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Purdue pathologists confirm tomato disease in Indiana

WEST LAFAYETTE, Ind. — Late blight of tomato, a serious plant disease caused by a fungus-like organism, has been found in Indiana for the second straight year, according to Purdue University plant pathologists.

Photo of tomatoes with Late Blight

A strain of the fungus, Phytophthora infestans, causes late blight in tomatoes. As the common name implies, Phytophthora is prevalent on tomato hosts in late summer, after the plants have bloomed. Late blight is more common in north central and northeastern states, but is observed in the Midwest when the humidity is high and temperatures are around 68 degrees F late in the growing season. Watch for the disease when cool, moist nights are followed by warm, humid days. (Photo and text - Missouri Botanical Garden)

Purdue’s Plant & Pest Diagnostic Laboratory (P&PDL) late Wednesday (June 30) confirmed that a plant sample from a home garden in Dearborn County near the Kentucky border was infected with late blight. The Dearborn County sample is the only known case of late blight in Indiana at this time.

“Late blight is a very damaging disease of tomato and potato,” said Dan Egel, Purdue Extension plant pathologist at the Southwest Purdue Agricultural Center in Vincennes. “The disease can spread very rapidly under cool, moist conditions, and this latest outbreak may have spread during recent rainy weather.”

Egel urged all tomato growers to inspect their plants for signs of disease and direct questions about late blight to their Purdue Extension county office.

Fungicides may slow the progress of late blight, Egel said. Retail products that contain the active ingredient chlorothalonil may reduce the spread of the disease if applied on a regular basis. Trade names include Bonide™, Daconil™, Exotherm Termil™ and PathGuard™.

Organic growers should use copper products, Egel said.

Late blight attacks a tomato plant’s leaves and stems. Infected plants develop brown lesions with whitish borders and sometimes discolored fruit. Late blight spores travel on storm systems and can be blown up to 40 miles from an infected plant.

The disease damaged tomato plants in at least 30 Indiana counties one year ago – the first outbreak of late blight since 1998. It is believed late blight entered Indiana in 2009 on tomato seedlings sold at retail businesses and later replanted in home gardens. The disease then spread into commercial tomato fields.

Already this season late blight has been confirmed in northeast Kentucky and eastern Ohio tomatoes and southern Michigan potatoes.

“I don’t think it will be as bad as last year because in Indiana late blight appears to be spreading by natural movement rather than multiple introductions via infected transplants,” said Tom Creswell, director of P&PDL. “Of course, it could be locally severe if weather is favorable for disease development and the pathogen is present in an area.”

Additional information on late blight for gardeners and commercial growers, including how to submit plant samples to P&PDL, is available online at

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