August 6, 1997 EMBARGOED UNTIL AUGUST 12
Terence L. Day - 509/335-2806 (office)
509/334-1619 (Home)
WSU Computer Models Give Early Warnings On Late Blight
PULLMAN, Wash. -- The disease that caused the Irish Potato Famine in 1845-1850 is stalking the world once again, and the fungus that causes it is meaner and nastier today than it was 150 years ago.
Dennis Johnson, a Washington State University plant pathologist who specializes in potato diseases, predicts severe late blight in this year's Northwest potato crop.
The culprits are the weather and the presence of aggressive strains of Phytophthora infestans fungus.
Fortunately, scientists have given vigilant farmers the means to control late blight. Johnson has developed computer models that help predict when farmers should spray their fields with fungicides and take other precautions to avoid the disease, which can devastate a potato field in a matter of days.
Today [August 12] Johnson is in Rochester, New York, reporting to the American Phytopathological Society on his system for forecasting late blight outbreaks.
The plant pathologist's computer models run on data from three automated weather stations in WSU's Public Agriculture Weather System -- PAWS for short.
Johnson runs the data through computer models and posts late blight warnings on an telephone answering machine in his Pullman office. Growers call 1 (800) 984-7400 to hear reports. Johnson updates reports as often as necessary, sometimes as frequently as twice a day.
In 1996 the machine received 1,512 calls.
Johnson developed his computer models after he and his WSU colleagues studied 25 years of weather and late blight data and found that the relative disease status of a crop in a given year can be predicted before 1 June. This is four-10 weeks after planting and 14 days before late blight has been observed in the Columbia Basin in any year.
This allows Johnson to alert growers in time for them to thoroughly monitor fields and spray fungicides.
Outbreaks correlate with cool, wet weather with humidity above 90 percent and temperatures of 45-70 degrees F. The number of rainy days in April and May, and in July and August, and total precipitation in May when daily minimum temperatures are above 41 degrees F also are good indicators of late blight risk.
Johnson says late blight in Washington usually begins near the Columbia River, either south or east of Prosser, and moves north as the growing season progresses.
Johnson says growers can reduce the risk of P. infestans infections by good management practices.
First, they need to buy seed potatoes that aren't infected with the fungus. They need to eliminate volunteer potato plants that have survived in the field from the previous year and destroy potato tuber culls. Infected tubers are the main means of enabling the fungus to persist from season to season.
"Over-irrigation, or too many applications of water, can make a bad problem worse," Johnson says. Potatoes are especially susceptible where sprinklers from adjacent fields overlap.
Fungus populations "explode" when leaf canopies close, creating an ideal micro-climate for the fungus. This occurs when leaves from plants in one row grow to the point they touch or overlap leaves from plants in adjacent rows.
Infections can be controlled by spraying fungicides, but only if applied before infection occurs.
Johnson says conditions this year favor the disease. So far late blight has appeared in several potato fields near Hermiston, Ore., and in about 30 in Washington's Columbia Basin.
"Late blight is a community disease," Johnson says. Farmers need to quickly report outbreaks to neighbors, and to WSU so other farmers can be protected. Unfortunately, some don't want to admit that their fields are infected. This only delays treatment of adjacent fields.
Farmers also can reduce the risk of outbreaks by buying uninfected seed pieces and vigorously fighting volunteer plants.
Farmers are going to have to learn to manage for late blight because they're going to live with it for a long time, Johnson says.
Total cost of managing late blight in the Columbia Basin in 1995 was estimated at $30 million. More than $18 million of that was spent to apply chemicals to control late blight.
In 1995, the last year for which data are available, Washington farmers harvested 80 million hundredweight of potatoes from 147,000 acres. They were Washington's fourth most valuable agricultural commodity, behind apples, wheat and milk.
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