UNIVERSITY PARK, Pa. -- The cool, wet weather that has enveloped Pennsylvania and the Northeast this summer has created ideal conditions for a serious plant disease long feared by potato and tomato growers, according to a specialist in Penn State's College of Agricultural Sciences.

Late blight, caused by the fungus Phytophthora infestans, can wipe out entire fields of tomatoes and potatoes within two weeks of infestation. The disease, which caused the famous Irish potato famine in the 1840s, occurs sporadically in Pennsylvania. In 1994, confirmed late blight outbreaks caused yield losses as high as 100 percent in some Pennsylvania fields, costing growers millions of dollars.

"Commercial growers apply fungicides in an effort to prevent or minimize the disease," says Barbara Christ, professor of plant pathology. "But home gardeners also should be vigilant. "Most gardeners and some small-scale vegetable growers do not use extensive disease-control practices, such as crop rotation, careful scouting for symptoms and fungicide spraying," she says. "If late blight develops in a garden, airborne spores can travel 20 miles or more, potentially infecting other gardens and commercial potato and tomato fields."

The disease thrives in cool, moist conditions. "The ideal conditions for late blight are nighttime temperatures of 50 to 60 degrees Fahrenheit -- accompanied by fog, rain or heavy dew -- and daytime temperatures of 60 to 70 F for at least four or five days," says Christ. "The fungus requires moisture on the leaf surface, and may die after an extended period of temperatures above 86 degrees."

The first symptoms are small, irregularly shaped dark spots on leaves that may expand rapidly in moist weather to form brown, blighted areas. Leaves may become limp. Under ideal conditions, the lesions will spread along the stem to other leaves. Should hot, dry weather kill the fungus, lesions will dry up and appear light brown and papery.

Left unchecked, the disease will attack the fruit or tubers. In infected potatoes, a corky, light brown dry rot will result. Infected tomatoes develop areas of black rot.

"If conditions are favorable for the fungus and you suspect late blight, inspect the underside of leaves that show lesions," advises Christ. "If it's late blight, you'll see a white fuzz containing the spores that spread the disease."

Home gardeners should check their potato and tomato plants for symptoms weekly, more often during cool, wet weather. "If symptoms appear, remove and bury or burn all parts of infected plants," says Christ. "If you bury tubers, first cut them in half so they will decompose and not survive.

"Tubers from infected potato plants can be eaten, but they should not be stored for long periods because the fungus may continue to grow," she says. "Mature green tomatoes from infected plants can be kept until they're ripe enough to eat, but they should not be stored in plastic bags or other places where humidity is high."

Weekly applications of protective fungicides containing chlorothalonil or mancozeb may discourage late blight development. But Christ urges caution when using chemicals. "Before spraying a fungicide, be sure to read and follow all directions and safety precautions on the label," she warns.

Christ is collaborating with U.S. Department of Agriculture scientists to develop potato varieties that are resistant to late blight, but until these new varieties are commercially available, she says, "vigilance and good sanitation are the best weapons we have."

###

EDITORS: Contact Barbara Christ by phone at 814-863-2068 or by e-mail at ebf@psu.edu.

Writer/Editor: Chuck Gill Office 814-863-2713 FAX 814-863-9877