Bioassay and baiting methods for determining occurrence of races of Phytophthora clandestina in soil

Abstract

The method currently used for determining races of Phytophthora clandestina requires isolation of pure cultures of the pathogen and testing of their pathogenicity on a range of differential cultivars. To date, the pathogen has not been isolated directly from soil and isolation of the pathogen from naturally infected seedlings is laborious. A bioassay involving the planting of differential cultivars in soil samples in small planting trays was developed to identify races of P. clandestina in soil. The specific races of the pathogen in the soil were determined by assessing the disease severity and the extent of sporulation of the pathogen on the roots of the differential cultivars. A more rapid baiting method using cotyledons of differential cultivars in flooded soil samples was also developed to determine the presence of different races in the samples. Both bioassays were used to confirm the presence of race 0 and race 1 in separate paddocks at Rutherglen, northern Victoria, over 4 seasons. The presence in field soil of another root rot pathogen of subterranean clover, Aphanomyces euteiches, was also detected using these techniques.