Montana State University

Gardeners advised to watch for late blight on tomatoes and potatoes

July 1, 2010

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Montana State University plant pathology specialists say weather conditions in Montana are ideal for late blight, the disease that caused the Irish Potato Famine in the 1840s, and they urge home gardeners to watch for signs of the disease on tomatoes and potatoes in home gardens to inhibit its spread to commercial crops.

"Gardeners should be aware that conditions have been ideal for the late blight disease over much of Montana the past few weeks," said Barry Jacobsen, MSU plant pathologist and Extension specialist.

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 lesions commonly occurring first on lower leaves, although under epidemic conditions all leaves may be infected. Jacobsen said the lesions will expand rapidly to large dark brown spots with a pale green to yellow water-soaked border. On a cool damp morning there may be a white, velvety mold growth visible on the underside of these leaf spots, he said. Lesions can also occur on stems, usually where infected leaves attach to the stem.

"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 for the past month 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.

Jacobsen said heavily blighted plants should be removed and disposed of in the trash. Spores of the late blight fungus are rain splashed or blown in wind currents from infected tissues and can go many miles.

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

Jacobsen advised that the presence of late blight could 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 USA during that time.

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

Barry Jacobsen (406) 994-5161,