Dieback is prevalent in many populations of invasive woody weeds globally. There are many preventative and contributing biotic and abiotic factors related to dieback occurrence¹. Previous dieback studies have focused on specific potential causative biotic agents, but the majority remain unexplained. Parkinsonia aculeata L. (parkinsonia), an invasive tree in northern Australia. It has naturalised across a wide range of climatic zones in northern Australia², many of which experience long periods of extensive drought interrupted by extreme rain events. Parkinsonia dieback has been observed in both drought-affected and regions prone to flooding³ but little is known about whether or not water availability has a role to play in dieback occurrence. 

In a glasshouse trial we tested the interactive effects of water availability and fungal inoculation on growth and pathogenesis of parkinsonia. We sampled roots, stems and stem tips from healthy and dieback-affected parkinsonia from northern Queensland. Fungal isolates from the samples were cultured and identified via ITS sequencing. A number were identified as known tree pathogens, some of which are associated with dieback in other host species. Eight of these were selected for a 10-week glasshouse pathogenicity trial on 1 year-old parkinsonia seedlings. Seedlings were also subjected to three different water treatments to simulate drought, normal water availability and inundation.

We observed lesion formation and gummosis in parkinsonia inoculated with Pestalotiopsis mangiferae, P. clavispora, Lasiodiplodia pseudotheobromae and Botryosphaeria dothidea and satisfied Koch’s Postulates, however we did not observe systemic infection, typical of dieback. Normal and inundated treatments were associated with larger lesion formation and increased gummosis. Lower water levels were also correlated to less plant growth, as indicated by plant height, dry weight and stem girth. 

Parkinsonia dieback is a complex phenomenon, occasionally attributed to specific fungal pathogens⁴. We have shown, however, that water availability and subsequent plant stress contributes to overall plant health and therefore susceptibility to infection by pathogens.