• By Tom Allen, Tessie Wilkerson, Nina Aboughanem, Jeff Gore, Don Cook, Angus Catchot, Whitney Crow, Darrin Dodds and Sead Sabanadzovic •
During 2017, a plant disease of cotton caused by a virus, cotton leafroll dwarf virus or for the sake of brevity — CLRDV, was observed and identified in several cotton fields not far from the Gulf of Mexico in Alabama. It’s also been referred to as Cotton blue disease.
The identification of CLRDV marked the first time the virus had been observed in cotton in North America. Since the initial observation and during the 2018 season, additional counties were determined to be infected with CLRDV throughout Alabama.
In addition, CLRDV was also detected in infected cotton plant material from Georgia and Mississippi; however, these observations occurred late in the 2018 season (October to November in the case of Mississippi) and were not considered situations where yield losses occurred. The Mississippi fields determined to be infected had in some cases been harvested and the virus was detected in regrown cotton.
However, most field situations were immediately before cotton was harvested and following defoliation where cotton leaves had regrown due to excessive moisture that delayed picking. The initial information regarding the detection of the virus was published in several Disease Notes and articles online. For the sake of this blog post, the initial information can be obtained from Alabama Cooperative Extension Service. For more information on the disease itself, read New South Wales, Australia, Exotic Pest Alert: Cotton Blue Disease.
Symptoms Associated With CLRDV
Typically, plant viruses induce outward symptoms that may appear in the form of plant stunting, misshapen leaves, or even a coloration difference in the younger leaves that oftentimes can be referred to as color breaking, chlorosis or mottling. The specific symptoms associated with CLRDV infections are currently difficult to detect visually, especially since some of the symptoms associated with this particular virus are not considered “classic” virus-like symptoms.
In addition, symptoms currently associated with the virus in young cotton plants (pre-bloom) have not been well characterized. Moreover, some of the symptoms associated with CLRDV can be misidentified as thrips injury (leaf cupping or even node stacking), severe aphid pressure (leaves cupping downwards) or herbicide injury (for example, node stacking as a result of sulfonyurea application/uptake).
Most importantly, be mindful that in fields of dicamba-tolerant cotton, there could be off-types that do not carry the herbicide trait that may produce symptoms that could be mistaken for virus infection.
More specifically, symptoms appear to differ between young cotton (pre-bloom) and cotton observed at the end of the season.
In young plants, symptoms may appear as stacked nodes (shortened internode distances), miniature leaves throughout the canopy, shortened plants (dwarfed plants), and additional dwarfed plant parts that include bracts and leaves, and leaves in the upper most canopy that appear crinkled.
In addition, one of the more specific symptoms that is difficult to ignore is a symptom that would be more often described as nutritional deficiency. Leaves with yellow leaf margins (see photos) appear to be one of the more classic symptoms associated with CLRDV.
However, leaves expressing red veins as well as a “bronze” or mild red coloration have also been described from other states in our region and did in fact test positive from samples collected in Coahoma County last week. In general, once plants are infected, new growth on the top of the plant may not produce bolls and the distance between nodes may appear shorter than on other parts of the plant (node stacking).
In some cases, leaves in the upper most canopy can appear thicker than normal and slightly distorted. As with almost all plant viruses, new, or young growth is most commonly affected.
Plant viruses are generally transmitted to their plant host via an insect vector. In the case of CLRDV, the virus is transmitted to cotton plants by cotton aphids. However, not all aphids likely carry the virus. At this time, it is not believed that more aggressive aphid management will prevent the disease.
Aphids should be managed according to threshold independent of the virus. Be mindful that even though cotton aphids transmit the virus, this does not mean that all fields that had an aphid infestation are infected with CLRDV.
CLRDV Impact On Yield
Currently no research-based information regarding the impact of CLRDV on yield exists in the United States. However, yield losses in some CLRDV-affected cotton fields in south Alabama in 2018 were significant.
With that in mind, the cotton field(s) in question were late-planted (June) and likely had high levels of CLRDV infection (virtually 100%) as a result of high aphid populations.
However, that was not the norm in Alabama and was limited to an extremely small geography east of Mobile Bay. Normally, viruses are not observed in such a widespread nature. Currently field trials are being conducted across the southern United States to determine the potential yield losses as a result of CLRDV to better understand the potential threat to cotton farmers.
Samples exhibiting CLRDV collected from Coahoma County last week were from a relatively small area within a field that was planted specifically for insecticide trials. A small percentage (27%; 11 out of 41 samples) of the samples collected tested positive for CLRDV suggesting that only a small percentage of any field exhibiting infection could contain CLRDV, likely much less than 1% at the current time (as of 7/24/2019).
With that in mind, management practices for any field of cotton should not be altered at this time, since there is little information regarding the impact of CLRDV on cotton yield.
Should you have any specific questions regarding cotton that may be presenting symptoms as described above, please contact us. However, processing samples requires a good deal of time (two-plus days) and involves a series of molecular technology procedures to determine the presence of CLRDV in infected plant material.
At the moment, high sample loads will be difficult to process. But by making that statement, we are not discouraging people from submitting samples.
Development of simple and reliable diagnostic test for large-scale CLRDV detection is a priority of the current research at MSU and several peer institutions in the region.
Should you have any specific questions regarding sample collection and how to ship samples, please let us know.
Dr. Tom Allen is Extension plant pathologist; Dr. Angus Catchot, Extension entomologist; Dr. Darrin Dodds, Extension cotton specialist; Dr. Don Cook, research entomologist; Dr. Jeff Gore, research and Extension entomologist; Dr. Whitney Crow, Extension entomologist; and Dr. Tessie Wilkerson, assistant research professor. All are with Mississippi State University. Allen may be reached at firstname.lastname@example.org.