Reniform nematodes are now lurking beneath the soils of four contiguous west Tennessee counties. University of Tennessee plant pathologists are trying to keep them from spreading further, with help from growers.
The microscopic worms attack cotton roots and can cause yield reductions of 10 percent to 75 percent. The reniform nematode is a tropical pest first discovered in Georgia and Louisiana in the early 1940s. Since that time, it's spread to all Southern states, including west Tennessee in 1992.
Damage is caused when the pest sticks its stylet into a root to suck out nutrients. The worm also uses its digestive system to inject toxins and diseases into the plant, causing mayhem in the metabolic processes of the cotton plant.
“For a while we were only concerned with reniform nematodes in Crockett County,” said Tom Stebbins, Extension assistant, University of Tennessee entomology and plant pathology. Stebbins spoke on reniform nematodes at the Milan No-Till Field Day recently.
“But we are now finding it in Gibson, Madison and Dyer counties, and we've detected it in over 100 fields. So far, it's confined to that area.”
No control measures will completely eradicate the pest, according to Stebbins. “You'll never get rid of it once you get it, so the thing to do is never get it. If you do get it, it's a numbers game.”
Populations of 10,000 nematodes per pint of soil may not cause a lot of problems, according to Stebbins, but populations of 100,000 per pint of soil could be very damaging.
Rotation to reniform-resistant crops like corn or grain sorghum for one or two years will help. “But this is not going to be the total answer,” according to UT plant pathologist Melvin Newman.
“It will just reduce the numbers enough to make one cotton crop. Then, you'll build the numbers back up and have to go back to corn again. Populations rebuild quickly.”
Newman noted that one female reniform nematode can produce 70 eggs in a 25-day cycle. Over four generations, the one female will have produced 24 million offspring.
Chemical controls of reniform nematodes include Temik and Telone. But sometimes the use of these chemicals might produce more nematodes by the end of the season because they produce a healthier root system, which creates a larger food source for the pests.
The fumigant Telone is used in Mississippi and Louisiana, “but I don't think we're going to get a chance to use it in the no-till areas of west Tennessee,” Newman said. “Telone has to be injected 8 to 10 inches deep into a bed. Then you have to wait two weeks before you can plant.”
Temik 15G has been tested across west Tennessee with fairly good results this year, according to Newman. “But you have to use a higher rate than the 3.5 pounds that you use for aphids and thrips. You have to go to at least 5 to 7 pounds in-furrow and then you have to sidedress with a another 5 to 7 pounds of Temik at pinhead or two applications of Vydate at 8 ounces each a couple of weeks apart on the foliage.”
If Temik is used, Newman suggests injecting the material 3 inches to 4 inches deep on either side of the plant “so a rain will dissolve it and push it down where the roots are.”
These approaches “have done pretty well across the Southern region where we've had high populations of nematodes,” Newman said. “On the other hand, 10 pounds of Temik required for the treatment costs $35 an acre, “and you're still not expecting a top yield. You still have the nematode problem.”
And you may not always get good results. In large west Tennessee test fields infested with at least 10,000 nematodes per pint of soil, 5 pounds of Temik increased yields from 350 pounds in the check to 399 pounds. The fields had a two-bale potential, according to Newman.
“Where we placed an additional 5 pounds of Temik side-dressed, we increased yields to 435 pounds per acre.
“This tells me two things. It's going to take a lot of chemical to address this problem, and we may still not get the yields. I'm hoping we can get a research project that shows us we can do a better job than what we did in the studies.”
Symptoms of damage from reniform nematode may include plant stunting, but there could be other causes for this symptom, including weather and disease. For that reason, Newman and Stebbins urged growers to begin sampling fields this fall. If reniform nematodes are present, implement practices to control and stop the spread of the pest.
“We don't have too many fields with the pest right now,” Newman said. “But Alabama, Mississippi and Louisiana are covered up. Fifty to 75 percent of their acres are infested, and it's causing some tremendous headaches. We need to prevent that from happening here.
“Each grower needs to sample his cotton land,” Newman said. “Find out what you have. A lot of farmers drive equipment from one field to another and could be dragging nematodes from one field to another — even on a planter and even if it's no-till. Any little bit of soil can carry some eggs. And that's all it takes.”
“Sample anytime,” says Newman, “and they'll show up if you have them. But the best time is just after you pick cotton. Cut the stalks, then get on the four-wheeler and start pulling soil samples.”
Newman suggests breaking fields into 25-acre blocks. “If you have a 100-acre field, break it into four blocks. Treat each block as a separate sample and take 20 sub-samples from each block. Dig down where the roots are, get a handful of soil and put it in a bucket. Mix that up real well, then take about a quart of soil from the bucket and put it in a Zip Lock plastic bag. Put your name on the bag and get it over to your county Extension office.”
Despite the damage reniform nematodes can cause, “they're very fragile,” Newman says. “You can kill them very quickly if they're in a plastic bag sitting in the back of a truck. Keep them cool, don't dry them out and get them to us quickly.”
You can refrigerate samples, according to Newman, “but don't freeze them. Treat it like a quart of milk.”
If you have a field that's infested, implement practices that will reduce numbers, such as rotating to a resistant crop. Then, make sure you don't spread the pest to other fields.
“If an infested field has to go in cotton, I would plant that field last,” Newman said. “Do the clean fields first. If I did go from a dirty field to a clean field, I'd find some way to wash equipment down, get every particle of dirt off. Take the time. It's worth it.”
Studies by Newman and other plant pathologists in the Mid-South give some indication of why reniform nematodes are virtually impossible to eradicate. “Significant populations of reniform nematodes can survive at 36 inches to 48 inches deep in the soil,” Newman said. “Some of the materials we talk about for control only go down a foot at best.”