Late blight disease attacks northeast U.S.

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.

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.

"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.

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.

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.