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Fall good time to take samples in cotton fields for nematodes

It's fall, and the best time of the year to focus on a pest that may very well be the next limiting factor for cotton yields. The pest is — drum roll, please — the nematode.

The pest “has been ‘discovered’ by everyone the last few years,” says Arkansas Extension plant pathologist Terry Kirkpatrick. “Of course, it's been here all along. But in the past, we had to deal with weevils taking the crop, and the morningglories covering it up. It was hard to worry too much about a nematode. It took the science and technology to solve other problems, and now we've bumped the next ceiling.”

The pests can slow development of the cotton plant and cause significant yield reductions. With root knot nematode damage, galls form on roots. Severely infected plants may have damaged taproots and lateral roots. Nematode-caused root problems grow progressively worse through the season. Water absorption is significantly decreased and translocation of nutrients is inefficient.

If you suspect that damage may be due to nematodes, Step 1 is to take nematode samples to figure out if the pest is present, and which pest is out there — root knot, reniform, or both.

“Root knot has always been the major problem in the upper Mid-South and it has been holding steady. Reniform nematode is a newcomer and it seems to keep moving north and spreading out,” Kirkpatrick said. “It is by far the greatest problem in Louisiana and Mississippi. We've found reniform as far north as the Missouri Bootheel. Wherever we grow cotton, we're going to wind up with reniform sooner or later.”

The best time to sample for nematodes is late summer or early fall, when populations are the highest, according to Kirkpatrick. “Obviously, weather is a factor. If it's dry and you can't get a soil probe in the ground, you're not going to do a good job sampling. So when you have moisture out there, get out there and get it done.

“Also, if you wait until the soil temperature drops, and you've been two to three months without live cotton roots in the soil, you won't have a good indication of what was there because the detectable population has declined.”

Site-specific control of root knot nematode shows good potential in the Mid-South because the pest is hardly ever uniformly distributed in a field, rather it resides in hot spots. Yield in these hot spots can be reduced by 25 percent to 30 percent, while 100 feet away, there are no yield-reducing populations.

Reniform nematodes are usually more uniformly distributed throughout a field, even though there will be some areas were populations are extremely high and other areas where they are not.

Kirkpatrick pointed to research in Louisiana which has demonstrated that in certain soil types, no matter how high the reniform nematode population is, there is no response by the crop to putting out soil fumigation as a control measure. “That's telling us that there are certain soil types where reniform nematodes are not going to be an economic problem whether the populations are high or low.”

But whether the correlation is root knot nematodes and hot spots or reniform and soil type, “the information begs to be harnessed and modeled as a site specific system, where a farmer can focus his expenditure for nematicide just in the zones in a field where the nematodes are the yield limiting factor.

“Our science needs to catch up. We can deliver any amount of nematicide that we want to any point in the field. But we don't quite know what to hang our recommendation on. Is it soil type? Is it population density, last year's yield? There are things we still need to work out.”

Deciding where to pull samples is an area where more research is needed as well, according to Kirkpatrick. “One thing we're looking at is electrical conductivity mapping carts to draw a soil EC map of each field. This gives you an indication of where natural changes in soil type occur in the field. We're looking at using those maps as a guide for deciding where to take samples.

“If that level of sophistication is not available, and you know where the differences in soil type are, sample them separately. If not, break the field into the smallest increments that you can practically sample.”

Nematodes can be found very deep in the soil profile, “but the largest populations will be concentrated where the cotton roots were. So make sure you're into the root zone.”

After you identify the type of nematode in a field, “match your crop to the nematode you're dealing with,” Kirkpatrick said. “If you have a root knot problem in your cotton and you grow corn, you're going to have a worse root knot problem when you come back into cotton.

“If you have a reniform problem in your cotton, you help tremendously by going to corn for a year. There are some reniform- and root knot-resistant varieties that will help. There is not much land that lends itself well to a cotton/rice rotation, but it would be a heck of a rotation if you could do it.

“If it's economically attractive to rotate, that's my recommendation. Get on a program where you can keep the nematodes at a low enough level where they're not going to be a factor in the years when you're going to grow cotton.

Unfortunately, there is a lot more land that has a bad problem and needs to be in cotton than is going to be rotated in any given year. That is going to have to be dealt with with a nematicide.”

Kirkpatrick noted that while root knot nematode is still far and away the more widespread pest, reniform nematodes are increasing in Arkansas fields. “In the late 1980s, a few Arkansas fields in Monroe, Jefferson and Lonoke counties had reniform nematode populations. Last year, based on thousands of nematode samples taken, there are now 11 counties in the state with reniform and they run from Mississippi County to Chicot County. They are on the move.”

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