Dr. Heather Kelly, associate professor and plant pathologist, University of Tennessee, announced to attendees at the recent UT Cotton Tour Field Day that a cotton disease transmitted by aphids, which is also in Brazil, has now been officially confirmed for the first time in a Tennessee cotton field.
Officially called the cotton leafroll dwarf virus, most people are referring to it as cotton blue disease. “In 2017, researchers at Auburn University noticed something was wrong with their cotton and suspected the virus,” says Kelly. “They confirmed it was leafroll dwarf virus last year.”
The virus has since been confirmed in Mississippi, Florida, Georgia and North Carolina. Unfortunately, there are many more questions than answers related to the disease.
Brazilian growers switched to resistant varieties attempting to control the virus, but the A-typical strain overcame the resistant varieties quickly. “Auburn researchers tested a high number of suspect cotton plants and had a lot of positives in 2018,” says Kelly. “In some fields, they reported up to 100 percent yield loss.”
Symptoms and management
Kelly is finding that offering advice about the disease is difficult because the symptoms are similar to other diseases, disorders, and pest damage. Some damage from an application of Sequence and heavy thrips injury were observed in the suspected field at the West Tennessee AgResearch and Education Center.
“It had thrips and possibly some herbicide damage, but there was something about it that just didn’t look right,” says Kelly. “Some of the leaves were crumpled up and curled under more than what Drs. Larry Steckel or Scott Stewart could identify as herbicide injury or thrips.”
Plant samples were sent for testing around the middle of June and 10 days later, results were positive for cotton blue disease. More samples from the research station were sent for testing but only one of the 15 sent came back as positive, and it was difficult to find any plants that could considered as having symptoms. “These early positive fields seemed to grow out of the virus symptoms and all are looking to have good yields. We had one field in Fayette County test positive,” says Kelly. “The other positives have been here in Madison County.”
Although the virus is aphid-transmitted, researchers are not advising any change to current aphid management control strategies. “They transmit the virus so quickly, a grower wouldn’t be able to affect control of the aphid population before they are able to transmit the virus,” says Kelly. “As many positives as they confirmed in Alabama last season, we thought they would have experienced significant yield loss, but in general, Alabama’s yields were good last year.”
A similar scenario occurred in Mississippi. The virus was found so late in the season, it is suspected that it did not have a chance to negatively impact yields. “We didn’t detect it in our fields here until June 2019, the earliest it had been detected in a season, and we’re looking at good yields in those fields where plants tested positive for the virus,” says Kelly. “We really don’t have a good handle on how it will impact yields moving forward.”
Kelly and the other researchers are asking all growers to monitor their cotton and contact them at the West Tennessee AgResearch and Education Center if they see something happening to their cotton they cannot identify. “The virus may give plant leaves a little off-color green with squares or nodes bunched up in the terminal,” says Stewart. “Plants that are under optimal growing conditions when there’s not a lot of stress may not exhibit symptoms, but they may start exhibiting symptoms once under stress or cooler temperatures.”