Cotton losses to nematodes have increased steadily over the past 10 years, primarily due to a corresponding decrease in rotation, says Bob Kemerait, University of Georgia plant pathologist.
“As cotton acres have increased, the ability to rotate with non-host crops has decreased,” said Kemerait at the Georgia Cotton Production Workshop in Tifton. “And if you look across the Cotton Belt, losses to nematodes continue to increase.”
It's important, he adds, that growers be able to identify the symptoms of nematodes in their cotton fields. “We had confusion early in the 2001 growing season about symptoms on cotton plants that had been affected by nematodes. Stunting is the most obvious symptom, but there are a number of other symptoms that might be mistakenly attributed to herbicide injury, nutritional deficiencies and other factors,” says Kemerait.
Nematodes, he says, are microscopic ground worms that have been a pest to cotton in the Southeast since the late 1800's. They first were identified on vegetables and then on cotton.
There are four parasitic nematodes in Georgia, he says. “The root-knot nematode causes stunting, and all farmers probably are familiar with this pest. It also causes galling on the root, which can be detected by digging up the plant.
“We also have reniform nematodes, which were not common in Georgia 10 years ago. But they're moving into the state, primarily from the west, including Alabama and Mississippi. The Columbia lance nematode still isn't widespread in Georgia, but it's moving down from North Carolina and South Carolina. Finally, we have sting nematodes. They are not common in Georgia, but they cause problems wherever they go,” says Kemerait.
All of these nematodes attack the root system of cotton plants, he continues. In most cases of nematode damage, the root system is severely injured, and the plant can't take up the water and nutrients required for optimum growth, he says.
“Once a field is infested with nematodes, you can't get rid of them. You can take steps to reduce their populations when you plant a cotton crop, but they'll always be there. Anything you can do to keep them from moving from field to field will be helpful, including cleaning machinery. Once they're in a field, you can't eradicate them.”
There are several indications that you might have nematode problems in your cotton field, says Kemerait. “If you have stunted plants in your field, you might have nematodes — that's the most common sign of nematodes, and it's symptomatic of problems with plant nutrition and water uptake.
“You also might have nematodes if you're having a poor response to management inputs. If you're trying to make two-bale cotton, and you're putting in all of the necessary inputs, but it doesn't make it, then you could have nematode problems. Again, the nematodes are restricting the uptake of essential water and nutrients.”
Scattered areas of wilt also might be an indication of nematode infestation, he says. “Most nematodes — sting, Columbia lance and root-knot — all tend to be found in clustered areas of a field. The reniform, however, tends to be more widespread in fields.”
Chlorotic foliage on cotton plants also could be a symptom of nematode damage, notes Kemerait. “Obviously, this could be the result of a number of factors, including low fertilization or drought. But it's another factor which often is over-looked when considering the possibility of nematode damage.”
Premature cutout also could be a sign of nematodes, he says. “If you have one section of a field which, for no obvious reason, is cutting out earlier than other sections, it might be due to nematodes. And, occasionally, you'll have a reduced fruit load where nematodes are present. The plant might be taller than others, because when you reduce the fruit load, more nutrients are available for vegetative growth. It's not a common occurrence, but we've seen it.”
Nematodes, says Kemerait, can affect plants very early, and some growers are not aware of this. When the root begins developing, it's subject to attack by nematodes, he adds.
“When numbers are high, nematode damage may resemble seedling disease, especially in the case of reniform nematodes. When nematodes chew on the taproot, seedlings are damaged from the start.”
Many people will argue that nematodes do not cause foliar symptoms on cotton plants, and they're right, he says. “You don't find nematodes in the leaves. Nematodes create a situation where nutrients aren't properly taken up by the plant. A leaf scorch pattern is associated with severe reniform damage of cotton plants.”
Symptoms on the plant's root system can positively identify nematode damage, he says. Root-knot nematodes often form galls on the roots. And, all nematodes will produce a poorly developed root system due to damage to the taproots and to lateral roots.
“Many growers will go out into a field and pull up a plant to detect nematode damage. But whenever you pull up a plant, you lose some of the damaged lateral roots. If you're going to look for nematodes in a field, it's best to dig up the plant with a shovel, thereby protecting the root system.”
Soil samples are the most reliable method of detecting nematode populations in a cotton field, says Kemerait. “Symptoms can give you an indication of problems in a field. But soil samples can tell you the type and population of the nematode species. The ideal time for taking samples is in the fall, after picking.
“Make sure the samples are handled properly. Nematodes are fragile, and they'll die if they become too hot or dry.”
The best management option for nematodes is to exclude them from your fields, he says.
“If they're not currently in your fields, try to keep them out. We don't have resistant varieties. If you can afford it, crop rotation is a very good option. Corn is a good non-host crop for reniform nematodes. And, it has been estimated that if you take out cotton for one year and plant corn, it's similar to putting out a good nematicide.”
Anything you can do to insure good, healthy growth of the cotton plant will help to combat nematodes, he says. Cultural practices such as subsoiling to break up a hardpan, irrigating and proper fertilization will reduce nematode stress, he says.
“Nematicides also are used by many growers. These include Temik, Vydate and Telone II. You need to apply these materials properly for the best results.”