Nematodes, foliar diseases and viruses, little more than an afterthought a decade ago, now demand attention from cotton farmers and consultants.
"The notion that we would be dealing with foliar diseases years ago and not concentrating almost exclusively on weevils and weeds was not on our minds," said Louisiana crop consultant Hank Jones during a nematode and disease update at the recent Beltwide Cotton Conferences in Austin, Texas.
But nematodes and diseases are taking a toll on cotton across the Belt and three Extension plant pathologists offered observations on limiting losses.
Variety resistance, they all agree, should be a key factor in managing for disease and nematodes. Crop rotation, production practices and chemistry round out the list of control or preventive options, according to panelists Travis Faske, University of Arkansas; Paul Price, Louisiana State University; and Kathy Lawrence, Auburn University.
Rotation offers the best option for nematode control for Arkansas cotton, Faske said. Root knot is the main nematode problem in north Arkansas and into the Missouri Bootheel. Reniform causes problems in south Arkansas, he says.
"Peanuts offer a good rotation with cotton," Faske said. "We get reports of a 200 pounds of lint improvement in cotton following peanuts."
He said peanut acreage is increasing in Arkansas, spurred somewhat by a new peanut company, Delta Peanut, in Jonesboro, Ark.
He said some nematode-resistant cotton varieties are available. "We would like to see better reniform resistance.
"Cotton growers probably have nematode populations every year; most know they have it and are looking for options."
He said some new chemistry is coming out but that some products work better in some locations than in others.
"We have a lot of root knot in sandy soils in Alabama," Lawrence said. "We have reniform in the heavier soils and are also seeing stubby root and lesion. We are looking for more resistance."
We have similar problems in Louisiana," Price said. "Our growers are not sampling enough. We have a lab and growers need to develop a baseline population in their fields, so they know where to start."
He said resistant varieties may offer help. "But do those varieties yield?"
Few chemical options are available, and Price says few producers want to use chemical boxes. "Seed treatments may be the most convenient option, although they may not be the most efficacious if you compare that to in-furrow options."
"In Louisiana, we have been managing nematodes with rotation," Jones said. "We've been adding corn acreage as cotton acres dwindled."
He warned that cotton behind sweet potatoes may be "a trap for farmers to bring in more nematodes."
He said a new nematode, Guava, has been identified on one field in one parish in Louisiana. "It has been quarantined."
Faske described the Guava nematode as "a root knot on steroids." Guava gets that reputation from its ability to overcome resistance and the number of hosts plants.
Panelists turned to target spot, a disease that has shown up intermittently across the Mid-South and into the Southeast for several years.
"We saw target spot for the first time in 2013," Faske said. "The following years it was worse. Variety selection was a key."
The disease has not been as bad in recent years. "It's still out there, just not as problematic. Using Pix and selecting resistant varieties help."
He said they sprayed a fungicide in 2016 but saw no yield benefit. "We are watching it. We want to see how Georgia and Alabama manage it. They have seen it more often. We don't want to see a year like 2016 again."
Lawrence said she saw little last year. "It was hot and dry."
"We saw a little target spot late," Price said. "It was a dry year, and target spot likes excess rainfall. This year was not a year for target spot."
He recommends that when farmers have scouts looking for plant bugs they also have them check for target spot symptoms as well. "Look at the lowest leaves in the canopy first. Also, look at your long-range weather forecast; if you're going to be showing rain over the next several weeks. you might consider putting something out."
In-season management also makes a difference. "You can't let cotton get rank," Price said. "Too high plant populations also may be troublesome."
He said PGR timing is a factor, as well as number of applications. "Producers ask, 'one shot versus two shots?' I say zero shots if you don't need them, but if you're going to put one shot out, my trial showed the best time is right at canopy closure."
Fungicide applications may not be justified in all cases. "An economic benefit from fungicide application is the exception, not the rule," Price said. "We have a 40% to 50% defoliation trigger, but that depends on when symptoms show up. If it comes on early, it might be a concern."
Jones said crop agronomics might be more important and suggested too much nitrogen, too much water and too high plant populations increase potential for target spot to develop. "Revenge spraying is unwise," he added. "The leaves do not go back on the plant."
Price said bacterial blight shows up on some susceptible varieties every year. Key to limit damage includes canopy management, debris management, variety selection and screening for resistance.
"We see it every year in Arkansas," Faske said. "It's usually light but some fields have serious infections. Resistance is the key."
Currently, no effective products are labeled.
They saved the newbie, cotton leafroll dwarf virus (CLRDV), for last. "I first saw it in 2017," Lawrence said. "Some symptoms were severe. Since, symptoms have been less and we're not seeing it in the same fields. It's never in the same place twice, which makes it more complex to study."
Complexity is compounded by the vector, an aphid. "We look at the number of aphids, the number of infected aphids and the number of infected plants. We've seen a few fields with significant losses, not most."
All three pathologists say symptoms vary dramatically and may include leaves in the top of the plant crinkling and then becoming deformed. Defoliation may occur; necrotic spots may show up; boll loss is possible.
Color may be a telling characteristic but that also varies from reddening to purpling to yellowing.
Lawrence recommends sampling but cautions that samples must be handled with extra care. "This is an RNA virus and is very fragile. Get it to a lab as quickly as possible." Dry ice, she said, could help preserve the sample.
Faske said he has not seen the classic symptoms of CLRDV in Arkansas. "It's there but at low incidence. Some consultants say they have seen similar symptoms for years. So, we wonder if it is a new problem."