Regeneration of Susceptible Native Species Following a Decline of Phytophthora cinnamomi over a Period of 20 Years on Defined Plots in the Grampians, Western Victoria

Regeneration of susceptible species following a major decline in the population and distribution of the pathogen Phytophthora cinnamomi Rands is reported from a reassessment of infested plots in open forest, woodland and heathland of the Grampians. The distribution of P. cinnamomi and changes in the vegetation were measured on quadrats set up in 1976 and 1977 and checked biennially until 1984. In 1995, P. cinnamomi was detected in only 15.6% of the 345 soil and root samples tested and was present in only 28.6% of the infested quadrats compared with 100% of the samples and infested quadrats in all previous assessments. Both the restricted distribution and the isolation of P. cinnamomi from susceptible species without symptoms were evidence of changes in the disease which may have been associated with factors such as a reduced supply of susceptible host roots or an alteration in pathogenicity or in the environment. Changes were recorded in structure and species composition of infested plant communities compared with those that remained pathogen-free, but few recent deaths or dieback symptoms were observed in 1995. On the infested plots, several mature stringybark eucalypts such as Eucalyptus baxteri and E. obliqua had died, and the survivors had reduced crown density and dieback of the major branches. However, regeneration in the form of 8-year-old saplings and young seedlings showed few or no symptoms despite the isolation of P. cinnamomi from the roots. The understorey of infested plots comprised a dense growth of field-resistant plants such as Leptospermum spp. and sedges entwined with dodder laurel, whereas the uninfested plots contained a species-rich heath flora of which 54% were known to be susceptible to P. cinnamomi. Regeneration and continuing survival of 24 susceptible species from 11 different families were recorded as small plants on various infested quadrats. However, 12 susceptible species from 7 different families had failed to regenerate. Pathogen populations may increase with the renewed supply of susceptible roots, but independent of pathogen potential, the emerging susceptible species may not be able to compete effectively against the dense field-resistant understorey in infested plots in order to re-establish the original diverse heathland understorey.