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Potato late blight lesion.  Photo: Ristaino, NC State University.
Potato late blight lesion. Photo: Ristaino, NC State University.

Late blight prevention and identification

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The cloudy, rainy conditions last month were ideal for the spread of late blight spores. Late blight is a plant disease that attacks potatoes and tomatoes. It has not been confirmed in the North Country, but has been found on a tomato plant in Oneida County. Amy Ivy, horticulturist with Cornell Cooperative Extension in Clinton and Essex Counties, says tomato and potato growers in the region should take action. There's no cure for inflected plants, but there are ways to prevent its spread.

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Reported by

Todd Moe
Morning Host and Producer

Late blight is a plant disease that attacks potatoes and tomatoes. The disease has been confirmed in some areas of New York State, and is working its way up to the North Country.

Late blight has appeared in Western Massachusetts, near Buffalo and in Madison County between Syracuse and Utica and south of Watertown. The disease is blown in on storms and air currents.

If you believe that your plant has been infected, it is important to confirm with the experts at Cornell Cooperative Extension. Every county has an office with someone that can provide support. Cornell is asking for samples to determine the particular strain of the late blight diseases are popping up. If you believe that you're plant has been infected, contact your local Cornell Cooperative Extension office and send them a sample.

The best source of tracking the latest confirmed locations of late blight can be found on The website has a map on the homepage that tracks the latest confirmed locations. The website also contains photographs of the disease, a video tutorial of how to find late blight in your garden, advice on how to handle the disease and contact information for the Cornell Cooperative Extension.

Late blight is devastating because within about a week or two the infected plants will die. The disease moves very quickly and cannot be stopped once it arrives. For the home gardeners, the disease jumps from one garden to the next.

Once the plant gets a spore it cannot be saved. While most spots have a clear edge or line around it, late blight shows up as a greasy purplish brown spot that bleeds like watercolor on paper. The blight will start as the size of a nickel or quarter. By the end of the day, the spot will have noticeably grown. The spots will show up on both the stems and the leaves of the infected plant.

Wet weather helps to spread the disease. Once the organism arrives, the disease can spread rapidly under the right conditions. If the dry spell we received this weekend stays, the spread will slow down.

The good news is that late blight is limited to potato and tomato crops. However, it doesn't matter where you got your potato or tomato plants from. The disease blows in so it doesn't matter if you started growing from seeds or not. If the spore lands on your plant it will eventually die.

Researchers at the Cornell Cooperative Extension have been trying to breed some resistant varieties, but there is no cure for the disease. Resistants' will not proof your plant. Even a resistant plant can eventually procure the disease.

If you're willing to protect your crop, you might try using a Foliar that you mix it with water and spray on the leaves. Foliar is caustic and very hard on your skin, therefore you want to use this with care. The conventional product has a couple of different brand names that all contain Chlorosilane. Chlorosilane is contained in products such as Funganil, Daconil and the commercial product Bravo.

The spraying is a protectant in case a spore blows into your yard. When it lands it gets burned by the copper or the product. Once the plant is infested to any degree at all, the Cornell Cooperative Extension insists that homeowners bag up the infected plants and take them to the dump. Remove them from the yard will help contain the disease. Because the spores are carried by wind currents, it's important to contain the spores so you're not infesting your neighbor's gardens.

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