By SHELLY STRAUTZ-SPRINGBORN
Michigan Correspondent

EAST LANSING, Mich. — The first late blight of the season was confirmed June 21 in potatoes near Nottawa, Mich., in St. Joseph County.

Willie Kirk, Extension specialist for potato diseases at Michigan State University (MSU) said this is the only reported occurrence so far in Michigan, but it has been found on tomatoes in Ohio, Pennsylvania and Maryland, and on potatoes in North Dakota and Washington.

Late blight is one of the most devastating diseases of potatoes and tomatoes worldwide. Since 1990, late blight has caused widespread damage throughout the U.S. and Canada. Left unmanaged, the disease, which caused the Irish Potato Famine in 1845, can result in complete destruction of potato or tomato crops.
Kirk said this year’s onset of the disease was earlier than usual. Last year, the first late blight was detected on June 28.

“The crop was destroyed by burning with propane burners and by disking the remnants back into the soil,” Kirk said of the St. Joseph County field.

The first symptoms of late blight are small, dark, circular to irregularly shaped lesions that appear on the leaves three to five days after they are infected. These lesions spread rapidly in cool, moist weather into brown to black spots that are often surrounded by a green border. The disease is spread from infected plants in one area to another by wind, splashed rain, animals and mechanical transport such as equipment.

According to MSU Plant Pathologist Mary Hausbeck, Michigan’s warm, wet spring along with a warmer winter that allowed for overwintering of volunteer potatoes that may have been infected with the disease are perfect conditions for the pathogen that causes late blight.

“We’ve had warm conditions with plenty of high humidity and rainfall in some areas of the state,” Hausbeck said. “These factors – along with late blight overwintering on diseased tubers – come together to create a disease outbreak.”

MSU Potato Specialist Chris Long said it’s time for home gardeners who grow tomatoes and potatoes to begin protecting their plants with fungicide sprays.

“This disease is on the march early this year, and we need to make everyone who grows tomatoes and potatoes aware of the problem,” Long said. “It has the potential to wipe out all tomato and potato plantings in the state.

“Late blight can take out a planting of tomatoes or potatoes in five days, so anyone growing these crops must be vigilant,” Long said. “We also have to care about our neighbor next door or down the road who may also be growing tomatoes or potatoes. We need to work together to keep late blight from ravaging everyone’s crop.”

Both conventional and organic products are available for gardeners to use to prevent late blight from attacking their plants. There are no products that can “cure” a plant once it has the disease, according to Jan Byrne, MSU plant diagnostic technician.

“There are several fungicides available to home gardeners that are effective in preventing late blight,” she said. “Gardeners need to read the fine print on the product label to make sure it is used on tomatoes or potatoes. Products that contain chlorothalonil or ethylene bisdithiocarbamate (EBDC) will protect plants from infection. Organic chemicals, such as copper-based fungicides, have some efficacy against late blight, and organic gardeners should check the OMRI (Organic Materials Review Institute) database [www.omri.com] to verify whether or not a particular fungicide meets organic standards.

“This is a nationwide problem,” Long says. “It is very important that home gardeners arm themselves with information about late blight in an effort to keep it from destroying not only their plantings, but their neighbors’ — and commercial growers’ — as well.”

For more information, visit www.lateblight.org or contact a local MSU extension office.