The variable-rate application of nematicides in some fields can reduce cost significantly and improve cotton yield, according to a study conducted by a team of plant pathologists from the Southeast.

The results of the study were presented by John Mueller, professor of plant pathology, Clemson University, at Cotton Incorporated’s Crop Management Seminar in Memphis.

A survey conducted over a seven-year period in the late 1990s indicated that for South Carolina, an annual average of 5.36 percent of the cotton crop was lost to nematode damage. That’s a total of 22,168 bales valued at $8.53 million. Nationally, the average annual loss to nematodes from 1952 to 2002 averaged 2.13 percent of the crop.

Mueller says that variable-rate applications of nematicides can be economically feasible in fields with varying soil types, especially those containing a percentage of sandy soils, where nematodes can thrive.

Currently, South Carolina researchers recommend two thresholds to guide control decisions. “A low threshold will result in a 10 percent yield loss if the nematodes are untreated. A high threshold will result in a 25 percent or greater yield loss if nematodes are not treated. In South Carolina, we have three species that we’re concerned with, root knot, reniform and Columbia lance.

“On our low threshold, our standard treatment is 5 to 6 pounds of Temik per acre. For the high threshold, we recommend that growers use 3 gallons per acre of Telone and 3 to 5 pounds of Temik. This is an expensive proposition, but there does seem to be a synergism there. Sidedressing Temik has worked well in certain areas, but it really needs to be irrigated cotton and it needs to be done on a timely basis.

“The best thing to do with a high threshold is to rotate crops,” Mueller said. “Of course, many of our growers got involved in continuous cotton for economic reasons and that will build up nematode populations in a hurry.”

Mueller says the expense of dealing with nematodes and the potential for loss are pushing researchers in several states in the Southeast and Mid-South “to try something different.”

“We knew that nematode distribution is determined by soil type, soil texture, soil chemical properties, and we knew we had some new tools like GIS/GPS and the ability to measure soil electrical conductivity. We know that to develop a system to put a nematicide only where the nematodes are, we would have to develop variable-rate application systems tied into GPS.”

They chose the Columbia lance nematode for the study. The pest “likes a little sandier soil than the root knot nematode. This nematode is a problem for us on about 50 percent of our fields.”

To map soil texture, a soil electrical conductivity mapping cart, or Veris rig, was run through the field at about 5 to 6 miles per hour. With GPS, the rig can develop a map of soil type in the field, specifically in regard to sand content.

“We know the nematodes are going to be in the lighter areas of the field,” Mueller said. “Can we afford to put a nematicide in there at a higher rate? How much do we need to put out where the good soils are?”

The researchers compared a standard rate of 6 pounds of Temik in a strip through two grower-cooperator fields, alongside variable rates of Temik ranging from 3 pounds to 7 pounds based on soil type. They also put out a standard rate of 3 gallons of Telone plus 3 pounds of Temik for thrips control and a variable-rate of Telone, from 0 to 3 gallons based on soil type. They also put out a control strip where no nematicide was applied, but was oversprayed for thrips.

The study found that as the percentage of sand dropped, nematode numbers also dropped. It didn’t matter when the samples were taken, at planting or at harvest. “We found a very linear relationship between percent sand and Columbia lance nematode. But this relationship will vary when you get into root knot nematodes or reniform nematodes.”

The research also indicated that there was little or no yield loss in better (less sandy) soils. “There is no reason to put Telone or Temik out for nematodes where we are projecting no yield loss from nematodes,” Mueller said.

Variable-rate Temik resulted in 5 percent higher yield and 34 percent lower nematicide use compared to a blanket rate of Temik. “Our variable-rate Telone also increased yield by 5 percent over the standard application with a 75 percent reduction in the amount of nematicide applied.

“The message is that if you have nematodes, you better start at the bottom and get your roots in order or you’re going to fight a losing battle all year long.”

Mueller said one issue is whether or not growers will adopt the technology. “How many growers are going to sit down in front of a computer and turn over what they do in the field to that computer? But there is hope. It took less than two days to get the research systems set up for two grower-cooperators. Both farmer fields where the tests were conducted have since adopted the variable-rate system on their farms. I think it has a future.”

e-mail: erobinson@farmpress.com