OREGON STATE UNIVERSITY

OSU scientists warn of possible late blight epidemic in gardens

06/22/1998

CORVALLIS - Gardeners beware. With a warmer than average winter and a wetter than average late spring, Oregon is poised for an epidemic of late blight of tomatoes and potatoes. Without protective fungicides, late blight can destroy an entire home garden crop of tomatoes and spuds.

OSU plant pathologists Cindy Ocamb and Mary Powelson are recommending that home gardeners check for signs of late blight on their tomato and potato plants.

Their basic instructions are simple: if you see the disease destroy the plants.

If you don't, start a fungicide spray campaign right away, as the disease has already been found in home gardens in the Willamette Valley.

"If you don't have the disease, you need to spray now unless you want to risk losing your potato and tomato crops," warned Powelson, a professor of plant pathology in OSU's Department of Botany and Plant Pathology.

"Late blight is a devastating disease," said Ocamb, plant pathologist with the OSU Extension Service. "It can move in and kill all the tomatoes and potatoes in your garden in just a few days."

Late blight is caused by the fungus (Phytophthora infestans) that caused the famous Irish potato famine of the 1840s, explained Ocamb. All strains are devastating to tomatoes or potatoes.

Late blight is carried on living tissue. The main way the disease can be introduced into your garden is from purchased tomato starts.

"Volunteer potatoes or tomato fruit or shoot material in compost piles can also be a disease source," said Ocamb. "Or spores can blow in from neighboring, infected gardens. Peppers, eggplants and related weeds, such as hairy (but not black) nightshade and bittersweet can also carry the disease."

Once it gets a foothold, late blight progresses rapidly and can leave your tomato and potato patch a slimy, foul smelling mess.

Check for symptoms early in the morning when the humidity is highest. By mid-day, symptoms are often difficult to see.

Symptoms of late blight on both tomato and potato plants start as irregular, water-soaked spots on leaves, petioles, or stems. Under cool, moist conditions, spots rapidly enlarge to form purplish-black lesions.

During periods of high humidity and leaf wetness, a cottony white growth is usually visible around the edges of lesions. In dry weather, infected areas quickly dry up, and the whitish growth can be difficult, if not impossible to detect. Lesions can girdle affected stems, killing foliage adjacent to the lesion.

On green tomato fruits, gray-green, water-soaked spots form, enlarge, grow together and darken, resulting in firm, brown, leathery-appearing lesions. If conditions remain moist, abundant white growth will develop on the lesions.

On ripened tomato fruits, the lesions have cream-colored concentric zones that eventually coalesce and affect the entire fruit. Secondary soft-rot bacteria may follow, turning the entire green or riper fruit into slimy wet rotting things.

What should home gardeners do if they find late blight?

"Pull up the plants, and either bury them or put them in a plastic bag and place them in the trash," recommended Powelson. "Do not compost them. Then plant new, disease free tomato plants or potato seed pieces in a different area in the garden. There's still time to grow a new crop."

If you don't find any signs of the disease, then treat tomato and potato plants weekly with an appropriate protective fungicide.