Montana State University

Late blight found in Montana: Gardeners advised to watch for disease on tomatoes and potatoes

August 18, 2010

Late blight, a devastating plant disease that caused the Irish Potato Famine, has been found in the Flathead Lake area. MSU plant pathology specialists urges home gardeners and commercial growers in the Flathead and other areas of western and southwest Montana to watch for signs of the disease on tomatoes and potatoes, which include spots like these. Photo courtesy of Barry Jacobsen.   High-Res Available

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Montana State University plant pathology specialists say late blight, the disease that caused the Irish Potato Famine in the 1840s, has been identified on potatoes and tomatoes in the Flathead Lake region.

Barry Jacobsen, MSU plant pathologist and Extension specialist, urges home gardeners and commercial growers to watch for signs of the disease on tomatoes and potatoes in home gardens and for commercial growers to apply protective fungicides.

"Gardeners should be aware that conditions have been ideal for the late blight disease over much of southwestern and western Montana this season," Jacobsen said.

Jacobsen explained that the disease is favored by temperatures in the 60s to 80s with frequent rains and heavy dews, and it can be devastating to both tomatoes and potatoes.

The first symptoms of the disease are small, dark, circular to irregularly shaped spots commonly occurring first on lower leaves, although under epidemic conditions all leaves may be infected.

The lesions will expand rapidly to large dark brown spots with a pale green to yellow water-soaked border, Jacobsen said. Brown to tan lesions will appear on stems, usually where infected leaves attach to the stem. On a cool damp morning there may be a white, velvety mold growth visible on the underside of these leaf spots or on stem lesions, he said.

"Plants can be totally blighted and killed in a week's time if cool wet conditions prevail," Jacobsen said. "This disease is quite uncommon in Montana, but the weather conditions this season have been near ideal."

There are no resistant varieties available to homeowners and the only control is to spray plants with a fungicide such as maneb, zineb, mancozeb or chlorthalonil on a weekly basis, Jacobsen advised. The fungicides are available at garden stores under a wide variety of names. Commercial growers should all have protective fungicides applied and applications should continue on a weekly basis till vines are killed.

Heavily blighted plants in home gardens should be removed and disposed of in the trash, Jacobsen said. If home garden potatoes are near harvest size, it would be best to remove plant tops to prevent infection and leave tubers in the soil for another two-three weeks to allow skin set. If potato tops are blighted, the fungus can infect tubers causing rot both in the soil and in storage.

Spores of the late blight fungus are rain splashed or blown in wind currents from infected tissues and can go many miles.

The source of infection was most likely infected potato seed or tomato transplants brought into the Flathead region from other states, Jacobsen said. Potato seed and tomato transplants produced in Montana should be free of this disease.

Jacobsen said that the presence of late blight has the potential to threaten Montana's seed potato industry.

"It is critical that home gardeners control this disease, since their infected plants could provide inoculum to our commercial seed production fields located primarily in the Gallatin and Flathead valleys," Jacobsen said. He added that commercial growers are already scouting for this disease and applying preventative fungicide sprays.

The late blight fungus caused the Irish potato famine in the mid 1840s and is responsible for much of the Irish immigration to the U.S.A. during that time.

For more information, contact Jacobsen at (406) 994-5161

Barry Jacobsen (406) 994-5161,