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'Late blight' alert: The Potato Famine disease found in St. Lawrence County crops

Posted 8/9/13

By PAUL HETZLER Cornell Cooperative Extension of St. Lawrence County Cornell’s plant pathology lab has confirmed in the last week several cases of late blight found in St. Lawrence County. Late …

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'Late blight' alert: The Potato Famine disease found in St. Lawrence County crops

Posted

By PAUL HETZLER

Cornell Cooperative Extension of St. Lawrence County

Cornell’s plant pathology lab has confirmed in the last week several cases of late blight found in St. Lawrence County.

Late blight is an airborne fungal disease unparalleled in its ability to destroy tomato and potato crops. Gardeners and market growers should be on the lookout for brown, greasy-looking but firm spots on tomato fruit, and on tomato or potato leaves and stems, for dark, watery lesions that look as though the tissue was frozen and then thawed. Late blight lesions may turn from dark green to black as they age. Yellow leaves or spots indicate another condition.

Late blight can’t be cured, and in most cases its progress can’t even be arrested, but if no symptoms are found, gardeners can protect tomatoes and potatoes with conventional or organic fungicide sprays. Products with the active ingredient chlorothalonil are available in garden centers and hardware stores. Organic growers can use copper-based fungicides, also widely available, or a spray containing Bacillus subtilis, which may need to be ordered. Keep in mind that in a very early-stage infestation, lesions are not visible to the unaided eye.

If late blight symptoms are found on any plants in the garden, the whole patch (with the exception of the new late-blight resistant varieties) should be assumed to be infected. Unripe, unblemished tomatoes should be dipped in a dilute (5 percent) bleach solution, brought indoors and spread out so they don’t touch each other. They’ll ripen best at room temperature in a dark or semi-dark place. Check them every day or two for signs of late blight, as the disease can progress fast.

Ripe tomatoes with late blight lesions are safe to eat once the spots are cut away, but it’s recommended they not be used for canning.

To help stop the spread of late blight to unaffected regions, tomato and potato vines should be either piled and securely covered with a tarp, or bagged and sealed. Once the plants have blackened, the organism will die out. A sunny location will expedite this process. Alternately, the plants can be sealed in plastic bags and placed in the garbage.

Potatoes can be left a week to harden up, but as late blight spores can wash through the soil and infect the tubers, they should be harvested no later than that. Bruising increases the chance of infection, so be careful as you dig. Since late blight cannot overwinter except in infected potatoes still in the ground, try to get them all, and be vigilant next year in destroying volunteer plants.

For more information, contact your Cornell Cooperative Extension of St. Lawrence County, 379-9192, ph59@cornell.edu.