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Published August 20, 2009, 07:30 PM

Late blight found in GF County

Potato growers to receive emergency notices today
Potato producers attending the Northern Plains Potato Growers Association’s “Field Days 2009” in Larimore were treated to a nasty surprise when North Dakota State University plant entomologist Neil Gudmestad announced that late blight has been found in north central Grand Forks County.

By: Matt Bewley, Grand Forks Herald

LARIMORE, N.D. — Potato producers attending the Northern Plains Potato Growers Association’s “Field Days 2009” in Larimore were treated to a nasty surprise when North Dakota State University plant entomologist Neil Gudmestad announced that late blight has been found in north central Grand Forks County.

The disease, also known as potato blight, was the primary culprit of the Irish potato famine of the 19th century. A 1999 outbreak in the Northern Plains destroyed $125 million worth of the tubers.

“We are in a situation that is not unlike 1999,” Gudmestad said. “We lost 10.5 (million) to 12 million bags of potatoes. We don’t need to go there.”

Early sightings

Late blight was first confirmed Wednesday morning at the potato association’s new irrigation test site in Inkster, N.D.

By Wednesday evening, Gudmestad had found seven fields with disease symptoms in Larimore, 20 miles away, and had received unconfirmed reports of its presence in areas generally on a north-south line along the Red River Valley.

“So, we have potato fields with late blight from the South Dakota border all the way up to Walsh County,” he said. “That’s really well-distributed and prepared to spread and explode if we get the kind of weather pattern that the National Weather Service is predicting for the next week.”

He immediately ordered all Inkster test plots with late blight destroyed to protect the other test plots at the irrigation site and also the production potatoes grown around the area.

“We want to be good neighbors,” he said.

Bad news

The reason is late blight’s ability to wreck entire crops in a very short period of time. A single late blight lesion about a half-inch in diameter disperses about 250,000 sporangia, or spores, per lesion per day. If the temperature is less than 60 degrees, the lesion is apt to multiply eight to 24 times its own area.

“You do the math and you can see that if you have a hot spot and a bunch of lesions, you’re throwing a lot of inoculum not only for your fields but for your neighbors,” he said.

It is transported by wind, rains and the movement of soils from one field to another. Studies have shown that the spores can be picked up by a thunderstorm’s updraft winds and dumped in several nearby fields with the rain.

How it got here

There are unconfirmed reports that a disastrous late blight outbreak on the east coast may have been caused when nursery-grown tomatoes from Alabama, Georgia and Florida that became infected with late blight were subsequently distributed across the U.S. by Kmart, Lowe’s, Home Depot, Wal-Mart and Menards.

In the Grand Forks County test plots, the origin of the disease is also still a matter of speculation.

“The only reason we have late blight in these small plots is that there’s a heck of a lot of inoculum out there,” he said. He suspects that late blight had been on some of the Larimore fields for “a long time.”

He is asking producers who suspect they have late blight to send samples to NDSU’s plant pathology laboratory in Fargo so they can map out its locations and possibly confirm a source.

Treatment

Gudmestad said his guess is that the Larimore fields were not properly protected with a regular seven-day fungicide treatment program. And now that late blight is in the area, he suggests stronger measures.

If late blight is discovered in a field, the grower’s first and most important responsibility is to destroy that “hot spot,” Gudmestad said.

Application of desiccants is the best method, as it very quickly prevents the disease from further growth. Discing infected plants is not recommended because it allows the spores to survive a lot longer, he said.

As for protecting the rest of the potato crops, he suggests stepping up fungicide programs.

“If you are in the immediate area, or if you are downwind, heading toward Minnesota, it probably would behoove you to go from a seven-day program to a five-day program through these next few weeks,” he said.

But preventing a major outbreak will be tough. The spores are apparently widespread and the weather forecasts are calling for the kind of conditions that help the disease continue to grow and spread.

“As soon as you get conducive weather, it’s going to explode,” Gudmestad said. “We’re in a really, really tough situation right now, where everyone has to do due diligence.”

Reach Bewley at (701) 780-1111, or by e-mail at mbewley@agweek.com.

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