May/June 2011
Late Blight By Bill Schaefer

Late blight. Two words that send chills through every potato grower’s spine. Most famous as the cause of the Irish potato famine in the middle of the 19th century, late blight to this day remains feared for its potential to lay ruin to and devastate potato crops.

However, potatoes are not the only vegetable susceptible to the late blight fungus. Plants in the solanaceae family – potatoes, tomatoes, pepino, tree tomato, eggplant and nightshade – all serve as hosts for the disease.

Two years ago, late blight was found in states from Maine to Washington. Researchers and plant pathologists from Maine, Wisconsin and Idaho all agree that the source of Phytophthora infestans, late blight, was tomato plants found in garden shops and some big box stores.

At a recent seminar in his new research building in Rupert, Idaho, Jeff Miller said that the source of a late blight outbreak in 2009 in Idaho was found in tomatoes shipped by Home Depot.

“We ran a genetic analysis and the genetic fingerprint was the same as the one on all the tomatoes that were being shipped by Home Depot,” Miller said.

Along with the standard advice of getting rid of cull piles, being on the lookout for volunteers and scouting for late blight in fields during the growing season, Miller advised growers to walk through garden centers and the big box stores when you’re shopping and inspect the tomatoes for evidence of late blight.

Miller said that he thought the potential for late blight in 2011 in Idaho is “pretty high,” at the same time he said that the ability to forecast is weak.

“All we’re doing is judging how favorable is the environment to the disease. You have to have the pathogen present, you have to have a susceptible host and you’ve got to have a good environment,” said Miller.

Amanda Gevens, an extension plant pathologist with the University of Wisconsin in Madison, said that Wisconsin had not had any reports of late blight from 2002 until 2009. Gevens said that weather conditions in 2009 – high relative humidity, high precipitation, extended periods of leaf wetness and cool days – were favorable for late blight’s reappearance. Gevens said that the strain they saw in Wisconsin, US22, was the identical strain found on tomato plants in Wisconsin.

Gevens that they also saw some late blight in 2010 but it was primarily a problem for small vegetable farms and home gardeners.

“History tells us when we’ve had a year of late blight, the following year there’s some of it kicking around and it just depends on how we conditions may be. If it stays hot and dry you may not see it, it may not be problematic. But if it is wet, likely you will see it again in 2011,” Gevens said.

Gevens recommends that growers use forecasting tools to dictate when to start protective fungicide spray programs.

The state of Maine reported incidences of late blight in 2009. According to Steve Johnson, crops specialist, Extension professor at the University of Maine, growers saw the same late blight strain come into the southern region of the state, once again brought in by tomato plants.

“It really didn’t affect commercial agricultural scale people,” Johnson said. “It was a lot of the home gardeners and the market gardeners that were badly hit.”

Johnson said there were no reports of late blight in 2010 but cautions that that doesn’t mean that Maine is late blight free.

“I need three years of no late blight before I start feeling comfortable in becoming less aggressive,” Johnson said.

Miller, Gevens and Johnson all had similar advice to growers. Remove your cull piles, watch for volunteers and weed control for all forms of nightshade. They all strongly advocated making sure your seed is clean and to include a seed treatment when planting.

Miller said current research results show that a post-harvest spray, using hydrogen peroxide or phosphorous acid in the field as they come out of the ground is the best way to protect your spuds, though the phosphorous acid seems to be better.

“If we wait an hour before we treat the efficacy drops a little bit, in fact the longer you wait to treat the more infection you get with the hydrogen peroxide based product. With the phosphorous acid we’re able to go as far as six hours after and still get good control,” he said.

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