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Published April 22, 2013, 09:49 AM

Changing focus on crop disease

Soybean cyst nematode is one of several threats to soybean growth in 2013.

By: Jonathan Knutson, Agweek

Last year, anthracnose and white mold were the major threats.

This year, rust and soybean cyst nematode are the crop diseases that pose the greatest danger to area bean growers, says Sam Markell, a North Dakota State University Extension Service plant pathologist.

“They (rust and soybean cyst nematode) can do a lot of damage,” says Markell, who’s responsible for disease management information on broadleaf crops, including dry edible beans and soybeans.

North Dakota is the nation’s leading producer of dry beans. Minnesota ranks fourth.

Minnesota is America’s third-leading producer of soybeans. North Dakota typically ranks among the top 10.

Rust will be the biggest disease challenge for area dry bean growers if the summer is hot and dry, Markell says.

In the past, the risk of a rust epidemic in area dry beans was low because of dry bean varieties that were resistant to the disease, which is caused by fungi.

In 2008, however, a new strain, or race, of rust developed in North Dakota to which dry beans are susceptible. The new strain subsequently spread across North Dakota and northwest Minnesota.

“We haven’t had a rust epidemic for a long time. But it’s out there. It’s lurking,” Markell says of the new strain.

In the past few years, many dry bean farmers applied chemicals to treat white mold. Those chemicals also help hold down rust in dry beans, he says.

If white mold is a smaller concern this summer, dry bean farmers will apply less of those chemicals, which could lead to more rust, he says.

Crop rotation and chemical application can reduce rust’s threat to dry beans.

‘800-pound gorilla’

Another disease threat this summer is soybean cyst nematode, which Markell calls “the 800-pound gorilla for soybeans.”

The disease first came to the United States in the 1950s, in North Carolina and slowly is spreading nationwide. It recently reached the Upper Midwest.

Soybean cyst nematode, sometimes known as SCN, is caused by parasitic worms called nematodes.

Female nematodes enter plant roots, where they’re fertilized by males. The females fill up with eggs and enlarge greatly, Markell says.

“You have living parasites right on the root. They don’t kill the root, but they live off it and suck all the nutrients out of it,” Markell says. “Yields can really take a hit.”

Soybean cyst nematode is a “sleeper disease,” he says. “The plants will look good, they’ll be green, they’ll look normal. You don’t see a lot of damage until you’re taking a 15 percent or 30 percent yield loss.”

A warning sign of the disease’s presence is small, yellow patches on soybeans in late July or August. A farmer who spots such patches should take soil samples for testing.

“The good thing is, we can manage it. The key is to find it early. If you can find it, you can manage it,” he says.

A year ago, Markell worried about anthracnose, a little known crop disease that can cripple both yields and quality in dry edible beans.

There also was concern that white mold, a disease dry bean growers have battled for years, could hurt soybeans.

But neither disease did significant damage in 2012. Markell credits farmers’ management practices and the hot, dry summer. Both anthracnose and white mold fare better in cool, wet conditions.

“Anthracnose needs water. If there’s not water, it’s not going to do much. And of course we were really dry. And we were warm,” Markell says. “I was really pleased with how little anthracnose we had. Frankly, I was pretty worried about this situation (going into the 2012 growing season.)”

This spring, farmers who plant dry bean seed grown in 2012 should be relatively safe from the threat of anthracnose, Markell says.

But the disease can live in dry bean seed for about five years, so farmers should be wary if they’ll be planting seed from 2011 or earlier, he says.

As for white mold, “there was some out there (in 2012). But there was a lot less (than in previous years),” he says.

White mold remains a concern, but area farmers generally recognize the disease and know how to deal with it, Markell says.

What’s shaping up to be a late spring could cause many area dry bean and soybeans to be planted later than normal.

Late planting, however, would have only limited effect on the incidence of rust, soybean cyst nematode, anthracnose and white mold this growing season, Markell says.

Weather and field conditions at specific times a in crops’ maturation will have the biggest impact on how common the diseases are, he says.

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