Nematodes cost cotton farmers across the U.S. cotton belt more than $450 million a year in lost yield — and control strategies may be limited.

Bill Gazaway, professor emeritus at Auburn University, says Rootknot nematode takes $238 million out of cotton farmers' pockets each year. Reniform nematode steals another $212 million.

Reniform is increasing in importance in Alabama, Arkansas, Georgia, Louisiana, Tennessee, Texas and Mississippi.

Gazaway and other plant pathologists discussed options for controlling Reniform nematodes during a World Cotton Research Conference breakout session recently in Lubbock, Texas.

Scientists from as many as 50 cotton-producing countries attended the conference, held every four years.

Gazaway said the Reniform nematode became a particular concern in Alabama in the early 1990s. “We saw an upward spiral in nematode losses in the 1990s and we increased efforts to control losses. Alabama farmers had $26 million in losses to Reniform nematode in 2006. We're concerned with its ability to spread.”

Rotation, he added, is a crucial aspect of nematode management. Two nematicides, Telone II and Temik 15 G offer chemical control potential.

Gazaway and other pathologists hope cotton breeders will identify new varieties with high levels of resistance to Reniform and other nematode species. “But even when we get resistance, we'll still need to rotate crops,” he said. “We can't rely on just one type of control.”

Gazaway has tested corn, grain sorghum, soybeans, and bahiagrass against continuous cotton and cotton treated with a nematicide. “Yield response after one year in rotation resulted in a good increase in all rotation crops.”

He said rotation works but not necessarily every year. “Reniform damage will vary with growing conditions. With good conditions, rotation offers little advantage. We can't predict how well rotation will work.”

Even with rotation, Reniform nematode populations increase significantly after one year back in cotton. A peanut rotation appears to be the best option and is effective with both Reniform and Rootknot. “We're looking for better resistance in soybeans.”

G.L. Ausmus, from Brazil, said crop rotation “is one of the most important factors in controlling nematodes. We have a lot of farmers with big acreage in various crops, but growers do not rotate well.”

Ausmus said Brazil's cotton farmers contend with Rootknot, Reniform and Lesion nematodes. “Lesion is the most frequent.”

His observations support Gazaway's claim about nematode population recovery following one cotton crop. “When growers go back to cotton the next season after a rotation crop (nematode) populations increase, regardless of the previous crop. Continuous rotation is critical.”

He said the best rotation crops include corn, grain sorghum, pasture, peanuts and resistant soybeans.

Brazilian farmers sometime get two crops from the same land in one season. “We often plant one crop as soon as we harvest another.”

Ausmus said Temik applications showed good yield increases “but nematode populations remain high.” He believes that planting between old rows instead of planting in the same row reduces nematode damage. He said test plots show a 21.9 percent yield advantage by planting between old rows.”

Kathy Lawrence, Auburn University plant pathologist, said weed control in rotation crops also affects nematode populations. “Corn is the primary rotation crop but (nematode) populations are still up following corn.” She said weed hosts that survive in corn may help nematodes survive. Many common crop weed pests serve as hosts to Reniform nematodes. Some weeds brought Reniform populations up, “but not as much as cotton.”

Sicklepod, ragweed, teaweed and morning glory are some of the possible Reniform hosts, she said.

“Non-controlled weeds will allow continuation (following corn rotation) of Reniform populations above acceptable levels.”

Remote sensing may offer nematode control possibilities, said Gary Lawrence, Mississippi State University.

Currently, the best way to determine nematode infestation is with soil analysis. “The average cotton farm in Mississippi is about 3,000 acres. Adequate sampling of that much land may be “too costly.”

But, the reflectance value of cotton plants infested with nematodes should be less than non-infested plants, he said. Using remote sensing to identify nematode hot spots could permit farmers to use variable rate technology to target infestations and treat fewer acres. He said one test plot, “the highest yield, used two gallons (of nematicide) per acre instead of the five gallon recommended rate.”

Remote sensing and variable rate technology demands regular data updates. “This is not an on-and-off system.”

Ken Lege, Delta and Pine Land Company, said resistant varieties may play a more significant role in nematode management in the future. Currently, very few varieties are resistant to nematodes.

“Rapid introduction of biotech varieties limited the amount of time breeders had to evaluate for nematode tolerance,” Lege said.

“We need continuous evaluation to find resistance or tolerance.”