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AFLP Analysis of Phytophthora cactorum Isolates from Strawberry and Other Hosts: Implications for Identifying the Primary Source of Inoculum

Forty-seven isolates of Phytophthora cactorum from North America and Germany were subjected to amplified fragment length polymorphism (AFLP) analysis to investigate genetic diversity among isolates and geographical populations; 42 isolates were recovered from cultivated strawberry plants (Fragaria × ananassa), and five isolates had been recovered from plants in four other genera (Syringa, Abies, Malus, and Panax). From all isolates evaluated, 226 out of 264 markers (85.6%) were polymorphic and provided 42 unique AFLP profiles. The genetic diversity among isolates of P. cactorum from strawberry was greater than that among isolates from the other hosts. Isolates collected during recent crown rot epidemics in strawberry fields in South Carolina were genetically diverse and scattered among isolates from other geographical areas in an unweighted pair-group mean analysis (UPGMA) dendrogram. Isolates collected during recent crown rot epidemics in North Carolina also were genetically diverse, but most isolates clustered with isolates collected in 1997 from Florida strawberry fields. These data suggest that recent outbreaks of Phytophthora crown rot in the southeastern United States resulted from use of transplants already infected or infested with P. cactorum rather than from endemic populations of this pathogen, which would affect recommendations for disease management.