Introduced Pests Outreach Project
PATHOGEN ALERT: Late Blight of Potatoes and Tomatoes
(June 29, 2009)

Late blight, a destructive disease caused by Phytophthora infestans, is a pathogen of tomato and potato plants that has recently been found in several states in the Northeast, including Maine, New York and Pennsylvania. The late blight has been identified on tomato transplants sold in big box stores and other garden centers under the brand name Bonnie Plants, and has also been found in a potato field in Pennsylvania. Because a few instances of late blight have now been detected on tomato plants in our state, the Massachusetts Department of Agricultural Resources (MDAR) is asking anyone growing tomatoes or potatoes to monitor them for signs of the disease, in order to prevent its further spread.

Late blight, the disease that was responsible for the Irish potato famine in the mid-19th century, is caused by a fungus-like pathogen that spreads through splashing rain or wind currents. Spores can disperse from one to several miles from the point of origin, with the infection spreading most efficiently in conditions of high moisture and temperatures ranging from 60 to 80F.

Symptoms of late blight include small olive green or brown lesions on the upper surface of the foliage or the stems. Under moist conditions, there is a white, fuzzy growth on the underside of the leaves where the lesions occur, but the absence of this growth does not rule out late blight. Eventually the lesions turn black, leaves start to die, and then the entire plant dies.

This is a serious, destructive disease that can spread quite rapidly when conditions are right, infecting an entire field within days. Any gardeners who suspect they have tomato or potato plants infected with late blight should dig them up, place them in plastic bags, and dispose of them in the trash. Commercial growers wishing to control late blight should begin spraying fungicide immediately, even before symptoms are spotted. Spraying must continue regularly, using a product containing chlorothalonil, a state restricted fungicide which requires certification to use. Growers should be prepared to destroy the plants if the late blight starts to become severe.

For more information about late blight of potato and tomato, including diagnostic images, see the following websites:

If you are a commercial grower with a suspected late blight infestation, you can report it on our website or contact UMass Extension. UMass Extension also provides advice for home gardeners who may be dealing with late blight.

To sign up for pest and pathogen alerts from the Massachusetts Introduced Pests Outreach Project, visit this page.

Massachusetts Department of Agricultural Resources
Massachusetts Introduced Pests Outreach Project is a collaboration between the Massachusetts Dept. of Agricultural Resources and the UMass Extension Agriculture and Landscape Program. This website was made possible, in part, by a Cooperative Agreement from the United States Department of Agriculture's Animal and Plant Health Inspection Service (APHIS). It may not necessarily express APHIS' views.