Nematodes have been a problem in Delta cotton for years. In their ability to cause damage, the microscopic pests run the spectrum from subtle plant changes to highly visible, extensive ravaging of a crop.
“Often, root-knot nematodes will interact with fusarium wilt. When the two get together, damage can be very severe. Thankfully, the varieties now available don't seem to be as susceptible to fusarium wilt as those in the past,” says Charles Overstreet, a nematologist with the LSU AgCenter.
Herbicides can also interact with nematodes. To illustrate, Overstreet shows a slide of a cotton crop “missing” plants.
“In this field, nematodes held the plants back. When it was time to go in with a lay-by herbicide, the small, stunted plants were taken out by the spraying. That led to major gaps in the field,” says Overstreet, who spoke at the Louisiana Cotton Forum in Monroe on Jan. 14.
There are two major nematodes in Louisiana: root-knot and reniform. Columbian lance nematode is a newcomer to the state and growers have yet to see it cause serious trouble. Thus far, Overstreet has found the Columbian lance nematode in about 15 fields in the state. The Columbian lance nematode is causing problems in some Southeastern states — South Carolina particularly.
“By far, reniform is the most important to producers,” says Overstreet. “It's found in much more of Louisiana than root-knot. And reniform nematodes aren't just a U.S. pest. Reniform is scattered throughout a vast portion of the world where cotton is grown.”
There are a number of management options in dealing with nematodes. Some of them are:
Tolerance and resistance
Unfortunately, there isn't a lot to offer under this category. Overstreet says two varieties — Paymaster 1560BGRR and Stoneville 5599BGRR — have a moderate level of resistance to root-knot nematodes.
“They work better than other varieties, but not nearly as well as we need.”
Against reniform nematodes, producers are out of luck. There are no resistant varieties.
“I don't see anything on the immediate horizon — say five years out — to deal with reniform in a strong way. There may be some moderate resistance coming, but nothing earth-shaking.”
In using the terms “tolerance” and “resistance,” Overstreet emphasizes the difference between the two.
“Tolerance is the reaction by the plant to a nematode attack. Resistance refers to how the nematode is affected when it attacks the plant.”
Producers have some materials (like Telone) which can be applied in the fall or spring prior to planting. The rate Overstreet and colleagues been looking at is about 3 gallons per acre.
“We've also been looking at the standard nematicide at planting — Temik. The rates we've studied have been between 3.5 and 6 pounds per acre. That's been standard for a number of years.”
As far as post-plant applications, Overstreet has studied applications of Temik sidedressed at 5 to 7 pounds about a month to six weeks after planting.
Also being looked at is 17 ounces of Vydate at pinhead square. Overstreet always applies Vydate after the use of a preplant application of Temik.
“You don't want to rely on Vydate alone. It gives a bit of suppression, but you need the up-front nematicide protection.
“Over the last 10 years or so, we've increasingly had to deal with the fact that these nematicides don't always work as well as we'd like. This is a field (shows slide of straggly cotton plants) with a standard rate of 3.5 pounds per acre and a very large nematode population. The application doesn't have a huge impact in such a field. The slide shows we're still getting high rates of injury from nematodes. When I did sampling, there were about 35,000 nematodes per pint of soil.”
Overstreet has used Telone on fields with very high nematode numbers. Sometimes he gets some “really nice responses” with the use of Telone. In one test field some rows were sprayed with Telone and others with Vydate. In later soil samples, the nematode population in the Temik side was 160,000 per pint of soil. In the Telone plot, the population was about 80,000.
Telone tends to work well in years when weather patterns are typical for the South. In the bad years — like the last couple in Louisiana with major rain events — Telone doesn't do nearly as well.
“I suspect the problem isn't the material acting on nematodes as it is adverse weather conditions affecting the material negatively.
“In data over 14 trials, it's obvious we can get a mid-season reduction of nematodes through use of Telone. The problem is by harvest the populations have jumped back up. There's no lasting effect from Telone.”
Does Temik really work? Over some 35 trials, the data shows that at the standard rate — 3.5 pounds per acre — Temik does work.
“We have a nice yield response from the product. It isn't that Temik isn't working, it just doesn't work as well in fields with really high nematode populations.”
“We've done some work with application of anhydrous ammonia. There wasn't a whole lot of difference seen in using it.” As anhydrous is toxic to nematodes, Overstreet was hoping to see a large impact from its use. That hasn't been the case. Telone had a much greater impact on yields than anhydrous.
There has been other work done in the South on anhydrous and nematodes, he says. It all seems to agree: anhydrous does a poor job of dealing with nematodes.
“Any improvement is very slight. There isn't any problem in using it, just not as active as we'd have liked.”
Crop rotation is also a way to combat nematodes. Research out of Mississippi shows that when a cotton/corn rotation was employed, a nice cotton yield bump occurred. Treated cotton gave a similar yield to that experienced after one year of corn. And where the field was in corn previously and was also treated, the yield jump was greatest.
Corn is a nice rotation crop with cotton. Corn knocks nematode populations down significantly. Rotations do help — particularly against reniform.
“What I recommend for Louisiana is to try and go with corn for more than one year. If you can only rotate corn for one year that's better than nothing. But if you want the full benefit of the rotation, two years is best.
“Against reniform nematodes, grain sorghum does about as good a job as corn. It seems to be better than corn against root-knot nematodes. In work I'm doing in northeast Louisiana, grain sorghum is doing extremely well in keeping nematode populations down.”
It would be extremely beneficial to locate areas of the field where nematodes are at their worst and treat those areas. In dealing with nematodes, variable rate application would be very welcome, says Overstreet.
“If you only had to treat areas of a field that are worst, that may mean only mean 50 percent of the field. Say you're applying Telone at 3 gallons on only 25 percent of a 48-acre field. That would cost $8.75. Treat the whole field and it'll cost you $33.”