Direct and indirect community effects of the invasive plant pathogen Austropuccinia psidii (myrtle rust) in eastern Australian rainforests

In 2010, the invasive pathogen Austropuccinia psidii was detected in Australia, posing a threat to vegetation communities containing susceptible Myrtaceae species. A large-scale field experiment tested the direct and indirect effects resulting from the infection of two highly susceptible rainforest species, Rhodamnia rubescens and Rhodomyrtus psidioides. Community-level impacts were assessed at three sites per study species in New South Wales, Australia. For R. rubescens, 20 plots containing an adult tree each were established per site. Each plot was designated one of four treatments: fungicide spray of the understorey only, canopy only, both or none (control). For R. psidioides, 10 plots containing only seedlings were established per site, with each plot designated to one of two treatments: fungicide spray or no spray (control). Richness and abundance of co-occurring understorey species were assessed every 4 months for a 24-month period, and changes in canopy transparency were assessed for R. rubescens. The R. rubescens control canopy plots were found to have greater canopy transparency (direct effect) which caused a reduction in the understorey richness and total abundance (indirect effects). For treated canopy plots, richness was similar but total abundance increased in fungicide treated understorey plots, suggesting a direct effect of the pathogen on understorey species. Understorey plots treated with fungicide had significantly greater abundance of R. rubescens and R. psidioides seedlings compared to control plots. This study shows that in a short time period, infection by an invasive fungal pathogen has resulted in changes in species richness and abundance in Australian rainforest communities.