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Seiridium cardinale is regarded as the most important agent responsible for the disease of cupressaceous hosts referred to as Cypress canker. The fungus was first described in California and is currently reported in all continents. A recent study based on seven SSR loci has suggested that California populations may represent the source of the epidemic in the Mediterranean. In this study, 185 AFLP markers were used on an expanded sample size of 125 isolates to determine whether the Mediterranean population may indeed be derived from the California one and to compare the reproductive modes of populations in the two regions of the world. Additionally, amova, NJ, STRUCTURE, principal component analysis and β-tubulin sequence analyses were employed to infer the presence of genetic structure within and between populations. The distribution of pairwise AFLP similarity coefficients suggests Mediterranean populations are reproducing only clonally, while California populations are reproducing both clonally and sexually. amova indicates Mediterranean and California populations are currently genetically isolated, but NJ and STRUCTURE analyses both suggest ancestral Mediterranean genotypes belong to the California population. No alleles were private to either population, and the presence of identical or quasi-identical genotypes at large distances supports the notion that movement of infected cypress plants is responsible for the global spread of the disease. Surprisingly, STRUCTURE identified a second cluster of Mediterranean genotypes distinct from the basal mixed California- Mediterranean cluster. This second cluster either may have originated from the first one under the selection pressure imposed on the pathogen in the new Mediterranean environments or may be the result of a further introduction from California or elsewhere. Cumulatively, the evidence presented here suggests that S. cardinale may be native or long naturalized in California and that two genetically distinct groups are present in the Mediterranean, with obvious implications for further studies on this disease. [ABSTRACT FROM AUTHOR]

Seiridium cardinale is the pathogenic fungus of unknown origin responsible for a world pandemic known as cypress canker affecting several species of Cupressaceae in both the Northern and Southern Hemisphere. In this study, a comparative genetic analysis of worldwide populations was performed using sequence analysis of a portion of the ?-tubulin locus and seven polymorphic simple-sequence repeat (SSR) loci on 96 isolates. Sequence analysis identified two distinct ?-tubulin alleles, both present in California. Only one of the two alleles was detected in the Mediterranean basin, while two isolates from the Southern Hemisphere were characterized by the presence of the allele absent from the Mediterranean. SSRs identified a total of 46 multilocus genotypes (MGs): genotypic diversity was always higher in the California population, and calculations of the index of association (I(A)) determined the presence of linkage disequilibrium associated with the absence of sexual reproduction only in the Mediterranean population but not in California. In 50 instances, the same MG was found at great geographic distances, implying a role played by humans in spreading the disease. Network analysis performed on SSR data identified three clusters of MGs: California, Morocco, and the rest of the Mediterranean. Both the Morocco and the Mediterranean clusters were linked to the California cluster. Coalescent analysis identified insignificant migration between California and Italy, as expected in the presence of a single introduction event, and very high migration from Italy into Greece, as expected of an outbreak still in exponential growth phase and starting from an Italian source.