Screening leaf-associated fungi from the critically endangered plant Rhodamnia rubescens suggests biocontrol potential against myrtle rust

Aims: Myrtle rust, caused by the obligate pathogen Austropuccinia psidii, is a significant disease of myrtaceous plants. Species in Australia, such as Rhodamnia rubescens, are now critically endangered as a result of this disease. This research aimed to evaluate the culturable fungal communities present on, and within, leaves of symptomatic and asymptomatic R. rubescens plants as inhibitors of myrtle rust disease. We hypothesized that microbes present in asymptomatic leaves may possess biocontrol activity, thereby providing a mechanism of resistance observed in the field.

Methods and Results: Fungal communities were isolated on three types of media and were identified using ITS sequencing. Of the 143 isolates obtained from both leaf types, germinating spores from nine isolates from the genera Nemania, Corynespora, Cladosporium, Pestalotiopsis, Quambalaria, Kalmanozyma, and Coniothyrium were found to inhibit the germination of A. psidii spores in vitro. However, no link was found between biocontrol activity of the isolates recovered and the degree of disease found on R. rubescens leaves. Isolates were also screened for antifungal activity during active hyphal growth using an unrelated pathogenic fungus Stagonosporopsis cucurbitacearum to test for broader bioprotective activity.

Conclusions: Isolates with antagonistic activities could be further explored in conservation efforts to protect plants against A. psidii symptoms, either in isolation, as part of a synthetic microbial community, or in more agricultural contexts to improve food security.