PATHOGENIC VARIABILITY IN AUSTRALIAN ISOLATES OF PHYTOPHTHORA-CINNAMOMI

Forty-two isolates of Phytophthora cinnamomi were obtained from native vegetation and horticultural crops within Australia. They represented a broad spectrum of geographical and host origins, both mating types, and all identified Australian isozyme genotypes. All isolates were tested for their pathogenicity to a single clone of Eucalyptus marginata by inoculation of soil in which plants were growing. Differences in pathogenicity were expressed as extent of root damage, reduction of plant growth, period to first visible disease symptoms and time to plant death. Significant variation between isolates was detected. Pathogenicity was unrelated to mating type and isozyme properties. A subset of these 42 isolates encompassing a range of virulence gave generally consistent rankings for pathogenicity variates when re-inoculated twice into plants derived from the original clone. This suggests that pathogenicity is a relatively stable characteristic. Detection of differences in susceptibility to P. cinnamomi between three selected E. marginata clones was influenced by the pathogenicity of isolates. Only the more pathogenic isolates were useful in this regard. Seedling stems of five eucalypt species were inoculated with virulent and less virulent isolates of P. cinnamomi. This method detected variation in both pathogenicity in the fungus and susceptibility in the host species. Variation in pathogenicity within Australian populations of P. cinnamomi should be taken into account by the choice of isolates of proven virulence when selecting for resistance in trees and other woody hosts.