Investigation of fungal communities associated with myrtle rust infection and the use of mycoparasites as potential biocontrol agents against the disease

Abstract

This study investigated the fungal communities associated with myrtle rust and identified potential mycoparasites for biocontrol. Leaf samples were collected from two native plant hosts, ramarama (Lophomyrtus bullata) and pōhutukawa (Metrosideros excelsa), in forests (Taranaki and Rotorua) and urban areas (Auckland and Christchurch). Metabarcoding revealed that Ascomycota (58.7%) and Basidiomycota (39.7%) dominated the fungal communities. At the genus level, an ASV assigned to rust pathogen Austropuccinia was relatively more abundant in M. excelsa than L. bullata, and higher relative abundance in urban than forest environments. From fungal isolation, 208 fungal isolates representing 64 genera were obtained. Among these, 23 putative mycoparasites from 11 genera were selected for antagonistic screening against A. psidii. Three in vitro assays including detached branchlet, spore germination, and pathogenicity tests were conducted. Two promising isolates (Cladosporium and Trichoderma) colonized 33–35% of rust pustules and inhibited 63–70% of urediniospore germination. Neither isolate caused disease symptoms on inoculated leaves. Greenhouse trials were carried out to evaluate their effectiveness under preventive (two days before rust inoculation) and curative (seven days after inoculation) applications. Preventive treatments significantly reduced the disease severity index to 19%, compared to 30% in the curative treatment and 33% in the untreated control. Trichoderma also promoted plant regeneration by increasing new shoot growth compared to Cladosporium and the control. These findings contribute to understanding the role of microbial communities in myrtle rust infection and inform the development of biocontrol strategies.