Monitoring the distribution, incidence, and symptom expression associated with cotton leafroll dwarf virus in the southern United States using a sentinel plot system

Cotton leafroll dwarf virus (CLRDV), transmitted by the cotton aphid (Aphis gossypii Glover), was first confirmed in upland cotton (Gossypium hirsutum L.) in Alabama, United States, in 2017. Subsequent observations of symptomatic plants followed by confirmation via reverse transcriptase PCR (RT-PCR) were made in neighboring states in 2018. To assess the distribution and incidence of CLRDV, and incidence of presumed symptoms across the southern cotton belt, a multidisciplinary team established sentinel plot survey sites at 16 experiment stations in 11 states stretching from Texas to Virginia and Tennessee to Florida beginning in 2019. Field trials were conducted over a three-year period using multiple cotton cultivars that were adjusted annually. Cotton plots were evaluated at each location by a single evaluator to attempt to correlate symptom severity across the cotton growing region with virus incidence in cotton plant tissues using RTPCR. Symptom incidence, based on visual estimation of plants in each plot with presumed symptoms, differed across the region and ranged from 0% to 75% with a low average from all locations of 11.4% in 2021 to an average high of 28.0% in 2020. Though symptom severity suggested the presence of CLRDV, there were instances where symptoms were observed but virus presence was not confirmed by PCR. CLRDV has since been confirmed from all locations, which suggests that it has become endemic in cotton production areas throughout the eastern range of the United States.