Researchers are testing a potato field north of Pasco, Wash., confirmed to have the disease late blight.
Late blight is a concern because it tends to spread rapidly.
Researchers found late blight in a different field in the same area last year, said Tim Waters, Washington State University Extension area educator for commercial vegetables in Franklin and Benton counties.
"The last few years we've had a strain of late blight that we don't find in other parts of the country," Waters said. "If we find that strain again, then we can be reasonably sure that it came from last year's infection."
Volunteer potatoes from fields that had late blight last year could have helped spread the disease, he said.
Waters and other researchers hope to get a sense of the depth of the infection.
"Fortunately we're at very hot, very low humidity days right now, so the risk is much lower than if it was cooler and there was more moisture in the air," he said.
As potatoes are harvested, the risk level falls even farther, he said.
Fields between Pasco and Mesa should be treated with a fungicide every 7 days, according to WSU.
Other fields in the Columbia Basin should be treated at a 10-14-day schedule and 1 to 7 days before any major rainfall.