COLUMBUS, Ohio — Late blight was confirmed in Pennsylvania and Kentucky in late May, generating concerns that this devastating disease of tomato and potato may soon show up in Ohio farm fields and backyard gardens.

Responsible for the Irish potato famine of the 1840s, late blight has caused widespread crop damage across the United States and Canada since 1990. If left unmanaged, this disease -- which is transmitted via spores -- can result in complete destruction of tomato and potato plants.

"We can assume that home gardeners have planted tomatoes with late blight in Kentucky at least, and that the inoculum (spores) is not far away," said Sally Miller, a vegetable pathologist with Ohio State University Extension and the Ohio Agricultural Research and Development Center (OARDC). "Therefore, we may see it sooner in Ohio than we did last year (when it was confirmed on June 25)."

Late blight thrives under cool, moist conditions. "Hot, dry weather (which was prevalent in Ohio last week) suppresses the disease," Miller pointed out, "but it does not make it go away entirely."

In northwestern Pennsylvania, late blight was detected on locally grown greenhouse tomato transplants. Since then, the affected plants have been destroyed and the grower has adopted a fungicide spray program to manage the disease.

In Kentucky, the disease was found on tomatoes in Boone County and Lexington retail operations. The plants in both locations came from Michigan. Miller said this operation may sell tomato seedlings in Ohio, mainly in the Cincinnati and Columbus areas, but its sales are reported to be fairly limited in Ohio.

What to do?

If late blight is found in your garden, destroy plants already infected -- pull out the entire plant, place it immediately in a plastic bag, and dispose of the closed bag in the garbage. Miller advised against putting infected plants on a compost pile or in a composter, or leaving them lying around.

"Live plant tissues serve as a source of inoculum, and uprooted plants may support active spores of the pathogen for some time," she pointed out.

Healthy-looking plants should be protected with a fungicide. Conventional gardeners can use fungicides containing chlorothalanil or copper; several brands are available in garden centers and other retail outlets. Organic gardeners can use copper-based fungicides; several OMRI (Organic Materials Review Institute) -- approved copper-based fungicides and formulations are available.

Miller added that another possible source of late blight inoculum at this time of year may be volunteer potato plants that were infected last season. Volunteer potatoes should be destroyed as soon as they emerge, particularly if late blight was observed in the area last year. Commercial growers planting corn after potatoes may consider several herbicide options.

Gardeners who spot potential late blight disease symptoms and are unsure of identification can submit samples to Ohio State University's C. Wayne Ellett Plant and Pest Diagnostic Clinic for a nominal fee. To learn more, log on to ppdc.osu.edu or call 614-292-5006.

What to look for:


Under cool, moist conditions, late blight lesions may appear on leaves within three to five days of infection, followed by white mold growth soon after. Spores formed on the mold are spread readily by irrigation, rain and equipment. These spores can also be easily dislodged by wind and rain and can be blown into neighboring gardens or fields within five to 10 miles or more, beginning another cycle of disease.

Late blight appears on potato or tomato leaves as pale-green, water-soaked spots, often beginning at leaf tips or edges. The circular or irregular leaf lesions are often surrounded by a pale yellowish-green border that merges with healthy tissue. Lesions enlarge rapidly and turn dark brown to purplish-black.

During periods of high humidity and leaf wetness, a cottony, white mold growth is usually visible on lower leaf surfaces at the edges of lesions. In dry weather, infected leaf tissues quickly dry up and the white mold growth disappears. Infected areas on stems appear brown to black, and entire vines may be killed in a short time when moist weather persists.

On potato tubers, late blight appears as a shallow, coppery-brown, dry rot that spreads irregularly from the surface through the outer 1/8-1/2 inch or more of tissue. On tuber surfaces, lesions appear brown, dry and sunken, while infected tissues immediately beneath the skin appear granular and tan to copper-brown.

Late blight can also develop on green tomato fruit, resulting in large, firm, brown, leathery-appearing lesions, often concentrated on the sides or upper fruit surfaces. If conditions remain moist, abundant white mold growth will develop on the lesions and secondary soft-rot bacteria may follow, resulting in a slimy, wet rot of the entire fruit.

A fact sheet with photos of late blight symptoms and additional disease information can be found at online.

OARDC and OSU Extension are the research and outreach arms, respectively, of Ohio State's College of Food, Agricultural, and Environmental Sciences.

SOURCE: Ohio State University.