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Soybean diseases costly, but could be worse

LITTLE ROCK, Ark. — About 11.3 percent of the total estimated Arkansas soybean production in 2003 was lost to 22 diseases, according to the University of Arkansas’ Division of Agriculture.

The Arkansas Agricultural Statistics Service estimates that the state’s farmers grew 2.9 million acres of soybeans and averaged 37 bushels per acre. The total estimated production was 107.3 million bushels.

About 12.1 million bushels were lost to diseases for a total monetary loss of $87.7 million dollars.

Eleven percent is a sizeable loss figure, but it’s in the loss range that soybean farmers experience in most years, says Rick Cartwright, plant pathologist with the UA Cooperative Extension Service.

“When soybeans are high priced, farmers don’t worry as much about these losses, but it’s critical in today’s economic climate to save every bushel you can.”

The leading diseases were soybean cyst nematode with 4 percent of the total, followed by root-knot nematode and charcoal rot, each at 2 percent. The remaining diseases were each less than 1 percent.

“Because a disease is not ranked high on the list doesn’t mean it’s not important,” noted Cartwright. “It’s a sign that farmers are doing better at controlling it.” He said the development of good resistant varieties has helped control many diseases.

The loss of bushels to a disease doesn’t tell the whole story. There are also losses from the expense of trying to control the diseases after an outbreak. For instance, in 2003, farmers had to spray 800,000 acres of soybeans at $18 an acre because of an outbreak of frogeye leaf spot and aerial blight.

But the control methods for nematodes and charcoal rot are more difficult for farmers to use, so they rank high on the disease loss list.

Cartwright said there used to be soybean varieties that could resist the most common races of cyst nematodes that attack the roots of soybean plants. But new races have developed that can attack resistant soybeans, and they’ve become a serious problem.

Cartwright said resistance to the root-knot nematode is not common in most high-yielding southern soybean varieties. Some varieties have resistance, but growers don’t typically switch to them “until they get burned” by the root-knot nematode.

“They call the nematode the hidden killer. It’s a stealth disease. You don’t realize how bad it is until harvest. It doesn’t kill plants, but it weakens production.”

Cartwright said if farmers have fields not performing up to expected, and there’s no other obvious reason, they should take soil samples late in the summer and send them off for a nematode check.

He said farmers can help break the nematode cycle by rotating grain sorghum with soybeans.

Charcoal rot, a fungal disease of roots and the lower plant, has a broad range of host plants. It thrives on stress caused by erratic rainfall, drought, thin soils and high temperatures and survives in the soil for a long time.

Growers who do a good job of irrigating and fertilizing soybean crops will have a better chance of avoiding charcoal rot problems. “Farmers who try to cut corners will find that charcoal rot catches up with them,” the Extension plant pathologist said.

Cartwright said Terry Kirkpatrick, Cliff Coker, Mark Trent and Kim Hurst screen varieties for diseases, and they “have a good handle on what’s resistant.” The information is available to soybean growers.

Lamar James is an Extension communications specialist with the University of Arkansas.

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