Identification of Ramulariopsis pseudoglycines causing areolate mildew of cotton in Georgia and first detection of QoI resistant isolates in the United States.

Abstract

Areolate mildew, caused by two Ramulariopsis species (R. gossypii and pseudoglycines), is an important re-emergent cotton disease in the Southeastern United States. This disease has been considered of secondary importance, but the recent prevalence and high incidence observed in cotton regions have raised concern about its importance. Timely applications of quinone outside inhibitor (QoI) fungicides have been suggested to manage this disease, but QoI resistance development is frequently detected for many fungal pathogens. In 2023, Ramulariopsis spp. isolates collected from Georgia were tested for species identification and for QoI resistance using molecular assays. Sequencing results revealed that all 6 isolates tested belong to R. pseudoglycines, confirming the presence of this species for the first time in Georgia. The partial amplification of the cytB gene showed that 83.3% of isolates had the G143A mutation, and no other amino acid substitution was observed. This also represents the first report of QoI-resistant isolates showing the presence of this amino acid substitution in R. pseudoglycines in the United States.