Home gardeners beware: This year, late blight -- a destructive infectious disease that caused the Irish potato famine in the 1840s -- is killing tomato and potato plants in gardens and on commercial farms in the eastern United States. In addition, basil downy mildew is affecting plants in the Northeast.

"[Late blight] has never occurred this early and this widespread in the U.S," said Meg McGrath, associate professor of plant pathology and plant-microbe biology.

One of the most visible early symptoms of the disease is brown spots (lesions) on stems. They begin small and firm, then quickly enlarge, with white fungal growth developing under moist conditions that leads to a soft rot collapsing the stem.

Classic symptoms are large (at least nickel-sized) olive-green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain). Sometimes the border of the spot is yellow or has a water-soaked appearance. Spots begin tiny, irregularly shaped and brown. Firm, brown spots develop on tomato fruit.

McGrath stresses the need to act quickly to protect garden-grown tomato and potato plants and to make sure that plants don't become a source of spores that could infect commercial farms, as late blight spores are easily dispersed by wind.

She recommends that gardeners:

• Examine their tomato and potato plants thoroughly at least once a week for signs of late blight;

• Spray fungicides preventively and regularly, and/or

• Be prepared to destroy plants when late blight starts to become severe.

"If you want to try to control late blight with fungicides, you need to begin spraying now -- even before you see symptoms -- and you need to continue spraying regularly," said McGrath. "Use a product that contains chlorothalonil. Copper is not very effective on late blight."

Petunias, which are closely related to tomatoes and potatoes, can also be infected by late blight and show similar symptoms.

Late blight is very destructive. Uncontrolled, it will kill plants faster than any other disease. And it affects tomato fruit -- especially green ones. Even with fungicide applied every week, there is no guarantee of success, especially if the rainy weather continues. McGrath recommends that gardeners consider growing more of other vegetables this year.

One source of late blight in New York has been traced to tomato plants imported to garden centers from production facilities in the South. If tomatoes were started from seed by a gardener or a farmer in the Northeast, plants are unlikely to be infected, at least initially, she said. If plants were purchased at a garden center and they show signs of late blight, McGrath recommends contacting a local office of Cornell Cooperative Extension or Cornell's Plant Disease Diagnostic Clinic to get confirmation and tell them where you purchased the plants.

Photographs of late blight on tomatoes are available online.

Another disease affecting gardens and farms in the Northeast is a relative newcomer -- basil downy mildew. In 2008 the disease was severe on many of the region's farms. It often went unrecognized because it was new and the major symptom -- leaf yellowing -- looks similar to nutrient deficiency. The downy spore-bearing structures only appear on the undersides of leaves.

For more information (including how to report basil downy mildew infestations), see Cornell's Vegetable MD Web site.

Craig Cramer is a communications specialist in the Department of Horticulture.