Genetic variation among Phyllosticta strains isolated from citrus in Florida that are pathogenic or nonpathogenic to citrus

Citrus black spot is an emerging disease in Florida since 2010. The causal agent is Phyllosticta citricarpa (teleomorph Guignardia citricarpa), but non-pathogenic P. capitalensis (teleomorph often referred to as G. mangiferae) is often isolated from black spot lesions. Florida isolates of P. citricarpa and P. capitalensis from citrus have not been characterized in detail. In this study, Phyllosticta species isolated from Florida citrus were compared with worldwide isolates using multi-locus sequencing of four conserved loci (rDNA ITS, TEF1, ACT, and GPDH genes). Moreover, the diversity within the two Phyllosticta species was compared based on the same four loci. DNA sequences of P. citricarpa and P. capitalensis were clearly distinct, coinciding with other P. citricarpa and P. capitalensis sequences from different continents. The species showed different population structures in Florida. P. citricarpa isolates did not exhibit genetic variation and were similar to strains from other continents. In contrast, Florida P. capitalensis isolates were distributed over five sequence groups. This study did not point to the potential origin of P. citricarpa and P. capitalensis in Florida. More variable genetic markers and isolates from various continents would be required to track the possible movement of these Phyllosticta species.