Impacts of the invasive fungus Puccinia psidii (myrtle rust) on three Australian Myrtaceae species of coastal swamp woodland

Exotic fungal pathogens can substantially affect individuals and populations of susceptible plant species, potentially resulting in changes in cummunity structure and composition. Myrtle rust (Puccinia psidii) is a pathogenic fungus native to South America that affects species in the family Myrtaceae. The pathogen was introduced accidentally to Australia and first detected in NSW in April 2010. Ecological impacts have been poorly studied in the native range of myrtle rust and even less for Australian native communities. Two experiments were conducted to assess myrtle rust impacts in three co-occurring species of coastal swamp woodland that are known to be susceptible: Meleleuca quinquenervia, Leptospermum laevigatum and Baeckea linifolia. Plants of each species were grown individually (Expt 1) and in mixed species assemblages (Expt 2), with half inoculated with myrtle rust and the other half remaining as a controls. Infection level was assessed and its impact on seedling survival and growth was recorded. In both experiments L. laevigatum and M. quinquenervia seedlings were heavily infected and showed high degrees of susceptibility with negative effects on growth (height, biomass, number of leaves). In contrast no B. linifolia seedling presented visible symptoms of disease. Melaleuca quinquenervia seedlings had greater infection levels and suffered greater growth reductions than L. laevigatum in both experiments. Biomass allocation was largely unaffected, with the exception of increased stem mass fraction. As M. quinquenervia is dominant in coastal swamp communities and highly susceptible to myrtle rust, changes in community structure are likely. This study provides better understanding of the potential impacts of myrtle rust in this ecological community and has significant implications for the conservation and management of Australian Myrtaceae-dominated plant communities generally.