The evolution of races of Phytophthora sojae in Australia

Isolates of Phytophthora sojae collected between 1979 and 1993 from three soybean-growing regions in eastern Australia and the United States were characterized for virulence and restriction fragment length polymorphisms (RFLPs). A total of 99 isolates were characterized: 84 from Australia and 15 from the United States. In the Australian population of P. sojae, five races (1, 4, 13, 15, and X) were identified. Five RFLP probes were selected that detected 30 fragments, of which 7 were polymorphic, leading to the identification of three multilocus RFLP genotypes amongst the Australian isolates. One multilocus RFLP genotype occurred in over 95% of all samples collected throughout the soybean-growing regions of eastern Australia. The 15 isolates obtained from the United States represented 12 races; the five RFLP probes detected 31 fragments, of which 13 were polymorphic, corresponding to 12 multilocus RFLP genotypes. The low levels of genotypic diversity (2.5 to 14.3%) observed in Australian fl sojae populations compared to the high level (60%) observed in the U.S. population indicates that the Australian population was most likely established by a single introduction of the pathogen. All five races in Australia occurred in the same multilocus RFLP genotype, suggesting that new races in Australia evolve from a common genetic background through mutation.