BOZEMAN – A disease of potatoes and tomatoes called late blight was discovered in the Gallatin Valley during the week of Aug. 17, according to Nina Zidack, director of the Montana State University Seed Potato Certification Program.

Late blight – caused by the pathogen Phytopthora infestans – is a very serious disease, Zidack said, and if left uncontrolled can cause tuber rot in storage.

Symptoms can appear on leaves as round, brown lesions, sometimes bordered by a light green halo, and infections can also invade the stem, ultimately infecting the tubers. In tomatoes, the fruit can be infected as well. Any infected foliage should be disposed of in plastic bags.

While late blight has been found very sporadically in other parts of Montana in the past, this is the first identification of the disease in the Manhattan area, where almost half of the state’s seed potatoes are grown, Zidack said.

“Since we live in a seed potato growing area, even small infestations in a garden could produce a source of infection for neighboring potato fields. It is very important to properly identify the disease and remove infected plants,” she said, adding that samples can be sent to the Schutter Diagnostic Laboratory at MSU for proper identification.

The pathogen is spread by windblown spores and requires long periods of free moisture and high humidity for the spores to germinate. The disease is best controlled by fungicides, and epidemics are significantly slowed with the onset of warm and dry weather.

“Gallatin growers are very optimistic that they are catching this disease early, and with the dry, warm weather and imminent harvest, that they can avoid having disease issues in the potatoes,” Zidack said.

Home gardeners and those with market gardens should scout their potatoes and tomatoes, and if they see late blight, the best thing they can do at this point of the season is to harvest their potatoes.

Zidack pointed to as an excellent online resource with detailed information on the disease and pictures of infected plants.