Montana State University

Late blight discovered in Gallatin Valley

August 26, 2015 -- MSU News Service

Brown late blight lesion on a potato leaf. Photo courtesy of Nina Zidack.

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MSU News Service
Tel: (406) 994-4571

Late blight, a disease of potatoes and tomatoes, was discovered in the Gallatin Valley late last week, according to Nina Zidack, director of the Montana State University Seed Potato Certification Program. 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,” Zidack said. She added that samples can be sent to the Schutter Diagnostic Laboratory at MSU for proper identification.

Late blight, caused by the pathogen Phytopthora infestans, is a very serious disease, and if left uncontrolled can cause tuber rot in storage, Zidack said. Infected tubers can also be sources of infection if planted the following year, as can volunteer potatoes and potatoes discarded in cull or compost piles. The pathogen is spread by windblown spores and requires long periods of free moisture and high humidity for the spores to germinate. Zidack noted there is currently a severe outbreak of late blight in commercial potato growing areas in southern Idaho. She said the disease is best controlled by fungicides, and epidemics are significantly slowed with the onset of warm and dry weather such as the Gallatin Valley is currently experiencing. 

"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 harvest potatoes. Any infected foliage should be disposed of in plastic bags. 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. 

Zidack pointed to as an excellent online resource with detailed information on the disease and pictures of infected plants. Gardeners can also check with their local MSU Extension agent or send a sample to the Schutter Diagnostic Lab, 119 Plant BioScience, P.O. Box 173150, Bozeman, MT, 59717-3150. For online information on packaging samples, go to

Contact: Nina Zidack, director of the Montana Seed Potato Certification Program, (406) 994-3150,