Population structure of Botryosphaeria dothidea from pistachio and other hosts in California

Genetic diversity was investigated among California populations of Botryosp haeria dothidea, causal agent of Botryosphaeria panicle and shoot blight of pistachio, with random amplified polymorphic DNA (RAPD) and microsatellite -primed polymerase chain reaction (MP-PCR). We surveyed 120 isolates, 112 o f which originated from the California Central Valley and included pistachi o isolates (n = 52) and isolates from other plant species (n = 60). Our-gro up isolates (n = 8) were obtained from pistachio in Greece. There was a str ong correlation (r = 0.99; P < 0.0001) between the RAPD- and MP-PCR dissimi larity data sets. Little genetic variation (haplotypic diversity [Hs] < 0.0 02) was detected among B. dothidea isolates collected from central and sout hern California pistachio plantings. We observed relatively high diversity for isolates from a northern California pistachio orchard (Hs = 0.0146), wh ere the disease was first diagnosed, and from the Chico U.S. Department of Agriculture Germ Plasm Repository (Hs = 0.0726), where the first pistachio trees were planted in California in 1929. Isolates obtained from other host s, especially those associated with the rare occurrence of the sexual stage of this fungus, showed the highest levels of diversity (Hs = 0.1689). Thir ty-eight pistachio isolates (73.1%) had DNA fingerprints identical to 28 py cnidiospore-derived isolates (56.0%) obtained from other host species. Gree nhouse inoculations demonstrated that all isolates obtained from other host s were capable of infecting pistachio and produced characteristic disease s ymptomology. Thus, California populations of B. dothidea from pistachio are , for the most part, genetically uniform, with the sexual stage rare to abs ent. However, the rare occurrence of the sexual stage of B. dothidra on oth er hosts, and more importantly, the capacity of these isolates to infect pi stachio, indicate that other host species may serve as sources of inoculum and genetic variation.