• By Anders Huseth •
In late August 2019, cotton leafroll dwarf virus awas confirmed in a cotton sample submitted to Auburn University’s CLRDV working group. The sample was collected from a variety trial located at the Sandhills Research Station in Montgomery County, North Carolina. This is the first detection of this insect-transmitted plant virus in NC.
Earlier in the growing season, we visited several field locations with reports of virus-like symptoms in cotton in several major cotton-producing counties (1A & B). As of December 2019, CLRDV has not been confirmed in these plant samples. However, follow-up disease diagnostics are ongoing.
Aphids are the only vector of this disease to cotton and the most common species is cotton aphid. Because many species of aphids can transmit CLRDV, the presence of large aphid populations in the field may not be an accurate measure of CLRDV risk.
CLRDV has a very short period of disease transmission from the aphid to the plant (seconds to minutes). As a result, insecticides are not effective tools to reduce disease transmission. Research conducted in Alabama and Georgia has shown that weekly sprays did not reduce CLRDV levels when compared to unsprayed plots.1 This means that insecticide sprays will have no impact to reduce disease incidence.
In 2020, we should be mindful that the benefits of intensive insecticide programs targeting aphids to limit CLRDV spread will not reduce end of season incidence of the disease.
What is CLRDV and how does it affect cotton?
Cotton leafroll dwarf virus is an aphid-transmitted cotton disease that was first documented throughout the southeastern region of the U.S. Cotton Belt in 2018 (Alabama, Georgia and Mississippi). Currently, very little is known about the yield or quality impacts of CLRDV in our region.
CLRDV can have many different symptoms including compact internodes near the terminal, leaf discoloration, tissue reddening, leaf puckering, reduced boll set, and late-season vegetative regrowth. Unfortunately, these symptoms can be variable and the intensity of symptom expression is likely driven by a combination of varietal characteristics and environmental stress.
Pictures and general information about the disease can be found here: New Virus Disease In Alabama Cotton
An in-depth collaborative review of current CLRDV information can be found here: Cotton Leafroll Dwarf Virus Research
How should growers respond to this disease?
Responding to insect-transmitted disease is a significant challenge in many different cropping systems. In cotton, we recommend that growers limit the use of insecticides targeting aphids. Although insecticide sprays may kill aphids, these applications are very hard to accurately time with aphid activity and will not reduce disease spread.
Moreover, aphid sprays may also disrupt natural enemies and flare spider mite populations. Finally, note that many North Carolina cotton aphid populations are resistant to neonicotinoids.
At this time, avoiding negative impacts of additional aphid sprays will be more important than trying to manage a disease/vector with an unknown distribution in North Carolina. We recommend that growers continue to scout and manage aphids only when populations reach the economic threshold.
Aphid scouting information can be found in the Insect Scouting Guide. Scroll down to the section titled “cotton aphids” for the threshold and aphid rating scale.
Dr. Anders Huseth is an assistant professor and Extension specialist, field crops and sweet potatoes, Entomology & Plant Pathology, North Carolina State University. He may be reached at firstname.lastname@example.org. Contributing authors were Drs. Dominic Reisig, Guy Collins and Lindsey Thiessen, also with NCSU.