When east Alabama farmer Melvin Taylor first began noticing that his cotton was yellow and stunted in places, he suspected a fertility problem. After all, he did plant during one of the wettest springs in recent history, and leaching was a distinct possibility.
“I put everything on that cotton but the kitchen dish rag,” says the Tallapoosa County grower. “We started out in April with about 30 units of nitrogen, 65 units of phosphate and 90 units of potash. We came back a month later with 60 more units of nitrogen, and by the end of July we had applied more than 120 units.”
But Taylor's aggressive fertilization program still didn't eliminate the yellow, stunted rows of cotton in his field. The nitrogen helped some, says Taylor, but so did advice from Bill Gazaway, retired plant pathologist and nematologist with the Alabama Cooperative Extension System.
“Bill Gazaway took some leaf samples and told me I had a nematode problem. In my case, it was reniform nematodes,” he says.
And Taylor is not alone in his misery, says Gazaway, who still works with producers throughout Alabama. “Since the mid-1980s, reniform nematodes have risen from a relatively obscure pest in the Southern states to one that threatens cotton production in Alabama, Louisiana, Mississippi, Arkansas and Georgia,” he says.
Over the past three years, cotton production losses to reniform nematode have exceeded 6 percent in yield or about $27 million annually in Alabama alone, says Gazaway.
“The reniform's rapid rise to prominence can be traced to its ability to spread from field to field and from region to region via soil on farm equipment,” he explains. “Once established in a field, nothing can be done to eradicate this nematode. Hidden from sight in the soil, it attacks and destroys cotton roots. By the time a grower realizes he has a problem, reniform is well established throughout the field and is causing substantial damage to cotton.”
The reniform nematode, says Gazaway, was first discovered in Alabama in the eastern part of the state in 1958. But it wasn't a problem, he adds, until about 1988, when it was found causing serious yield losses in a cotton field near Huxford, in southwest Alabama.
“That's when we put in a test to study the nematode. We knew reniform was a relatively new problem because we had seldom seen this nematode in soil samples taken from cotton prior to that time,” he says.
From that point, Gazaway began conducting loss studies in cotton. “We found some interesting things, including the fact that some nematicides worked well against the reniform and some did not. Temik, at various rates, worked quite well, and Telone also did a good job.”
From these tests, researchers found they were getting yield losses as great as 75 to 80 percent. They also found that losses would vary from year to year depending on growing conditions.
“We learned early on that nematodes did their greatest damage to the plant when there were stressful conditions — whether it was excessively dry or wet weather. In some years, we would get losses of as little as 10 percent. Generally, the losses would be about 20 to 25 percent of total yield.”
It wasn't long, says Gazaway, before surveys revealed that the reniform nematode was spreading in Alabama. “Statewide surveys conducted in 1990 showed that the reniform nematode was concentrated in south Alabama, including Escambia and Monroe counties, where a lot of cotton is grown. Looking north to Alabama's Tennessee Valley, we found only one or two fields infested with reniform nematodes.”
When the survey was repeated later, the results were much different, he says. “The reniform nematode indeed had spread, and it had spread quite rapidly. It continued to spread in south Alabama, going into Baldwin and Mobile counties. The intensity of the infestation also was greater in central Alabama. But most surprising of all, we found that Limestone and Colbert counties in the Tennessee Valley were heavily infested, all within a 10-year period. We were seeing serious problems.”
The movement of the reniform, he continues, appeared to be from south to north in the state.
“This movement is by equipment. The reniform has an uncanny ability to survive in dried soil for months, making it an ideal candidate to be spread by dirt clods clinging to equipment. Farmers rent land, they borrow equipment, they swap equipment, and they buy used equipment. It's imperative that growers in any of these situations clean their equipment.”
Reniform nematodes also can be spread through drainage, when water from an infested field drains into an non-infested field, he adds. The reniform prefers heavier soils but thrives in most soil types in which cotton is planted, he says.
“What makes the reniform particularly damaging is that it reaches huge populations, even within a single growing season. It simply overwhelms the cotton plant's root system. It is very competitive. Generally speaking, once reniform is introduced, it drives root-knot nematodes out of a field. We've seen this in field studies and in the greenhouse.”
Damage to cotton inflicted by the reniform nematode varies according to growing conditions, in addition to the nematode population, notes Gazaway.
“The reniform generally stunts cotton by feeding on the root systems. With its ability to produce extraordinarily large populations in a single growing season, reniform can seriously impair cotton's ability to take up nutrients and water from the soil. So you'll see quite a lot of nutrient deficiencies with this nematode.
“Even though you might have applied adequate nutrients in a field, the root system is impaired and cannot take up the moisture and nutrients needed for a healthy plant.”
The reniform nematode produces no visible symptoms on the roots, says Gazaway, except that the root systems of infected plants are poorly developed.
Above ground, reniform produces non-distinctive symptoms on cotton foliage resembling nutrient deficiencies, stunted vegetative growth and reduced fruiting.
“Cotton damaged by reniform usually matures one or two weeks later than healthy cotton, causing a delay in maturity of 15 to 20 days. Stunted cotton plants usually occur in localized areas of the field where the nematode is just getting started. Above-ground symptoms of damage often resemble a potash deficiency late in the season, with fewer and smaller bolls.”
In areas of moderate infestation, infected cotton plants actually may be larger than non-infected plants at the end of the season, says Gazaway. “This is by virtue of the fact that there are no bolls to take up the nutrients, so the vegetative growth is greater. I always caution growers not to look at the vegetative growth toward the end of the season. Instead, look at the number of bolls and the fruiting.”
The only sure means of determining if you have a reniform nematode problem is by sending a soil sample to a nematode diagnostic laboratory for a nematode analysis, says Gazaway. There are several private and state-supported nematode diagnostic laboratories that can conduct the analysis, he says.