First report of oomycetes associated with the invasive tree Parkinsonia aculeata (Family: Fabaceae)

Phytophthora species have caused the decline and dieback of multiple tree species in Australia and around the world. Dieback in invasive trees in Australia has been observed for decades, motivating research into the potential causes of dieback to be used for biological control of these invasive species. Despite wide-ranging and ongoing research into invasive plant dieback, Phytophthora species have been largely ignored as potential causal agents of dieback, with the focus more on latent fungal pathogens living as endophytes. We conducted the first survey of Phytophthora and other oomycetes to determine their association with dieback of the invasive tree, Parkinsonia aculeata L. (Fabaceae). Using zoospore baiting, we recovered 37 oomycete isolates from roots and soil of healthy and dieback-affected P. aculeata in Kununurra, Western Australia and Charters Towers, Queensland. Using molecular taxonomy, we identified ten unique oomycete taxa, predominantly composed of Phytophthora palmivora, Ph. nicotianae and Phytopythium vexans. Parkinsonia dieback occurs across multiple climatic zones including those experiencing severe drought. We recovered fewer oomycete isolates from soil and roots in drought-affected Charters Towers than Kununurra, which had experienced recent rainfall. This may be because oomycetes require soil moisture for the dispersal of zoospores. None of the genotypes identified were consistently isolated from dieback-affected trees suggesting that any association with parkinsonia dieback may be localised. More extensive surveys and pathogenicity screenings of isolated oomycetes are required to evaluate their role in the parkinsonia dieback phenomenon.