Genetic diversity in Populations of Phytophthora citricola associated with horticultural crops in California

California populations of the plant pathogen Phytophthora citricola were examined for amplified fragment length polymorphism (AFLP), pathogenicity on almond, and sensitivity to mefenoxam. The characterizations of AFLP variation and mefenoxam sensitivity were based on 86 isolates (44 from almond, 11 from avocado, 3 from strawberry, 18 from walnut, and 10 from six other hosts). Cluster analysis of the AFLP data using the unweighted pair group method indicated a high level of genetic diversity among the isolates, and four main clusters were identified-one dominated by isolates from almond, another including all isolates from avocado, and two including isolates from several hosts other than avocado. Analysis of molecular variance revealed that 38.4 and 24.9% of the AFLP variation were associated with host and geographical factors. respectively. Of 24 isolates, including those from almond, avocado, strawberry, and walnut, 22 were aggressive on almond shoots; there was no evidence of host specificity. All but 1 of the 86 isolates grew at different rates on V8 juice medium amended with mefenoxam at 1 ppm, indicating partial tolerance to the fungicide. Isolates of P. citricola from California populations are genetically diverse, and much of the variation is associated with host and geography. These populations are all potentially pathogenic on almond and tolerant to mefenoxam.