Potato Blight is Back

By Jack Ruttle

This is a real horror story. Late blight, the disease that caused the great potato famine in Ireland, has reappeared in a more dangerous form. Worse, the new strain devastates tomatoes. Early in 1993, it wiped out virtually all tomatoes in an area 150 miles wide in northern New York. The disease has popped up on tomatoes and potatoes from Michigan to Vancouver. It's the number-one potato research priority, according to Stephen Fry, a disease expert at Cornell University. Fry and other researchers are trying to produce resistant varieties.

Late blight is a fungus, (Phytophthora infestans). Spores of the original strain live only a few hours in the air. To survive winter, the fungus has to move into a potato tuber -- it cannot survive on dead leaves or stems. This has made the disease relatively easy to control by cleaning up and destroying damaged or culled tubers.

In its new form, the fungus has multiple strains and is able to reproduce sexually. The resulting spores can live for several seasons on debris and in the soil. Researchers say the new strains appear to have almost completely supplanted the old one. To date, the only crossing between strains in North America seems to have been in British Columbia, but researchers fear that the stage is set for a coast-to-coast epidemic.

Gardeners should be on a close watch for the disease during cool spring weather. Look for irregular dark spots. They expand rapidly and become brown, blighted patches, especially after damp weather. The disease travels along the stems to other leaves.

A fungicide can stop the disease, but that's not a good idea, especially in the garden. Instead, destroy infected tomatoes or potatoes. Stems and leaves can be safely composted. Tubers should be thoroughly chopped or heat-treated before composting.

There are no tomatoes resistant to this disease, and few potatoes, but researchers across the U.S. are hard at work. Today, the most blight-tolerant potatoes are Elba, Alleghany, Kennebec, and Sebago.

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