Exploring the role of fungal endophytes in the sudden death syndrome of the invasive shrub Chrysanthemoides monilifera subsp. rotundata in Australia

Pathogens that attack invasive plants can positively affect the integrity and functioning of ecosystems. Stem-tip dieback and extensive wilting followed by sudden death have been observed in Chrysanthemoides monilifera subsp. rotundata (bitou bush), one of Australia’s worst invasive shrubs. Metabarcoding and culturing methods were used to investigate if fungi are implicated in this syndrome. Metabarcoding results revealed significantly different endophytic fungal communities within healthy and diseased bitou bush, and co-located native plants. There was no difference in fungal communities between soil sampled in the root zone of healthy and diseased bitou bush at the same site. Two Diaporthe sp. operational taxonomic units (OTUs), dominant at sites with extensive wilting, explained 30% of the similarity between diseased bitou bush across all sites. Two other OTUs, Austropleospora osteospermi and Coprinellus sp., explained 20 and 40% of the similarity between diseased plants, respectively, and were only dominant at sites with dead or stunted, partially defoliated but not wilted bitou bush. A Penicillium sp. OTU explained 90% of the similarity between healthy bitou bush. Various Diaporthe spp. dominated isolations from diseased bitou bush. Manipulative experiments confirmed Diaporthe spp. pathogenicity on bitou bush excised and in-situ stems. In another experiment, Diaporthe masirevicii infected flowers and from there colonized stems endophytically, but wilting and sudden death of bitou bush did not occur within the experimental timeframe. Our study provides circumstantial evidence that bitou bush sudden death syndrome is the result of a shift in the composition of its endophytic fungal community, from mutualist to pathogenic species.