Weekly treatments needed on potato fields; tomatoes may harbor disease

By MITCH LIES
Capital Press

HERMISTON, Ore. -- Researchers are recommending growers treat potato fields once a week through July to help fight the spread of late blight.

The disease has been found in four fields in Washington.

Oregon State University plant pathologist Phil Hamm said conditions are ripe for spread of the disease and growers need to keep new tissue protected.

"There's so much new growth," Hamm said. "(Potato plants) are just not protected unless you protect them every week."

In two of the four fields where late blight has been found, Hamm said growers were treating every two weeks.

Hamm estimated the disease has been festering in at least two of the infected fields since the first week of June, which is the earliest late blight ever reported in the area.

"We knew we were set up for late blight," he said.

Late blight favors cool, wet conditions.

Researchers from OSU and Washington State University are unsure how the blight entered the area. Generally, Hamm said, late blight emerges from infected seed, from cull piles containing infected tubers or from spores left over from the previous year's infection.

This year researchers also are considering a fourth source: Growers back East last year found late blight spreading from infected tomatoes to potatoes.

One reason incident has been low to date in the Northwest is disease incidence was extremely low last year, Hamm said.

Two of the infected fields are just north of Pasco. A third is in Horse Heaven Hills and the fourth is in the Burbank area of Washington.