Nematodes continue to frustrate producers throughout the Cotton Belt. The average crop loss due to nematodes has been reported to be as high as 10 percent in some states, although losses in individual fields can be 50 percent or higher.

These losses occur despite the widespread use of nematicides, says Richard Davis, USDA research plant pathologist in Tifton, Ga. "Losses caused by seedling diseases are highly variable and hard to predict," says Davis. "But losses can be devastating when conditions are favorable for disease. For example, Tennessee reported a statewide average loss to seedling diseases of 20 percent in 2002."

Crop rotations, he adds, can help minimize damage from nematodes and diseases, but potential rotation crops often are less profitable than cotton.

"A rotation crop that is profitable in one area may be economically unsuitable in another, so rotation recommendations must be evaluated with due consideration of local experience," says Davis. Crop rotations, he continues, may have beneficial effects regardless of nematode and disease problems if they improve soil fertility, soil tilth, cation exchange capacity, soil organic matter or other factors that likely will increase the yield of a subsequent cotton crop. "Various legumes, sod-forming grasses and crops that leave a lot of organic matter in the field are the most likely rotations to improve soil quality," says Davis.

"The greatest cotton losses often occur in varieties and in fields with the highest yield potential. If the percent loss is the same for two varieties, the variety with the greatest yield potential will lose more actual pounds of lint. For example, a 25 percent loss is greater if the yield potential is 1,000 pounds per acre rather than if the potential is only 600 pounds."

There is some evidence, he says, that varieties with the highest yield potential may suffer the greatest percent losses, but it hasn't been documented conclusively. "However, it is clear that nematode and disease management become more important as yield potential increases."

Rotation crops will reduce the damage caused by nematodes in a subsequent cotton crop if nematode reproduction is sufficiently limited on the rotation crop, notes Davis.

"So the best rotation crops must be very poor hosts for the nematode you're seeking to suppress. To illustrate why a rotation crop must be matched to the specific problem in a field, consider this example: corn is a very poor host for the reniform nematode and a good host for the Southern root-knot nematode, so corn could reduce reniform damage but increase root-knot damage in a subsequent cotton crop."

A few examples of crops that have been shown to be useful rotations for reniform nematode suppression in cotton include corn, peanuts, bahiagrass and coastal bermudagrass, says Davis. Crops useful for Southern root-knot nematode suppression include peanuts, sorghum and coastal bermudagrass, he adds.

"Some crops that generally are susceptible to a specific nematode may have certain cultivars that are highly resistant, and the resistant varieties may be suitable for rotation with cotton. For example, most soybean varieties are very susceptible to the reniform nematode, but a few varieties have a high level of resistance and are effective at suppressing reniform nematode population levels."

In most instances, one year of a nematode-resistant rotation crop will have a significant effect, says Davis, and two years will be sufficient to achieve the maximum benefit. Unfortunately, he says, nematodes typically rebound to pre-rotation levels after one year of cotton.

"Cotton seedling diseases may be suppressed slightly by a rotation crop that is a poor or non-host for the fungal pathogen, but the fungus Rhizoctonia can grow on dead plant residue in a field and on some weeds. So several years of a poor host may have a limited effect."

Rhizoctonia, he says, may possibly be suppressed by growing several years of corn, bahiagrass or bermudagrass. But growing peanuts or soybeans, which are good hosts for Rhizoctonia, will increase problems.

Direct effects of rotations on the fungus that causes Fusarium wilt are not known, says Davis, but suppression of the Southern root-knot nematode will reduce the amount of Fusarium wilt in a field because the nematode and the fungus have a synergistic interaction.

"Winter cover crops that are good nematode hosts can increase nematode population levels in a field if soil temperatures are sufficient, and this can reduce the beneficial effect that a summer rotation crop might otherwise have had.

"Nematode-resistant cover crops will not reduce nematode levels compared to winter fallow, but they should be used when nematodes are present in a field because they will not increase nematode levels as a susceptible cover crop might."

As with all plants, a cover crop may be resistant to one nematode species and susceptible to others, says Davis, so the resistance must be matched to the nematodes present in a field.

"Weeds that are good hosts for nematodes also can reduce the nematode-suppressive effect of a rotation crop. For example, Florida beggarweed is a good host for the reniform nematode and it is one of the most common weeds in Southeastern peanut fields. So, it has the potential to allow significant reniform nematode reproduction when peanuts are grown, even though peanuts are virtually a non-host."

e-mail: phollis@primediabusiness.com