The Impact of Phytophthora cinnamomi on Six Rare Native Tree and Shrub Species in the Brisbane Ranges, Victoria

Abstract

The impact of Phytophthora cinnamomi was measured on six native tree and shrub species occurring in the Brisbane Ranges, Victoria, that are considered significant, rare or vulnerable and likely to become endangered. Seedlings of Eucalyptus yarraensis (rare) and cuttings of Grevillea chrysophaea (significant), Olearia pannosa, Pultenaea graveolens (both vulnerable) and Prostanthera decussata and Rhagodia parabolica (both rare) were grown in disease-free soil and inoculated with P. cinnamomi. After 60 days, all plants of G. chrysophaea and P. graveolens were dead and, hence, were regarded as highly susceptible. Inoculated plants of P. decussata developed severe symptoms and 13% died, while O. pannosa showed decreased vigour with 11% dying. Both species were classified as susceptible. Some plants of R. parabolica developed symptoms and were slightly susceptible, butE. yarraensis was resistant. A foliage spray of the phosphonate, Foli-R-Fos (dipotassium phosphonate) on healthy susceptible species, followed by stem inoculation with the pathogen, reduced both stem infection and mortalities, but did not eliminate the pathogen. The concentration of 3% was slightly phytotoxic to G. chrysophaea and P. decussata. The distribution of the six species and of disease due to P. cinnamomi was mapped. Stands of G. chrysophaea, O. pannosa and P. graveolens are diseased and at risk. One of the few known stands of P. decussata was invaded recently. When measured on replicate quadrats located in areas of recent disease, old disease and areas free from disease, P. cinnamomi was shown to produce significant reductions in plant density, plant growth, flowering and regeneration, particularly in areas recently invaded by the pathogen, but less on old infested areas. One-third of the population of O. pannosa is dying due to P. cinnamomi and its seed production is inhibited. There is a strong decline in populations of G. chrysophaea, and P. graveolens, which may place these species at risk.