Late Blight Comes Early, Hitting Tomatoes Hard, Experts Say

By Adrian Higgins
Washington Post Staff Writer
Friday, July 10, 2009

Plant scientists are asking home gardeners in the region to check their tomato plants for a disease that could wipe out much of this year's crop.

In spite of its name, late blight has appeared early in the tomato growing season, which runs from April to October, and threatens not just homegrown fruit but commercial crops, said Jerry Brust, an expert on vegetable pests at the University of Maryland's cooperative extension service.

If the disease takes hold, consumers might see "a little less quantity and a higher price" for tomatoes as growers spend extra on fungal sprays, said Meg McGrath, a plant pathologist at Cornell University. So far, outbreaks have been sporadic but spread over a wide geographic area, with infected plants reported from South Carolina to Maine and west to Ohio. Late blight is the same fungal disease that wiped out potato crops in Ireland in the 1840s, leading to the infamous potato famine.

The disease is sometimes seen at the end of the tomato growing season, but after the fruit has been harvested. Its appearance now has the potential to cause widespread plant losses before harvest, said Brust, "especially if we start getting driving rains and some good storms that will pick up the spores," which can travel miles to infect healthy plants. Scientists attribute its early appearance to an unusually cool, wet and cloudy spring. McGrath, who works from Riverhead, N.Y., said that in 21 years she has only seen five outbreaks "and I've never seen it this early before."

In Maryland, the disease was discovered in mid-June when a Howard County gardener brought a sick plant to extension agents for diagnosis. Brust said many of the infected plants were sold unwittingly by big-box retailers -- he declined to name them -- raising fears that diseased plants could be widespread.

The initial symptoms are small, watery, gray or brown lesions on the leaf that quickly spread to the entire leaf. The fungus causes dark-brown lesions on the stems. The tomatoes turn brown and rot. The Maryland cooperative extension service has posted images of infected plants on its blog, Grow It, Eat It, at http://www.growit.umd.edu.

The fungus is not toxic to humans, but quickly renders an afflicted fruit rotten and inedible. Blighted plants should be placed in a sealed plastic bag and thrown out. Don't put them on the compost heap, where the spores will continue to spread, according to Brust.

The disease can be prevented if healthy plants are sprayed with a fungicide containing chlorothalonil. "That would be very helpful at that point and it would help with other foliage diseases," he said. Tomato vines are often afflicted with another disease called early blight, in which lower leaves yellow and shrivel.

Gardeners who want to spray organically could use neem oil, he said, but would have to reapply the oil weekly.

Word is getting out among farmers, McGrath said, but she is worried that the disease will spread quickly because home gardeners, many of them novices at growing food because of the recession, won't monitor their plants closely enough. "It's amazing how quickly this disease can devastate plants and that's something people need to realize," she said. "It's not like any other diseases on tomatoes that we can live with."

Hiu Newcomb and Ellen Polishuk of Potomac Vegetable Farms said they have not noticed late blight on their tomato crops. They are growing approximately 2,000 plants on their farm near Vienna and an additional 3,000 on their farm near Purcellville.

"There's not a lot to be done when you're an organic grower," Polishuk said, "so we are going to wait and see what happens."

The blight, Phytophthora infestans, also causes potato plants to collapse and the tubers to rot. Potatoes and tomatoes are both in the nightshade family.

"We are growing potatoes but they're pretty much done already, and we'll be harvesting shortly," Polishuk said.

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