The deadly fungus that caused the Irish potato famine destroyed crops worth $9 million in Pennsylvania last year.
Confirmed outbreaks of Phytophthora infestans, commonly known as late blight, occurred in 16 Pennsylvania counties in 1994, destroying 5,000 acres of the state's 20,000-acre potato crop. Unconfirmed cases were reported in six additional counties.
The fungus, which killed thousands of acres of potatoes in Ireland 150 years ago, has been controlled in the United States by fungicides. But recently, a fungicide-resistant strain has developed, and it also attacks tomatoes.
The new strain showed up all over the United States last year, from Maine to Florida to California.
"Pennsylvania was at the top extreme," said Barbara Christ, professor of plant pathology at Penn State University. "Few states were hit as hard."
The late-blight outbreaks began in home tomato gardens in 1993 and spread to commercial farms the next year, Christ said. In tomato plants, the first late blight symptoms appear on the leaves--small, dark spots that expand rapidly during moist weather. Leaves become limp and the lesions turn brown, spreading along the stem to other leaves.
Late blight thrives in wet conditions when night temperatures drop into the 50s or 60s.