Endangered species face an extra threat: susceptibility to the invasive pathogen Austropuccinia psidii (myrtle rust) in Australia

Austropuccinia psidii (myrtle rust) is an invasive fungus native to South America that infects the young growing tissues of species in the Myrtaceae family, one of the dominant plant families in Australia. To date, 360 native species from 49 genera have been found to be susceptible in Australia, but the vast majority remain untested (81%). The aim of this study was to test a range of plant species whose susceptibility status remains unknown, including endangered species, species with a large distribution overlap with A. psidii and species from a genus that has not been previously tested. Different sub-species and provenances were also tested to assess for intra-specific differences. Of the 24 tested species/sub-species, 18 (including 12 endangered) were found susceptible to A. psidii to varying degrees (including the first species record within Triplarina being susceptible), while one presented a hypersensitive reaction and six were resistant. The most susceptible species were the critically endangered Melaleuca megalongensis, and the endangered Eucalyptus copulans, E. parvula, E. scoparia and Melaleuca irbyana. No significant differences in intraspecific susceptibility were found between sub-species or provenances. We suggest that the susceptible species we have identified will be vulnerable to infection in their native ranges in the future, if they have not already become infected. For highly susceptible species, A. psidii should be considered as a major additional threat and appropriate control measures incorporated into existing threatened species plans. Monitoring of susceptible species' populations in the wild and seed collection for seed banking are vital steps for ensuring their conservation.