The ubiquitous oomycete pathogen Phytophthora cinnamomi is reported to be the most common and economically important causal agent of avocado root rot worldwide. Surveys from avocado producing regions in Latin America and the Mediterranean have identified other oomycetes belonging to the genera Phytophthora and Phytopythium, as well as some species of nectriaceous fungi as causal agents of avocado root rot. In Australia, there are ad hoc reports of Phytopythium vexans and Dactylonectria spp., both known root rot pathogens of avocado, being frequently co-isolated with P. cinnamomi from commercial orchards. To investigate this, we surveyed 144 symptomatic and asymptomatic trees from 36 orchards distributed across eight avocado growing regions of Australia. Oomycetes and putative fungal pathogens were isolated through soil baiting and/or direct root plating, assigned preliminary identification based on morphology, and species identities were confirmed through endpoint PCR-based molecular diagnostics. P. cinnamomi and Pp. vexans were the two most frequently isolated oomycetes, being present in all eight growing regions, with P. cinnamomi isolated from 82% of symptomatic trees (n=108) and 89% of asymptomatic trees (n=36) sampled, in 90% of the orchards. Pp. vexans was present in 52% of all trees sampled, and in 85% of all orchards, with 85% of the isolates co-isolated with P. cinnamomi. Dactylonectria spp. were isolated from the roots of 15% of trees sampled (n= 80), and in 35% of the orchards (n=20), and in 92% of cases co-isolated with P. cinnamomi. Other Pythium spp. and Ilyonectria liriodendri were also frequently isolated alone or together with P. cinnamomi. Soil baiting resulted in a higher level of pathogen isolation for P. cinnamomi and Pp. vexans than root plating. Pathogenicity testing on avocado seedlings confirmed representative isolates of P. cinnamomi from Queensland and Western Australia were highly pathogenic and significantly reduced plant growth parameters and increased root necrosis (P < 0.001) compared to inoculation with Pp. vexans, Pythium spp., Dactylonectria spp. and mock-inoculated control. This study demonstrated that P. cinnamomi is the most common, widespread and the most pathogenic causal agent of root rot in Australian avocado orchards. Its high incidence in soil and root samples from apparently healthy trees underscores the need for proactive and preventative integrated disease management programs in all orchards.