The Carlisle Mosquito Online

Friday, September 11, 2009

Tomato late blight: What do we do now?

The fungus-like pathogen Phytophthora infestans (late blight) hit New England hard this year. In spite of its name, it infected tomato and potato crops early in the growing season, largely because of our cold, wet, early summer. The blight usually appears toward the end of the growing season with limited damage to plants and fruits.

According to the Boston Globe, by late July, late blight had affected every Massachusetts county, 100 to 200 farms had the disease on their tomato and potato crops, and all of the 100 or so community-supported agriculture (CSA) farms had lost their tomatoes. CSA farms sell weekly shares of produce to members. Some have compensated. According to this week’s newsletter, Dragonfly Farms in Pepperell is growing tomatoes in its greenhouse where it seems to be keeping the late blight at bay with warm temperatures.

P. infestans can affect tomatoes, potatoes, petunias, and some other members of the Solanaceae or nightshade family. Those susceptible are all “New World” plants, that is, native to the Americas, as is the pathogen. The center of origin of late blight is central Mexico. In the 1840s it caused widespread devastation in the northeastern United States before it appeared in Ireland, causing the potato famine. By the early 1900s late blight had spread worldwide. A number of more aggressive exotic strains of P. infestans are now also present in the U.S.

The University of Massachusetts Extension describes the symptoms of late blight to 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.” The white growth is the site of spore production.

Why this has been such a bad year?

A combination of weather and infected seedlings helps explain the severe outbreak in the Northeast. The University of Massachusetts Extension indicates that 2009 is the earliest the blight has been reported over so much of the country. In addition, infected tomato plants were distributed to large local retail stores and other garden centers from Maine to Ohio. This was the most extensive distribution of late blight-infected seedlings in history. P. infestans, which is extremely contagious and can kill plants faster than any other disease, then moved to plants from other sources in garden centers.

According to the Plant Pathology Department at Cornell University: “Severe late blight epidemics occur when P. infestans grows and reproduces rapidly on the host crop. Reproduction occurs via sporangia that are produced from infected plant tissues and is most rapid during conditions of high moisture and moderate temperatures (60°-80°F).” Spores disperse through rain splash or on wind, which can transport them up to a mile. Our cool, rainy early summer provided ideal conditions for the early outbreak.

Are the tomatoes and potatoes edible?

Barbara Ingham, a food safety specialist with the University of Wisconsin Extension, says that unblemished tomato fruits or potatoes from plants showing signs of the disease can be safely eaten, and even preserved. But she advises against just cutting off the bad section of an infected tomato or potato, indicating that while there is no documented harm it “will taste bitter and may be harboring other organisms that could cause food-borne illness.” She also says not to can infected tomatoes because P. infestans can affect the acidity of the fruit – which is important for safe preservation.

Disposal and prevention

Preventing spread of the late blight disease means eliminating the spores. Checking plants that still appear healthy frequently, and immediately removing diseased individuals, is important. While some experts advise bagging and disposing of infected plants in the trash, Superintendent of Public Works Gary Davis says that infected plants should not be taken to the Transfer Station because “State regulations do not allow the Town to send leaves and yard waste to the incinerator.”

Disposal methods Carlisle gardeners might use include burying all plant parts well below the frost line (two feet deep) so they will rot, leaving plant remains on the ground or under a tarp in hot weather (heat will kill the spores) or exposing them to freezing temperatures through the winter. Plants and fruits or tubers should not be composted because the internal temperatures of the piles may allow spores to survive.

Looking ahead to the 2010 growing season, there are ways that could help limit late blight damage and loss such as choosing resistant varieties of tomatoes, potatoes and petunias, and removing all parts of any “volunteer” plants that come up because they may be infected. Other strategies include rotating crops, planting in areas with full sun, spacing plants to provide good air circulation and minimizing moisture on the foliage by watering the soil with a hose rather than a sprinkler.

References: Carlisle Mosquito, July 31, 2009, “Blight hits Carlisle.” Cornell University Plant Pathology Department ( Massachusetts Department of Agricultural Resources (

The Boston Globe ( including

University of Massachusetts Extension (

University of Wisconsin Extension ( ∆

© 2009 The Carlisle Mosquito