Population Structure of Botryosphaeria dothidea from Pistachio and Other Hosts in California.

ABSTRACT Genetic diversity was investigated among California populations of Botryosphaeria 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 of which originated from the California Central Valley and included pistachio isolates (n = 52) and isolates from other plant species (n = 60). Out-group isolates (n = 8) were obtained from pistachio in Greece. There was a strong correlation (r = 0.99; P < 0.0001) between the RAPD- and MP-PCR dissimilarity data sets. Little genetic variation (haplotypic diversity [Hs] < 0.002) was detected among B. dothidea isolates collected from central and southern California pistachio plantings. We observed relatively high diversity for isolates from a northern California pistachio orchard (Hs = 0.0146), where 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 hosts, especially those associated with the rare occurrence of the sexual stage of this fungus, showed the highest levels of diversity (Hs = 0.1689). Thirty-eight pistachio isolates (73.1%) had DNA fingerprints identical to 28 pycnidiospore-derived isolates (56.0%) obtained from other host species. Greenhouse inoculations demonstrated that all isolates obtained from other hosts were capable of infecting pistachio and produced characteristic disease symptomology. Thus, California populations of B. dothidea from pistachio are, for the most part, genetically uniform, with the sexual stage rare to absent. However, the rare occurrence of the sexual stage of B. dothidea on other hosts, and more importantly, the capacity of these isolates to infect pistachio, indicate that other host species may serve as sources of inoculum and genetic variation.