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WSU Issues Late Blight Warning to Potato Growers

PULLMAN, Wash. — Northwest potato growers were warned today of high risks of late blight, the disease that caused the Irish potato famine.

Washington State University Plant Pathologist Dennis Johnson said the risk of late blight increased last week because of cool, wet weather that placed the crop at high risk. Seven to 10 days of dry, warm weather have been forecast. Johnson said this will reduce risks somewhat; but more cool, wet weather is expected.

Risks vary within the region, depending on weather and other local factors. Washington’s potato industry has a farm-gate value of nearly half a billion dollars.

In a message posted today (May 16) on a telephone alert system, Johnson estimated the risk at 83 percent in the Hermiston, Ore., area; 74 percent near Pasco, and 71 percent around Othello. The risk in the Prosser area is much lower, at about 35 percent.

Johnson bases his estimates on a computer model that uses April and May rainfall. “Any more rainfall in May will increase the risk,” Johnson notes.

Irrigation practices are important. Johnson warns that late blight often develops where sprinklers overlap, and within the first 80 to 100 feet out from the center of pivot-center sprinkler systems.

Potatoes should not be grown in those areas, Johnson says. Late blight can be prevented by protective fungicide sprays, which need to be applied several times during the growing season. Johnson urges potato growers to consult fieldmen and WSU county extension faculty for advice on fungicides.

Control is a regional problem, Johnson says. Cull piles are an especially important problem because they provide a place for the fungus to grow.

Late blight first appeared in Washington in 1947, near Prosser. It was next reported in the area in 1974, near Hermiston. It struck crops during seven of the next 16 years and every year since 1990.

“It is controllable if it is managed,” Johnson says, “but it’s a disease that can ruin a grower financially.”

Late blight is kept in check by good management practices based on research and extension work. Johnson also has a research program aimed at development of resistant plants.

“We will always have to live with late blight,” Johnson says, “but we will be able to manage it more easily, at a lower cost in the future.”

Potatoes are Washington’s third most valuable crop.

Johnson’s telephone alerts are posted on an answering machine at 1-800-984-7400. The line will operate through October.

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