A new virus disease of cotton found so far in several southeastern US states has attracted a lot of attention lately. This is the “cotton blue disease”, which is caused by the cotton leaf roll dwarf virus. Symptoms are seen in the new growth following virus infection and include stunting, blistering or crinkling of leaves (Figure 1), and downward cupping of leaves (Figure 2). Other symptoms can include reddening, shortened internodes, upward cupping of leaves and abnormal top growth. Plants may also exhibit a green-blue leaf color, for which the disease is named. The virus can cause boll drop and although yield losses have been associated with it, there are also examples of no obvious effect on yield. To further confuse recognition of this disease, some symptoms may resemble phenoxy herbicide injury.
The virus is transmitted by aphids and additional hosts, such as henbit and pigweed, have been identified in the United States. Persistence of the virus in regrowth cotton is also possible.
The virus occurs on four other continents where cotton is grown. In the United States, it was first identified in Alabama in 2017 and was subsequently found in Florida, Georgia, Mississippi, and South Carolina. This is still a relatively new disease in the United States and we are and will be continually learning new things about it as each season progresses.
Figure 1. Blistering symptom in new leaf growth associated with cotton leaf roll dwarf virus. Photo courtesy of Dr. Judith K. Brown, University of Arizona.
It is not inevitable that this disease will show up in Texas, or if it does, that it will have a significant impact on production. However, people should be on the look-out for the symptoms described here and report them to Texas A&M AgriLife county agents or specialists. Diagnosis cannot be made solely from symptoms and requires molecular techniques. Dr. Olufemi Alabi, extension plant pathologist in Weslaco, has the primers of the virus needed for sequence comparison.
Figure 2. Downward cupping symptom in new growth associated with cotton leaf roll dwarf virus. Photo courtesy of Dr. Judith K. Brown, University of Arizona.
Apparently, most US cotton varieties are susceptible, but the timing of infection and other factors will ultimately affect yield loss. Current control recommendations for the southeastern US include controlling weeds where the virus could be harbored. In general, with virus diseases, plants that are young when infected tend to sustain greater damage than older plants, so weed control could help to delay infection. At this point, there is no indication that efforts to control aphids will successfully manage the disease. Looking at the example of virus diseases of cucurbit vegetables, aphids transmit the virus immediately upon feeding, before the insecticide can kill them.
Thomas Isakeit, Professor and extension plant pathologist
Texas A&M AgriLife Extension Service, College Station
firstname.lastname@example.org; cell 979-229-4976