Scouting essential for fighting soy diseases

“Bottom line, we just want to help growers make more money,” says Wrather, a University of Missouri plant pathologist.

Wrather’s concern is understandable. In 2001, a variety of diseases hit Missouri soybeans and caused serious loses. Charcoal rot resulted in almost one million bushels lost in the state. Phytopthora root-rot and soybean cyst nematodes each reduced yields about 4 million bushels. Also last year, Sudden Death Syndrome (SDS) likely reduced yields in the state about 200,000 bushels.

“These diseases aren’t found in every field that has disease. Farmers in one town may not have charcoal rot but do have SDS. It just depends,” says Wrather.

While there is no cure-all, there are some ways to manage these diseases. In the Missouri Bootheel, charcoal rot can be managed by planting cotton for two years or more. Wrather says following cotton, soybeans aren’t damaged by charcoal rot “very much.”

But if you’re in an area of Missouri where you can’t raise cotton, what can be done?

“Our soybean breeder in Columbia has developed a soybean line with some tolerance. That could help some, although it hasn’t been available for this part of the state yet.”

Phytopthora root rot can be problematic for the Bootheel. Some six weeks ago, in a 10-mile square area around Portageville, growers received about 4 inches of rain over a single weekend. Wrather says Phytopthora got into clay soils and killed a bunch of soybeans.

“This disease can be best managed several ways,” he notes. “First, treat the seed before planting with something like Apron or Apron XL. Those products will protect the seed for a short period of time. Second, plant varieties with some resistance.”

Soybean cyst nematode, like charcoal rot, can be managed in southern Missouri by crop rotation. “We’ve learned through years of research that a year of corn will greatly reduce a cyst nematode problem. That isn’t the case in north Missouri, however. There, farmers might have to plant corn for two or three years before getting the same benefit.”

Wrather says before growers go out and plant massive corn acreage, however, the best way to deal with soybean cyst nematode is to plant a resistant variety.

SDS typically shows up in a plant through yellowing leaves that later turn brown. If the SDS is bad enough, pods will then fall off. There are no methods to control this disease except for a few tolerant varieties.

“With SDS, crop rotation is of no benefit and we know of no chemicals that can protect the plant. While there is some variety tolerance to SDS leaf symptoms and pod abortion, nothing is resistant to the infection. The germ moves into the roots and then, about the time the plants bloom, we suspect a poison is produced that eventually moves into the plant tops, causing leaf yellowing and pod abortion.”

At his tour stop, Wrather spoke often of resistant varieties. That’s the best way to deal with disease management, he says. But while there are resistant varieties, none are resistant to all diseases. So what to do?

“Breeders are trying to develop varieties with resistance to many diseases. But that is very difficult. That means producers need to select current varieties properly. The only way to know what varieties to plant is to know what diseases may develop in your fields.

“I don’t have a crystal ball, but I can scout a soybean crop each year. That’s tough because most of our soybean crops are on rows of 15 inches or less. But fields need to walked three times: once about four to six weeks after planting, second at bloom and third when the pods are full of seed.”

While walking, try and find disease. If a disease is there this year, chances are it’ll be there the next time you plant soybeans. So pick a variety with some resistance.

A few years back, when conservation tillage studies were being run at the Delta Center, farmers were told no-till slowed down the spread of cyst nematodes. Does that still hold true?

“That’s a phenomenon observed primarily in the southern U.S. We’ve done work on the effects of tillage, and we’ve found that cyst nematode populations won’t increase during the season as rapidly as in conventionally tilled fields. It’s an interesting finding and while it helps, it doesn’t do a great deal with control.”

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