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Cotton nematode threat increasing

As Georgia's cotton acreage has continued to increase in recent years, crop rotation — or the lack thereof — has contributed greatly to worsening problems with nematodes.

“We cannot over-emphasize the importance of nematodes and the damage they cause to our cotton crop,” says Bob Kemerait, University of Georgia Extension plant pathologist. “As our acres increase, rotation becomes more and more important to the success of our cotton crop.”

Nematodes cost Georgia cotton growers more than $30 million in losses in 2001, says Kemerait, with most of the losses attributed to control costs and yield reductions. “The cost of nematicides, loss of yield and reduction in crop value due to poor quality all contribute to these losses,” he says.

There probably are several reasons for the increase in nematode problems, says Kemerait. First, many growers do not recognize the symptoms of damage to their cotton fields from nematodes. Damage from nematodes, such as stunting, premature cut-out and foliage symptoms often can be misdiagnosed as fertility problems, pH imbalances, drought or herbicide injury.

Therefore, growers may not be taking the steps needed to minimize losses.

“Also, anytime you continue to grow more and more of one crop on the same land without proper rotation, you risk increasing the population of soil-borne pathogens — such as nematodes — to damaging levels.

“As cotton acreage increases, land devoted to other crops decreases. And, in some cases, you may not be able to grow desirable rotation crops. Your options for rotating may not help you with nematodes,” says Kemerait.

A comprehensive nematode survey had not been conducted in Georgia since the mid-1990s, says Kemerait. So, this past year, researchers — with the cooperation of Georgia county Extension agents, the Boll Weevil Eradication Program and Dow AgroSciences — initiated a new survey.

“Thanks to the dedication and efforts of county agents from 67 Georgia cotton production counties, ‘Nematode Roundup 2002’ was very successful.

Between October 2002 and January 2003, nearly 1,800 samples were submitted to the Nematology Diagnostic Lab in Athens.

“These samples were from fields that had been arbitrarily chosen with the help of maps provided by the Boll Weevil Eradication Program. Because of this, the survey was not biased by samples collected only from fields known to have had damage from nematodes in the past. Thus, it provided a more realistic look at the scope of the nematode problem on cotton in Georgia.”

The survey not only was an opportunity to improve the understanding of the distribution of parasitic nematodes throughout the state, but it also allowed county agents to provide additional education to growers on the importance of nematodes on all Georgia crops, says Kemerait.

As data from the nematode survey continues to be compiled and summarized, initial results are revealing, says the plant pathologist.

For example, root-knot nematodes — likely Southern root-knot nematodes — were recovered from samples from 61 of 67 counties that participated in the survey, and they're probably present in all counties. Perhaps more importantly, root-knot nematodes were found to infest at least 69 percent of the sampled fields.

Root-knot nematodes were above economic threshold values (100 per 100 cc of soil) in more than 25 percent of the fields that were sampled. The incidence of root-knot nematode becomes even more serious when one considers samples from specific regions of the state. For example, of the samples collected from the Worth-Tift-Doughtery-Colquitt area, root-knot nematodes were recovered from 80 percent of the samples.

In the Lowndes-Brooks-Lanier-Cook area, root-knot nematodes were recovered from 94 percent of the samples. In both cases, sandy soils likely make these areas specifically hard-hit by root-knot nematodes. Clearly, the root-knot nematode is a very serious problem in Georgia and will continue to increase in importance unless growers can increase their crop rotations.

Reniform and Columbia lance nematodes were much more localized in their distribution compared to root-knot nematodes, being recovered from only about 5 and 3 percent of all samples respectively. However, the low figures can be deceiving based upon the limited distribution of the nematodes.

For example, the reniform nematode is most commonly found along the northern edge of the Coastal Plain from Burke County and down to Early County on the Alabama line. It is found only sporadically south of the fall line. In counties like Jefferson, the reniform nematode was recovered from more than 50 percent of the fields that were sampled. In Morgan County, reniform nematodes were recovered from 45 percent of the fields while root-knot nematodes were found in only 11 percent.

Also, reniform nematodes are more likely than root-knot nematodes to be above threshold levels if they are found at all in a field. Distribution of Columbia lance nematodes is geographically similar to that of the reniform nematodes, though perhaps not as extensive. And, like the reniform nematode that is found in relatively few fields across the state, in some areas — such as Burke and Jefferson counties — the number of infested fields rises to more than 30 percent.”

The bottom line of this survey, says Kemerait, is that parasitic nematodes are all too common throughout the crop production regions of Georgia.

The next phase of educational efforts to reduce the impact of these parasites will be to provide information on management strategies such as the use of crop rotation and cost-effective nematicide treatments. In the absence of suitable long-term rotation, the use of nematicides such as Telone II and Temik is our next-best solution for dealing with fields where nematodes are a problem, he says.

In the past, the question from many has been, “Can I afford to treat my cotton with a nematicide?” The better question today appears to be, “Can I afford to grow cotton without managing the nematodes in a field?”


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