of Agriculture, Trade and Consumer Protection is tracking the extent of the problem in the state. If your plants are diagnosed with late blight, or you’re confident that your plants have the disease, gardeners and growers of all sizes are asked to contact Adrian Barta, plant pest and disease specialist, 608-224-4592 or email Adrian.barta@wisconsin.gov.

Articles in the weekly Wisconsin Pest Bulletin will also track the disease at http://pestbulletin.wi.gov.