Backyard gardeners: beware of blight

Late blight, the disease that caused the Great Irish Potato Famine, has spread across the Northeast and can ruin tomato and potato crops. Late blight, the disease that caused the Great Irish Potato Famine, has spread across the Northeast and can ruin tomato and potato crops. (Photos By Meg Mcgrath/Cornell University via Reuters)
By Carol Stocker
Globe Correspondent / July 16, 2009
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If you bought a tomato plant at a big-box store this spring, you may have a ticking time bomb in your backyard.

Known as late blight, the fungus that caused the Great Irish Potato Famine in the 1840s has spread across the Northeast this summer in large part by big box store delivery trucks. Infected tomato plants, grown by the giant Bonnie Plants company of Alabama, have been removed from Lowe’s, Wal-Mart, Home Depot, and Kmart stores in all six New England states.

So what is it, and how do you know if your plants have it?

Phytophthora infestans attacks only members of the nightshade or potato family, including tomatoes and petunias. Brown spots, or lesions, appear on the stems and leaves of infected plants and spread quickly, producing a white fungal growth in moist weather. The fungus is not toxic to humans, but it quickly rots tomatoes and potatoes. Most other vegetables, including lettuce, peppers, squash, carrots, green beans and broccoli, are not affected.

The fungal infection first appeared in the Northeast in early June, according to Scott Soares, Massachusetts’s commissioner of agriculture. He’s asking home gardeners to check their tomato plants daily for the highly contagious windborne fungal spores that can spread to commercial crops.

“There’s always some of it around,’’ said Soares, adding that Massachusetts is faring better than many states. “I’d describe [the spread of the disease] as light but all over the state. We think it came in on starter tomato plants sold in big box stores. If you bought your tomato plants from local growers or grew your own from seed, your plants are less likely to be infected.’’

The fungus thrived in June’s cool wet weather. If this month turns sunny, problems will abate, Soares said. In the meantime, gardeners should dispose of all parts of diseased plants in sealed plastic bags, put in the garbage. You can also bury them at least two feet underground so they do not resprout.

“But they should not be composted,’’ warned Nancy Garragrants, director of the University of Massachusetts cooperative extension. The spores can be carried on the wind, infecting more plants.

Some farms in New York and Connecticut have been heavily affected, said plant pathologist Robert Wick of the UMass-Amherst Department of Plant, Soil, and Insect Sciences.

“The problem is that these big box stores have sold infected plants throughout the Northeast, so there are a lot of tomato plants just sitting in people’s backyards giving out spores that could infect our farms,’’ said Wick, adding that the state has 2,000 acres of potatoes under threat. If infected, the crop will be lost in five days. Many of the state’s 7,600 farms also grow some commercial tomatoes.

Plymouth County Cooperative extension educator Deborah C. Swanson said the irony is that many families are growing vegetables for the first time to try to save money. They picked a tough year.

“I haven’t had a lot of calls about potato blight,’’ Swanson said. “I’m getting complaints about the weather in general. It’s been too wet and cool for tomatoes to grow even if they’re healthy.’’

If you want to try to protect your plants with fungicides, begin spraying now, before symptoms appear. Use a product that contains chlorothalonil, such as Daconil, every week if it stays rainy. And don’t water tomato plants with an overhead sprinkler, as damp leaves invite fungus.

If you think you have seen late blight of potato and tomato, report it at, or call the MDAR Plant Pest Hotline at 617-626-1779, or the state Department of Agriculture at 617-626-1700.

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