Phylloplane fungal community composition shifts with Austropuccinia psidii infection status and reveals candidate fungal antagonists for future biocontrol research

Aims Myrtle rust, caused by the biotrophic fungal pathogen Austropuccinia psidii, is a significant invasive plant disease affecting members of the Myrtaceae worldwide. This research aimed to characterise the diversity and composition of phylloplane fungal communities on symptomatic and asymptomatic leaves of two native myrtaceous host plants in New Zealand, with particular emphasis on their association with myrtle rust infection. Methods and Results Field surveys were conducted on Lophomyrtus bullata and Metrosideros excelsa across urban and native forest environments. Both epiphytic and endophytic fungal communities were characterised using metabarcoding. The findings revealed that host plant identity, rust infection status and geographic location (with associated environmental differences) did not affect alpha diversity (richness and evenness) but did influence beta diversity and fungal community composition. Random Forest analysis identified Puccinia as the strongest indicator taxon associated with symptomatic leaves across all regions, alongside additional site-specific indicator taxa. In contrast, asymptomatic leaves were associated with other fungal genera, including Trichoderma and Sphaerellopsis, both of which have previously been reported as antagonists of rust fungi. Complementary culture-dependent isolation resulted in a living collection of 201 fungal isolates spanning 59 genera, serving as a resource for functional studies. Conclusions Asymptomatic leaves harboured microbial communities distinct from those of symptomatic leaves, with these differences driven by key distinguishing taxa that may serve as a source of fungal antagonists against myrtle rust.