Cotton Leaf Roll Dwarf Virus (CLRDV-AL), which causes cotton blue disease, is now considered part of the environment, according to Alabama Extension specialists and Auburn University researchers. The same disease that was first recognized in 2017, damaged fields near the Gulf Coast in late 2018 and was widely detected throughout the state in 2019, will likely impact cotton crops in 2020.
Most researchers agree: the symptoms are sporadic and impact on yield is not consistent. However, there are many things researchers now know about CLRDV since its recognition in Alabama cotton in 2017.
The aphid-vectored disease was first observed in cotton planted in southern Alabama during the 2017 growing season. The disease has since been detected across the United States cotton belt. The virus now appears to be present at some level in most cotton fields in the state, according to Austin Hagan, an Alabama Cooperative Extension System plant pathologist.
Between the 2018 and 2019 growing seasons, researchers detected the virus in multiple winter weeds. The abundance of host plants, like henbit and white clover, allowed the disease to overwinter and have a dominant presence at the offset of the growing season.
Kassie Conner, also an Alabama Extension plant pathologist, took on the task of confirming the virus in samples sent to Auburn from many other cotton-growing states.
Thus far, professionals have identified the virus in the following states.
- North Carolina
- South Carolina
“It can be a difficult disease to see and diagnose visually,” Conner said. “In some cases you could sample every plant in the field and all could be infected—but not showing obvious symptoms.”
In addition to the symptoms that may be difficult to pinpoint, most researchers agree there are many unknown factors contributing to the spread of CLRDV-AL.
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Aphids as vectors present many questions for growers, as well as researchers. Researchers are still actively working to determine whether the cotton aphids are the sole vector, or if other species can spread the virus.
Connor said in an indicator plant study in Brewton, Alabama, they have observed plants showing symptoms shortly after they come out of the ground. She says this is the primary spread of the virus from the weed hosts to cotton.
“The secondary spread of the virus occurs throughout the season as aphids move from infected plants to healthy plants in a cotton field,” Connor said. “The timing of infection explains the variation in symptoms. The earlier in the season plants become infected, the more severe the symptoms will be.”
Alabama Extension cotton agronomist, Steve Brown, said aphids are difficult to control even with intense management.
“In research plots entomologists can never completely eliminate aphids, even with very aggressive insecticide applications,” Brown said. “So, it is clear that we cannot manage the disease through attempts at aphid control.”
Brown said both Auburn University and University of Georgia researchers are diligently watching cotton for symptoms and have seen several symptoms that are likely linked to the disease.
Both Brown and Hagan agree on the assessment of symptoms.
“It is difficult to train your eyes to see symptoms,” Hagan said. “There are a variety of symptoms in different plants, with cotton cultivar and fertility status influencing symptom expression and severity.”
At this point, Hagan said he has observed a reduction in the number of bolls produced, fruiting nodes and open bolls. However, CLRDV-AL affects cotton plants in different ways at varying stages of crop growth.
The potential for yield loss is high. The earlier the symptoms show in a plant, the higher the yield impact will be.
Brown said the impact on cotton is erratic.
“We don’t understand why one field has significant yield loss, while others with CLRDV symptoms may be minimally affected—or not affected at all—in terms of loss,” Brown said.
Unfortunately at this point, there is no magic solution to the Cotton Leaf Roll Dwarf Virus. However, Auburn University and Alabama Extension professionals are looking at immediate measures—such as crop management practices—as a means of reducing the impact of the disease.
Auburn University’s long-term goal is to breed resistant cotton varieties. In Africa, Brazil and Argentina, where similar viral diseases have attacked cotton, the best and longest-lasting remedies have come through varietal resistance.
“Though other countries control the disease with excessive insecticide use, we have shown that this option cannot eliminate aphids and the virus entirely,” Brown said.
In addition to resistant varieties, researchers are looking closely at current agricultural practices to determine ways to minimize risks.
- Plant early. Researchers have observed reduced virus effects in cotton planted during the first week of May. Late plantings tend to be more at risk for significant yield effects.
- Stalk destruction. Cutting stalks to the ground helps eliminate some of the overwintering habitat for aphids and virus, while cutting stalks to the ground kills the stalk and prevents growth where aphids could also visit and deposit the virus.
- Winter weed control. Take extra care to control and remove winter weeds. Henbit, white clover and chickweed are likely winter hosts. There are likely many more alternate hosts for researchers to identify.
- Cover crops. Researchers are working on cover crop options as a means of virus control. Research is still underway to determine whether certain cover crops sustain aphids and CLRDV, or if those crops minimize the threat of the virus.
There are many researchers dedicating lots of time in order to prepare producers for the challenges that will inevitably arise in the field during the growing season.
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