If you grow tomatoes or potatoes, it is extremely important to protect them from late blight now

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Gardeners who grow tomatoes and potatoes need to be protecting their plants from late blight (Phytophthora infestans) now with fungicide sprays. This disease, which caused the Irish Potato Famine beginning in 1845, has been found in southwestern Michigan and five other states so far this year. Chris Long, potato specialist at MSU, said that this disease has the potential to wipe out all tomato and potato plantings in the state. Long also says that this year’s wet, cool spring has favored the development of the disease.


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. Potatoes and tomatoes are members of the Solanaceae family of plants, which is why both are susceptible to late blight. Because conditions for this disease are so favorable this year, vegetable specialists at MSU and the Michigan Potato Industry Commission are urging growers to be proactive by spraying preventative sprays on their plants as soon as possible and to keep up the regimen over the summer months. Plants that show signs of the disease as well as plants surrounding the diseased plants should be removed and completely destroyed. Late blight can take out a planting of tomatoes or potatoes in five days, so anyone growing these crops must be vigilant. Also take care about neighbors next door or down the road who may also be growing tomatoes or potatoes.

Commercially grown potatoes in Michigan represent a $138 million farm gate crop. Though commercial growers have registered products to help slow the spread of the disease, home gardeners do not, and the care they take to keep the disease out of their gardens will ultimately help the commercial growers as well. 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. 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 to verify whether or not a particular fungicide meets organic standards. This is a nationwide problem. 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, contact your local MSU Extension office or visit the following resources:

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