This occurs when leaves from plants in one row grow to the point they touch or overlap leaves from plants in adjacent rows. Potatoes are especially susceptible where sprinklers from adjacent fields overlap.įungus populations “explode” when leaf canopies close, creating an ideal micro-climate for the fungus. “Over-irrigation, or too many applications of water, can make a bad problem worse,” Johnson says. Infected tubers are the main means of enabling the fungus to persist from season to season. They need to eliminate volunteer potato plants that have survived in the field from the previous year and destroy potato tuber culls. infestans infections by good management practices.įirst, they need to buy seed potatoes that aren’t infected with the fungus. Johnson says growers can reduce the risk of P. 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.
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. Outbreaks correlate with cool, wet weather with humidity above 90 percent and temperatures of 45-70 degrees F. This allows Johnson to alert growers in time for them to thoroughly monitor fields and spray fungicides. This is four-10 weeks after planting and 14 days before late blight has been observed in the Columbia Basin in any year.
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. In 1996 the machine received 1,512 calls. Johnson updates reports as often as necessary, sometimes as frequently as twice a day. Johnson runs the data through computer models and posts late blight warnings on an telephone answering machine in his Pullman office. The plant pathologist’s computer models run on data from three automated weather stations in WSU’s Public Agriculture Weather System - PAWS for short. Today Johnson is in Rochester, New York, reporting to the American Phytopathological Society on his system for forecasting late blight outbreaks. 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. The culprits are the weather and the presence of aggressive strains of Phytophthora infestans fungus.įortunately, scientists have given vigilant farmers the means to control late blight. 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.ĭennis Johnson, a Washington State University plant pathologist who specializes in potato diseases, predicts severe late blight in this year’s Northwest potato crop.
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