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.
Potatoes are especially susceptible where sprinklers from adjacent fields overlap. 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.