Triggering dieback in an invasive plant : endophyte diversity and pathogenicity

Dieback causes a progressive reduction in plant population health, resulting in the death of plant parts and often plant death. It is prevalent in many invasive woody weeds in Australia and has been suggested as a potential mechanism for biocontrol of these species. Parkinsonia aculeata one such invasive tree in northern Australia. It has naturalised across a wide range of climatic zones and some populations have been heavily reduced by dieback occurrence. The cause(s) of dieback in parkinsonia remain elusive, although fungal endophytes have been previously implicated. In this study, we characterised the culturable fungal endophyte community of healthy and dieback-affected parkinsonia using culture-based techniques, and identified cultured isolates via amplicon sequencing of the internal transcribed spacer (ITS) of the rDNA operon. Eight isolates, identified as pathogens, were selected for a 10-week pathogenicity trial, including water stress treatments, on parkinsonia seedlings. We isolated a taxonomically diverse fungal community from parkinsonia, representing 54 unique species from 25 families. Communities were similar across healthy and dieback-affected plants, but differed by plant tissue. Of the eight putative pathogenic isolates tested in the pathogenicity trial, inoculation with Lasiodiplodia pseudotheobromae, Botryosphaeria dothidea and Pestalotiopsis mangiferae resulted in the largest lesions, but systemic infection or dieback-like symptoms were not observed in any treatment despite plant stress being induced by drought or inundation. We concluded that inoculation of parkinsonia with the tested putative fungal pathogens is unlikely to result in dieback, which has implications for future work in biocontrol of parkinsonia.