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Host range and phylogenetic diversity of Corynespora cassiicola, cause of target spot of cotton in the southeastern USA. 
L. SUMABAT (1), R. Kemerait (2), M. T. Brewer (1). (1) University of Georgia, Athens, GA, U.S.A.; (2) University of Georgia, Tifton, GA, U.S.A.

<i>Corynespora cassiicola</i> is a ubiquitous pathogen causing emerging plant diseases worldwide including target spot of cotton, which has significantly increased in incidence and severity throughout the southeastern USA. Target spot has also increased on tomato and soybean within the same region. Our aim is to understand the emergence of target spot by comparing phylogenetic relationships of isolates from cotton with other hosts and determining their host range. Thirty-nine isolates were collected from different hosts in the southeastern USA and a total of 1709 nucleotides were sequenced. Across the four gene regions, <i>C. cassiicola</i> from the southeastern USA clustered based on host of origin, regardless of geographic location. Results revealed no genetic diversity among isolates from cotton in the southeastern USA and showed them to be genetically distinct from isolates from different hosts of origin. Thirty-two isolates from different hosts were tested for pathogenicity on cotton, soybean, tomato, and cucumber cultivars revealing significant differences. Isolates originally from cotton were more aggressive on cotton than isolates from other hosts. Soybean, tomato, and cucumber were only susceptible to isolates that originated from the same host, indicating host specialization among these isolates. These results suggest that emerging epidemics in the USA are caused by introduction of host specialized isolates or the evolution of more aggressive strains on each host.

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