LAKE COUNTY — Local potato farmers want to urge home gardeners to be on the lookout for signs of late blight, which has reared its ugly head in the Mission Valley this month.

“Even one inoculated plant from a small home garden can sporolate so fast that within days, it can take out the whole valley,” potato farmer Jack Lake, of Lake Farms Inc., said.

Characterized by black colored wounds or abnormalities on the leaf and stem, late blight is the same devastating pathogen responsible for the Irish potato famine of the late 1840s. According to Lake, potatoes, peppers, eggplant and wild nightshade can get, and spread, the disease.

Typical lesions have a necrotic center surrounded by collapsed pale green or chlorotic tissue. Under moist conditions, the pathogen sporolates from a host tissue, producing a white “downy mildew.” Lesions on stems are water-soaked and dark green to black, commonly infecting the stem at the apex of the shoot or at the juncture of the petiole and the stem.

Late blight was last spotted in Lake County about 10 years ago, Lake said. Most growers learned from that experience that without continual application of the appropriate fungicides, the onslaught of late blight can be devastating. Those who didn’t learn that lesson, however, have already been hit hard by the outbreak this time around.

“In Montana, we have the growing weather, the soil and are somewhat isolated,” he said. “The million dollar question for us is, ‘where did it come from?’ The more people with gardens makes the inoculum somewhat easier to spread.”

Since the fungus can’t survive on dead tissue, Montana State University extension agent Jack Stivers advises potato growers to scout their fields or home gardens for abnormalities on the leaf and stem.

If lesions are found, leave potatoes in the ground, but put all vines in a plastic bag to cook in the sun and die. This effectively kills the pathogen, while preserving the crop and allowing the skins to thicken for two to three weeks, as usual. If potato tops are blighted, the fungus can infect tubers causing rot both in the soil and in storage.

“Get [the vines] killed early enough so they’re dead and there is no tissue carrying the fungus,” Lake said. “It can’t translocate through the plant into the tuber; it has to make contact. Sometimes there can be a problem if there’s a crack in the ground during harvest.”

Finally, and perhaps the most important step for any home garden is to buy certified seed from a local grower each season and resist the urge to replant seed, especially seed that has been infected.

“Uncertified seed is going to be carrying something,” Lake said. “When you plant your own garden, be responsible and get certified seed.”

Even local growers who did spray fungicides, which must be applied prior to the arrival of the pathogen to prevent germination and penetration of plant tissue, are now taking precautions to preserve their crops.

“We’re not devastated, it’s just a management thing,” Lake said. “We were lucky that we had our fungicide on. Lucky and smart at the same time. It’s a challenge.”