Host Range and Phylogenetic Diversity of Corynespora Cassiicola, Cause of Target Spot of Cotton in the Southeastern U.S

Corynespora cassiicola is a ubiquitous fungus causing emerging diseases on crops worldwide including target spot of cotton, which has been rapidly increasing in incidence and severity throughout the southeastern U.S. since 2005. This increase in target spot has also been evident in tomato and soybean within the same region. Our aim is to understand the emergence of target spot by comparing the phylogenetic relationships of isolates from cotton with other hosts and determining the host range of these isolates. Isolates were collected from cotton, soybean, tomato, pepper, cucumber, Hydrangea, and Mandevilla. A total of 1709 nucleotides from four gene regions among 39 isolates were sequenced to determine genetic relationships. Across the four gene regions, C. cassiicola from the southeastern U.S. clustered based on host of origin, regardless of geographic location. No genetic diversity among isolates from cotton in the southeastern U.S. was found and showed them to be genetically distinct from isolates of different hosts of origin. Thirty-two isolates from seven host species were tested for pathogenicity on cotton, soybean, tomato, and cucumber cultivars. Greenhouse experiments revealed significant differences among isolates. Isolates originally isolated from cotton were more aggressive on cotton than isolates from other hosts. Soybean, tomato, and cucumber isolates were only aggressive toward the same host from which they originated, indicating evidence of host specialization among these isolates. These results suggest that emerging epidemics of target spot are caused by the introduction of host-specialized isolates or the evolution of more aggressive strains.