Rush to prevent late blight in Southern Wisconsin

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Thursday, August 13, 2009
— Tomato death is blowing on the wind.

Don’t panic. But stop and think about your garden as well as produce crops across Wisconsin. The advice from one Rock County organic farmer and a Wisconsin plant disease specialist is:

Get a fungicide.


The problem is late blight, a disease caused by Phytophthora infestans, a fungus-like organism. The disease has been confirmed on tomatoes in Dane, Rock, Portage and Langlade counties.

Specialists have not figured out if this particular late blight strain can transfer to potatoes, but it might, said Adrian Barta with the Wisconsin Department of Agriculture, Trade and Consumer Protection.

One strain of late blight was the biotic cause of the Irish potato famine in the mid-1800s.

The disease is not harmful to humans, but it is devastating to crops. That’s why home and commercial gardeners alike should be vigilant, Barta said.

In other states, late blight quickly made the leap to potatoes. State officials have met with Wisconsin potato growers to explain the signs and to prepare growers for a potentially devastating crossover. Wisconsin—the nation’s third largest potato producer behind Idaho and Washington—last year harvested 2.3 billion pounds of potatoes.

“It has now landed in the center of commercial potato production in Wisconsin, so the risk is great,” said Amanda Gevens, a plant pathologist with the UW-Madison and University of Wisconsin Extension.

Rock County organic farmers Steve Pincus and Beth Kazmar have had to cut down rows of tomato plants on their produce farm in Magnolia Township south of Evansville. The goal is to prevent the spread of the disease, which was confirmed on their farm, Tipi Produce, Aug. 1.

The disease has not yet been confirmed in Walworth County as of Wednesday afternoon, Barta said.

Unlike some other common tomato diseases that limit yield, late blight is a fast killer, Barta said.

“(Late blight) can cause plant death within a week,” Barta said. “It can cause some really dramatic losses in a home setting or a commercial setting.”

The disease has not been seen in Wisconsin for six years, and Barta has never seen it hit tomatoes to this extent, he said. A cool July followed by wet, humid weather made conditions right for the spread of late blight, Barta said.

Late blight should not be confused with other diseases. After all, tomatoes are prone to disease and can have all sorts of relatively non-threatening spots, Kazmar and Pincus said.

“A whole lot of things can go wrong with tomatoes,” Pincus said.

Kazmar advised that late blight lesions are “bigger and faster moving” than other kinds of lesions. Keep in mind that the disease wasn’t confirmed in Rock County until August, she said. Spots that were visible before that were caused by something else, she said.

Home gardeners should cut an affected tomato plant off at the roots, bag it and leave it in the sun to kill the blight, Kazmar said. Throw out the plant and do not compost it, she said.

It’s not practical for commercial farmers to bag that many plants, Kazmar said.

At Tipi Produce, workers have cut affected plants at the base and left them to wither in the sun. The fungus won’t thrive on dead plants, Pincus said.

Pincus estimates Tipi has lost 20 percent of its tomato crop so far. But a lot could happen in the next two months—for better or for worse, he said.

Kazmar has been walking through the tomato crop that hasn’t been destroyed and using a propane torch to burn individual leaves and kill the fungus. As organic farmers, Kazmar and Pincus can’t use commercial fungicides. They have used copper-based, organic fungicides, they said.

The two disagree on whether home gardeners should use fungicide of the organic or inorganic type.

Kazmar says, “No,” because the chemicals are too powerful for home use.

Pincus doesn’t think gardeners “should give up on their tomatoes so easily.” He says, “yes” to whatever type of fungicide gardeners are comfortable using.

Think about your neighbor’s gardens and fields, too, Barta said.

“We just hope people will be vigilant,” Barta said. “If they let an infection go unchecked, it’s their neighbor’s problem. On commercial farms, it could be really bad.”

What it looks like: The disease starts as a pale green lesion on stems or leaves of tomato plants. It turns brown to black and can grow big enough to destroy entire leaves. Within a week, entire plants or crops can die.
To prevent it: Take action now by using a commercial fungicide or copper-based organic fungicide.
Step one: E-mail or call Amanda Gevens with UW-Madison at gevens@wisc.edu or (608) 890-3072 if you need help with diagnosis.
Step two: Report the blight to Adrian Barta with the department of agriculture at adrian.barta@wi.gov or call (608) 224-4592.
Step three: In your home garden, bag up the affected plant, leave it in the sun to kill the blight and throw it away. Do not compost the affected plant.

Keep in mind that the disease could spread to commercial tomato or potato farms. So, even if you’re not too worried about your tomato plants at home, keep your neighbors in mind.

To learn more: Visit www.datcp.state.wi.us. Information about late blight is available on the home page.

Keep in mind: Don’t feel nervous about bringing tomatoes home from the store or farmers market, said Steve Pincus, organic farmer in Magnolia Township. Affected tomatoes wouldn’t be for sale, he said. But, remember, farmers could be using more fungicides than normal on their plants. Wash your tomatoes well, Pincus said.

Last updated: 11:10 am Thursday, December 13, 2012

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