Molecular analyses indicate that both native and exotic pathogen populations serve as sources of novel outbreaks of Cypress Canker Disease.
Invasive pathogens may follow stepwise spread pathways in which novel disease outbreaks become themselves sources of genotypes initiating novel infestations. Due to its worldwide distribution, Cypress Canker Disease (CCD) provides an opportunity to understand patterns of global spread of a forest pathogen. A comparative genetic analysis was performed on 110 isolates of the causal agent of CCD, the fungus Seiridium cardinale, from Europe, North America, Africa and Oceania using β-tubulin gene sequences and repeat numbers of seven nuclear SSRs. SSR data were used to calculate genetic indices for each geographic population, including those calculating allele and private allele diversity; clonal genotype, haplotype, and gene diversity; expected heterozygosity; and presence of linkage disequilibrium. Additionally, SSR data were used to calculate migration rates between regions, and to draw a minimum spanning network to visualize genetic relatedness among genotypes. Results identified the California population as a sexually reproducing, highly diverse, native population, and the Mediterranean population as a large, clonally reproducing, and exotic population directly derived from the California population. The New Zealand population appeared to be non-native, with intermediate values of genetic diversity and an asexual reproductive mode. Unexpectedly, two sources were identified for the New Zealand population: an older source was identified in California, while a more recent one was identified in the Mediterranean. Results allowed to infer migration intensity between continents, and are among the first to show that an exotic outbreak of a forest disease can become a source of pathogen genotypes for novel outbreaks in disjunct geographic regions.