Approximately 45 percent of West Texas cotton acreage is infested with root-knot nematodes (RKN) particularly the counties to the south and west of Lubbock, according to plant pathologist Terry Wheeler, Texas A&M University.

Extensive USDA research over a 16-year period in West Texas indicates that RKN causes an average yield reduction of 26 percent in cotton.

“You can find RKN north of Lubbock in certain counties. Where we have sandy soils, we have RKN,” says Wheeler, who is based at the Texas Agricultural Experiment Station in Lubbock. “Many area growers don't recognize the yield loss that can occur from RKN. Growers in the counties with the worst problem are aware that they should be concerned, but there are not a lot of tools to attack RKN effectively. We basically have just a few chemical, cultural and varietal options.”

Temik provides one control option. Most Texas growers apply Temik to control early season insects, particularly thrips. “Depending on how much RKN pressure is present in the field, we recommend growers increase Temik rates,” Wheeler says. “They normally use around three pounds for early season insect control, and we'd like for them to put out five pounds, sometimes six, for RKN.”

Wheeler conducted surveys in 1995 and 1996, and found the only West Texas farmers who used Temik rates higher than those for early season insect control had both RKN and black root rot fungus (Thielaviopsis). On those fields, farmers doubled average Temik.

“Growers who had only RKN stayed with the insect rates,” she says. “You get some effect on RKN at the insect rates. However, the more nematode pressure you have, the less response you will see at the insect rates.”

Peanut rotation

Peanuts provide the only rotational crop immune to RKN. “The root-knot nematode cannot reproduce on peanuts,” Wheeler says. “The nematode population drops after a year's rotation with peanuts. Nematodes will hatch out slowly over that year, but they won't have anything to feed on. The following year should be great for a cotton crop.”

However, simply rotating to peanuts is not enough. Growers need to eliminate nematode food sources such as nutsedge and other types of weeds while the field is in peanuts. Sampling has shown high levels of RKN populations in peanut fields infested with weeds.

“If you're not careful about weed control, you'll lose all your benefits from a peanut rotation and you'll get hammered when you rotate back to cotton,” Wheeler says. “We struggle with some weeds out here, especially purple and yellow nutsedge in peanuts. Some of the better peanut herbicides are not labeled for use in some of our counties.”

The southern High Plains normally has about two million acres of irrigated cotton and only 200,000 acres of peanuts, so peanut acreage is never adequate to provide ample rotation. Water quality and quantity pose another limiting factor. “You can't grow peanuts on many High Plains fields, which is why we don't have more than 200,000 acres,” Wheeler says. “Many of our RKN fields are not candidates for peanuts.”

Varietal selection

Varietal selection offers a third root-knot nematode control option for West Texas cotton growers. Stoneville's ST 5599BR, a stacked Bollgard/Roundup Ready picker variety includes RKN tolerance. Wheeler began looking at ST 5599BR in 2000.

“I have studied RKN-tolerant ST 5599BR in all sorts of field and weather conditions, and it has been the top yielding variety in every one of my tests,” she says. “The more nematode pressure we have, the more ST 5599BR separates itself from other high yielding varieties.

“ST 5599BR is the first new option that I've been able to recommend as a solution for growers who have a RKN problem,” Wheeler says. The stacked variety also produces well under limited water as well as plentiful water.

“I wouldn't mind seeing ST 5599BR planted on 500,000 to 1 million acres out here. I would like to see it as a standard in fields that have root-knot nematode,” Wheeler says.

Wheeler says future RKN control options might include biological products and seed treatments.