Rehabilitation of Phytophthora cinnamomi-infested jarrah (Eucalyptus marginata) forest sites using pelletised seeds of resistant species

Shanika S Harshani

The soil-borne plant pathogen Phytophthora cinnamomi Rands has severely damaged the community structure and composition of the jarrah (Eucalyptus marginata) forest in the southwest of Western Australia. Complete eradication of the pathogen from infested sites is not possible. This project examines techniques to rehabilitate P. cinnamomi-infested forest sites with native resistant species that can replace susceptible species lost to the pathogen. A vegetation survey evaluated the impact of the pathogen on five reserves in Mundaring, approximately 35 km east of Perth, Western Australia. This demonstrated that the pathogen has had a significant impact on forest structure (understorey cover), taxonomic diversity (species richness, Shannon diversity, Pileou evenness), functional diversity (nutrient acquisition traits, carbon acquisition traits and reproductive traits) and species composition. Seed-based rehabilitation requires information on the resistance of native species during seedling emergence and early survival. This was assessed in a glasshouse experiment for 50 species, and the resistance of 24 selected species was evaluated further when plants were 5-7 months old. Out of these, two resistant and six moderately resistant species were identified. The results demonstrated the need for information on the disease susceptibility of young seedlings as well as mature plants as species resistant as adults in the field may be susceptible as seedlings. To overcome the limitations of seed-based rehabilitation, including seed predation, low seedling establishment and difficulty in distributing seeds with different shapes and sizes, the use of extruded pellets was assessed as a seed enhancement technique. Five pellet formulations were tested using seeds of four resistant species, and the optimum pellet formulation was determined to be 25% calcium bentonite, 15% diatomaceous earth and 60% sand. When two pellet preparation methods were tested, field-deployed pellets resulted in highest seedling emergence in Acacia acuminata and Hakea laurina, while for Calothamnus sanguineus and Melaleuca seriata, laboratory-produced pellets were better. Seedling establishment was evaluated in 18 rehabilitation plots in P. cinnamomi-infested areas of three reserves using six species (A. acuminata, A. saligna, C. sanguineus, M. seriata, Banksia sessilis and H. laurina). Non-pelleted seeds and two pellet types: laboratory-produced and field-deployed, were used, incorporating rhizobium bacteria for the two Acacia species and spores of ectomycorrhizal fungus for the two myrtaceous species. Good levels of seedling emergence (22.8-88.4%) and survival after 9 months (16.3-83.6%) were obtained for most species except for M. seriata which showed no survival. Non-pelleted seeds resulted in the highest emergence and survival and the provision of beneficial microbes in the pellets did not affect seedling emergence, survival or shoot growth of acacias or myrtaceous species. Quick disintegration in the field may have diminished the possible beneficial effects of the pellet, while burial of control seeds may have resulted in better emergence and survival from this treatment. This study demonstrated the feasibility of rehabilitating P. cinnamomi-infested forest sites with native species resistant to P. cinnamomi which can help restore the ecosystem functions and services of the susceptible species lost due to the pathogen.

Rehabilitation of Phytophthora cinnamomi-infested jarrah (Eucalyptus marginata) forest sites using pelletised seeds of resistant species

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