Mating type and simple sequence repeat markers indicate a clonal population of Phyllosticta citricarpa in Florida

Phyllosticta citricarpa, the citrus black spot pathogen, was first identified in Florida in March 2010. Subsequently this pathogen has become established in Florida but can be easily confused with the endemic nonpathogenic citrus endophyte P. capitalensis. In this study, the mating-type (MAT) loci of P. citricarpa and P. capitalensis were identified via draft genome sequencing and characterized at the structural and sequence levels. P. citricarpa was determined to have an idiomorphic, heterothallic MAT locus structure whereas P. capitalensis was found to have a single MAT locus consistent with a homothallic mating system. A survey of P. citricarpa isolates from Florida revealed that only the MAT1-2 idiomorph existed in the Floridian population. In contrast, isolates collected from Australia exhibited a 1:1 ratio of MAT1-1 and MAT1-2 isolates. Development and analysis of simple sequence repeat (SSR) markers revealed a single multilocus genotype (MLG) in the Floridian population (n = 70) and 11 MLGs within the Australian population (n = 24). These results indicate that isolates of P. citricarpa from Florida are likely descendent from a single clonal lineage and are reproducing asexually. The disease management focus in Florida will need to be concentrated on the production and dispersal of pycnidiospores.