Effect of Austropuccinia psidii inoculum concentration on myrtle rust disease incidence and severity

Austropuccinia psidii (myrtle rust) is one of the most significant threats to plant industries and biodiversity in the Australasian region. Susceptibility phenotypes of Australian native Myrtaceae to A. psidii have varied significantly since the pathogen was first detected in Australia in 2010, with some species appearing to increase in susceptibility over time. We hypothesise that increased inoculum concentrations in natural ecosystems may be contributing to observed changes in susceptibility phenotypes. A study was undertaken to determine the effect of A. psidii inoculum concentration on myrtle rust disease incidence and severity on five native (Rhodamnia rubescens, Syzygium hemilamprum, S. leuhmanii, S. moorei and S. oleosum) and one exotic (S. jambos) species of Myrtaceae under controlled conditions. Disease incidence and severity were found to increase across all species as inoculum concentration increased, with notable increases for species considered to be tolerant or of lower susceptibility to A. psidii. This improved understanding of the influence of increasing inoculum A. psidii concentrations on susceptibility phenotypes can now be integrated with current management and research plans, to predict and mitigate the impact of A. psidii on Australian native biota and ecosystems.