Late blight has been found in several potato fields near Paul, Idaho, the Pacific Northwest Pest Alert Network reports.
Sporulation was observed on diseased tissue. The University of Idaho is testing to determine the type or strain found.
The blight is extensive in some of the fields, Jeff Miller of Rupert-based Miller Research said in the alert notice. Weather conditions as of the report date were not favorable for spreading the pathogen, but risk of spread would increase greatly if thunderstorms develop. He advised growers to maintain an effective fungicide program.
“Late blight under the right conditions completely defoliates a crop,” Miller Research Field Trial Manager and agronomist Trent Taysom said in an interview. “It takes leaves off and can kill a potato crop in a field not protected by proper fungicide applications.”
This summer’s storms, though sporadic and punctuated by dry periods that were fairly long, may have contributed, he said. The last significant incidence in Idaho occurred in 2015, which had wet conditions in late July and August.
“Fortunately, based on the climate, it is not something we deal with every year,” Taysom said. But vigilance is required constantly because “when you have the right conditions, it happens very quickly — in a matter of days, whereas with other diseases it can be a matter of weeks.”
Symptoms can include dead rings on leaves, or lesions that show a pale green halo around their perimeter, he said. A white, cotton-like material, where the fungus can grow, may appear on leaf tissue in early mornings or late evenings when humidity is higher.
“When you see the fungus actively forming, you know sporulation is actively going on,” Taysom said.
Spores released from the cotton-like fungal growth can land on healthy tissue and begin to infect it, he said. In severe cases, spores that hit the ground can be watered into soil, grow onto roots and damage the tuber.