The origin, distribution, and disease association of P. cinnamomi in native plant communities in Australia has been examined.

Soil and plant root samples from a variety of native forest habitats throughout Australia were examined for potentially pathogenic Pythium and Phytophthora spp. by lupin baiting.Those It is suggested that disease of native plant species normally attributed to Ph. cinnamomi could be caused by other Pythium and Phytophthora species, acting singly or alone, with or without Ph. cinnamomi.

The soil-borne fungus Phytophthora cinnamomi Rands is a particularly impor-tant pathogen in Australia because of its consistent association with root-rot disease of a wide variety of exotic and native plant species. It was thought originally to have been introduced from south-east Asia (Crandall and Gravatt 1967), but evidence recently obtained (Pratt, Heather, and Shepherd, unpublished data), suggests that it may be indigenous to eastern Australia and may have been partly instrumental in determining the distribution of certain susceptible species, particularly Eucalyptu8 spp.

Six mono‐uredospore isolates (races) of Melampsora medusae Thiim. produced qualitatively distinct reactions, in vitro, when incubated at 15 °C on leaf disks of certain cultivars of Populus deltoides Marsh. The infection types in some race‐cultivar combinations were very temperature sensitive with less distinct reactions when incubated at 20 or 25°C. The significance of differential race‐cultivar‐temperature interaction in the epidemiology of M. medusae is discussed.

No abstract