Emerging Disease Issues in Cotton

Paul Price, Emfinger, Karla D., Purvis, Myra, Ezell, Dustin, Towles, Tyler, Walker, Wade

Trey Price, Tyler Towles, Karla Emfinger, Myra Purvis, Dustin Ezell and Wade Walker

Monitoring Continues for Cotton Leafroll Dwarf Virus

The cotton leafroll dwarf virus causes yellowing and distortion of leaves (Figure 1) and is believed to be vectored by the cotton aphid (Aphis gossypii) during feeding (Figure 2). A litany of other symptoms has been attributed to the virus including missing fruit and bolls, over-tall plants, reddening of veins, dwarfed plants and leaf cupping. In some situations in the southeast United States, significant yield losses have been attributed to the virus.

After the much-publicized discovery of the virus in Alabama in 2017, the cotton leafroll dwarf virus has been reported in many other cotton-producing states, including Louisiana. For the past several seasons, and in collaboration with other cotton scientists, sentinel plots, funded by Cotton Incorporated, have been established at the Macon Ridge Research Station in Winnsboro to monitor for the virus. Plots consist of multiple varieties with a range of susceptibilities, according to studies from Auburn University and the University of Georgia. Half the plots are treated on an insecticide spray schedule to exclude as many aphids as possible, while the other half remain nontreated. Periodically throughout the season, plots are rated for symptoms, plants are sampled and leaf tissue is tested for presence of the virus using molecular methods. Symptomatic and asymptomatic plants are tracked to compare yields at the end of the season.

Aphid populations have been markedly lower in sprayed plots versus nonsprayed. However, weekly applications of insecticides have not completely prevented aphid infestation. Virus incidence has been low (0-5%) in the sentinel plots, and there does not appear to be any correlation with symptom expression. No yield differences have been observed between symptomatic and asymptomatic cotton plants.

So far, there have been few suspected and confirmed samples of the cotton leafroll dwarf virus received from Louisiana stakeholders. Based on LSU AgCenter research and research from other states as well as observations throughout the seasons, the cotton leafroll dwarf virus seems to be more of a threat in Georgia and Alabama. Possible explanations for this include differing environmental conditions and production practices (planting date, seed treatments, varieties, rotation options, etc.) in the region. No yield losses have been attributed to the virus in Louisiana to date. However, we will continue to monitor cotton fields in Louisiana and deploy sentinel plots annually as resources allow.

Prevalence of Target Spot Increases

Although target spot of cotton was discovered in the Midsouth during the 1960s and has been a sporadic problem since, the disease has increased in prevalence in Louisiana over the past 10 years. This foliar fungal disease is caused by Corynespora cassiicola. Symptoms begin as water-soaked green lesions (Figure 3) at canopy closure, progressing from the lower to upper portions of plants. As the disease progresses, lesions take on a target-like appearance, and significant premature defoliation may occur during periods of frequent rainfall. A toxin produced by the fungus has been implicated in contributing to defoliation. The fungus survives as a saprophyte (decomposer) on crop debris from season to season.

The same fungus also causes target spot in soybeans. However, soybean isolates are less virulent on cotton and genetically distinct, while cotton isolates are less virulent on soybean and genetically distinct.

Cotton varieties vary in susceptibility to target spot. Although there are no truly resistant varieties, some varieties are considered tolerant. Crop rotation or tillage may reduce target spot incidence by reducing debris harboring the pathogen. Because rank (tall, spindly) cotton is more likely to have severe target spot, canopy management through plant growth regulators and optimum nitrogen fertilization are key to reducing defoliation from target spot.

Based on many years of research, fungicides will reduce defoliation due to target spot, but significant yield preservation usually is not realized unless disease pressure is extremely high. Thus, the odds of a return on fungicide investment are exceedingly low. A good rule of thumb: if target spot is not observed until August, the disease will likely function as a free defoliant. On the other hand, the closer to Fourth of July that the disease initiates, the higher the odds of a return on fungicide investment.

Trey Price is an associate professor, Tyler Towles is an assistant professor and Myra Purvis, Dustin Ezell and Wade Walker are research associates at the Macon Ridge Research Station, Winnsboro, Louisiana.

(This article appears in the winter 2022 issue of Louisiana Agriculture.)

yellowing cotton leaves

Figure 1. Yellowing and distortion of cotton leaves caused by cotton leafroll dwarf virus. Photo by Trey Price

aphids on the underside of a cotton leaf

Figure 2. Cotton aphids (Aphis gossypii). Photo by Tyler Towles

dark lesions on a cotton leaf

Figure 3. Young and maturing lesions of target spot (Corynespora cassiicola) on cotton. Photo by Trey Price

3/15/2022 10:33:12 PM
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