Late blight infection on peeled potato tuber

An early warning system developed about 15 years ago at WSU is helping protect the state’s $685 million potato industry from late blight, a disease caused by a fungus-like organism involved in the 1845 Irish potato famine.

The disease, which causes potatoes to rot, is spread by spores from infected tubers left in the ground from the previous harvest, cull piles, infected seed potatoes or volunteers. The spores can be dispersed by wind and rain when springtime weather conditions are favorable.
“The reason why late blight is so important to forecast is that it can spread very rapidly over a large geographic region,” said Dennis Johnson, a WSU plant pathologist. “It only takes one or two infected potatoes to start an epidemic.”
The good news is that the disease can be controlled with timely applications of fungicides. That’s where the late blight hotline, field observations and a computer model developed in the early 1990s by Johnson and Richard Alldredge, a statistician at WSU, come into play.

Late blight lesions on potato leaflets

The model calculates late blight predictions based on two variables: inoculum potential and number of rainy days in the early spring.

Field persons, who scout potato fields during the growing season, alert Johnson when they see signs of late blight. “If they are not sure what it is, they will send me a sample to confirm a diagnosis, or I’ll go and take a look at it,” he said.
The warnings are made available via a 1-800, toll-free telephone number. The message may be changed daily when circumstances warrant. The number may receive 1,100 and 1,800 calls during the growing season.
When conditions change dramatically and growers must take immediate action, warnings are distributed to growers by the Potato Commission via e-mail. Information is also posted on a website.