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Late blight pathogen spreads to state’s potato crop

August 19, 2009 By Nicole Miller

The plant pathogen best known for causing the Irish potato famine — Phytophthora infestans — was just discovered in two commercial potato fields in two separate Wisconsin counties. Before this, the outbreak of late blight, as the disease is known, had been confined to tomato plants.

To date, late blight-infected tomato plants have been reported in eight southern Wisconsin counties, with a number of cases coming from the Madison area. In tomatoes, the disease often starts in the plant’s leaves — causing brown, expanding lesions to appear — and then travels through the stems to the fruit.

For home gardeners wondering if it’s safe to eat healthy-looking fruit from diseased tomato plants, the answer is yes, says University of Wisconsin–Madison plant pathologist Amanda Gevens. However, she points out, these tomatoes won’t store well and shouldn’t be used for canning.

Gevens and her colleagues in the Wisconsin Department of Agriculture, Trade and Consumer Protection are asking home gardeners to please destroy all of the diseased plants they find, as small outbreaks have the potential to impact Wisconsin agriculture writ large.

“This is a community problem,” says Gevens. “Every little bit counts. Each plant that produces more spores is contributing to the overall [level of pathogen in the air], making it worse for the larger growers in the state.”

The proper way to dispose of late blight-infected plants is to cut them off at the ground, seal them in a plastic bag and leave them in the sun until the plants are clearly dead, and then toss the whole package in the garbage.

For more information, contact Amanda Gevens at 608-890-3072, 608-575-3029 or