CHANGES IN THE RACIAL COMPOSITION OF PHYTOPHTHORA-SOJAE IN AUSTRALIA BETWEEN 1979 AND 1996

Surveys of commercial soybean fields, disease nurseries, and trial plo ts of soybean were conducted throughout eastern Australia between 1979 and 1996, and 694 isolates of Phytophthora sojae were collected and c lassified into races. Fourteen races, 1, 2, 4, 10, 15, and 25, and eig ht new races, 46 to 53, were identified, but only races 1, 4, 15, 25, 46, and 53 were found in commercial fields. Races 1 and 15 were the on ly races found in commercial fields in the soybean-growing areas of Au stralia up until 1989, with race 1 being the dominant race. Race 4 was found in central New South Wales in 1989 on cultivars with the Rps1a gene, and it is now the dominant race in central and southern New Sout h Wales. Races 46 and 53 have only been found once, in southern New So uth Wales, and race 25 was identified in the same region in 1994 on a cultivar with the Rps1k gene. Only races 1 and 15 have been found in t he northern soybean-growing regions, with the latter dominating, which coincides with the widespread use of cultivars with the Rps2 gene. Ch anges in the race structure of the P. sojae population from commercial fields in Australia follow the deployment of specific resistance gene s.