Late blight, the plant disease of tomatoes and potatoes that devastated those crops last year, has just been found in St. Lawrence County again this year, according to Steve VanderMark, Senior Extension Resource Educator with Cornell Cooperative Etension of St. Lawrence County.

The infected plants were found last Friday at a farm a few miles south of Canton, and the diagnosis was confirmed a few days later by Cornell's plant pathology lab.

It may be fortunate for most gardeners and growers that the disease has not appeared in the county until now, since most of the tomato and potato crops have been harvested, Cooperative Extension says. Cooler fall temperatures are not very supportive of the disease. Its development is favored by temperatures of 60 to 80° F and wet or very humid conditions.

Extension warns there is still some immediate concern for any plantings still in the field as the season finishes up. This is especially important for any potato plantings. Normally late blight disease cannot survive the winter here since it must have live host plant material to continually grow in. While potato plants die down at the end of their season, infected plants can still release disease spores that can reach the underlying potato tubers in the ground. Rain often promotes this. If any infected tubers were not harvested and survived the winter, they could sprout next year and bring the disease up to the soil surface. From there, fresh spores could become airborne and start new local crop infections. These infections could spread quickly with winds and rain to start an early-season epidemic across a wider area. Harvested potatoes may rot in storage if contaminated with live spores during harvest from an infested planting also. Seed potatoes from an infested planting must not be saved for use next year.

VanderMark says it will be very important next season for all gardeners and growers in the St. Lawrence County area to destroy any “volunteer” potato sprouts that happen to come up from potatoes left over in the ground from this season. Besides any tubers inadvertently left during harvest, this also applies to any discarded potatoes in cull or compost piles.

With most of these crops finished anyway, most plantings may not need fungicide of sprays at this point, but those with tomatoes still undercover such as plastic tunnels or otherwise still being grown should check their plants closely for late blight. In a few such cases the disease may still ruin the final tomatoes.

Many gardeners will recall from last year’s infestation that the blight appears quickly, causing dark, dead areas of leaves and stems that are usually edged with a fine white fuzzy fungal growth. Infected tomatoes themselves show dark greasy brown lesions usually around the stem end. Such tomatoes are not to be used for preserving.

While late blight is not capable of overwintering here unless in potatoes, it's wise to practice prevention and clean up plantings well this fall, clean and disinfest growing equipment before next season, and rotate plantings. These general steps are good protocols against most vegetable diseases, including ones common to tomatoes and potatoes here each year.

For further information on the late blight situation for our area contact Cornell Cooperative Extension at 379-9192.