Matteo Garbelotto
Forest Pathology and Mycology Laboratory
Department of Environmental Science, Policy, and Management, UC Berkeley
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Additional key words: Phytophthora cinnamomi, Arctostaphylos myrtifolia, microsatellites
We sampled soil in and near Arctostaphylos myrtifolia mortality centers to determine the overall geographic distribution of Phytophthora cinnamomi within the range of A. myrtifolia. Green pear fruit were placed in flooded soil samples to bait P. cinnamomi from the samples. P. cinnamomi was found in mortality centers near the northern end of the A. myrtifolia range near Carbondale Road. P. cinnamomi was also found associated with A. myrtifolia mortality centers between Ione and Buena Vista. No mortality centers were seen in A. myrtifolia stands south of Buena Vista as of June 2004. Genetic data, based on DNA microsatellite analysis, indicated that at least three clonal lineages are represented among the P. cinnamomi isolates baited from mortality centers. From the geographic distribution of the genotypes represented among the P. cinnamomi isolates, it appears that the P. cinnamomi infestation is the result of multiple introductions of the pathogen into A. myrtifolia habitat, with subsequent local spread from points of introduction. Baiting was also used to assess the local distribution of P. cinnamomi around mortality centers. P. cinnamomi was most reliably recovered from samples taken near the leading edge of well-defined mortality centers. It was infrequently recovered (1 of 20 samples) amid asymptomatic plants located 2.5 to 5 m beyond the edge of mortality centers. Hence, it appears that at least by early summer, the distribution of dead plants closely approximated the distribution of the pathogen in the soil. Measurements of disease progress along permanent transects in one mortality center showed only about 0.5 m of disease spread uphill but at least 2 m of disease spread downslope after 2 years. P. cinnamomi was uncommonly baited from soil samples collected from the upper 1 cm of the soil profile compared with samples collected at 2-10 cm depth in May 2004, suggesting that few viable propagules of the pathogen were present on the soil surface by early summer.