Late Blight Hits North Country

Submitted by Paul Hetzler, CCE Horticulture and Natural Resources Educator

Late blight, a disease which ravages tomatoes and potatoes, has been confirmed in the Colton area on August 27, and very likely will soon show up across the region.

“Better late than never” is a common saying, but when it comes to late blight, it should be “Better Never Than Late.” Historically, late blight was rare. But ever since infected tomato plants were shipped from southern greenhouses to stores across the Northeast in May 2009, essentially destroying the tomato crop that year, it has recurred every year.

Gardeners and produce farmers should scout their tomato and potato crop for signs of late blight. The first symptoms are large, watery lesions on leaves and stems, especially near the top of the plant. The leaves will have the appearance of having been frozen and then thawed. In moist and humid conditions, white, fuzzy fungal growth may be seen at the margins of such lesions. Since late blight is airborne, symptoms will show up throughout the plant, not just near the bottom. On the tomatoes themselves, late blight causes brown, greasy-looking patches that are quite firm to the touch.

Home gardeners can use protective fungicides with the active ingredients chlorothalonil or copper on potatoes and tomatoes. Once late blight hits, these fungicides cannot stop the disease. If late blight is already in your garden, you can try to salvage ripe tomatoes, and unripe ones of mature size, by immersing them in a 10% bleach solution and laying them out indoors on a counter top or baking sheet where you can keep an eye on them. Tomatoes with small late blight spots are safe to eat after removing affected areas, but the USDA recommends not using them for canning.

Late blight can spread on the wind from one garden or farm to the next. To protect other growers, diseased plants should be placed in clear plastic bags and left in the sun until no green tissue is left. Once the plants are completely dead, the late blight organism can no longer produce spores and it is safe to compost those plants, bury them, or discard them in the trash.

The only way late blight can overwinter is if potatoes from infected plants are left in the ground. When these infected volunteers sprout in the spring, late blight can work its way up the stem from the tuber and produce spores which could touch off an epidemic early in the season. It is important that gardeners and farmers destroy all volunteer potato plants next spring.

For more information on late blight, go to usablight.org, or call your local Cornell Cooperative Extension office.