Understanding the mode of action of phosphite in avocado for enhanced management of Phytophthora root rot (AV19005)

Glasshouse and laboratory trials investigating the modes of action of phosphite and their relative contribution to the suppression of Phytophthora are progressing well. A repeat trial looking at the activation of defence responses in avocado roots by phosphite has been completed, and analyses of gene expression are underway. Preliminary glasshouse trials indicate that there is no significant stimulation of root or shoot growth in seedlings maintained in the glasshouse by phosphonate in the absence of Phytophthora cinnamomi.

Initial co-analyses of phosphorous acid and major nutrients show a trend for increasing potassium with increasing phosphorous acid in fruit pulp, and further sampling and analyses of leaf and root tissues will explore this further. There are clear indications of interactions between phosphonate application and the accumulation of starch and soluble sugars, which may be seasonal.

Field trials focusing on the optimal timing of phosphonate application for effective levels in roots and minimal levels in fruit are progressing well. In particular, the analyses to date from the field trial at Ravensbourne show that sprays to active leaf flush (rather than when leaf flush has hardened) may be more efficacious for root accumulation of phosphorous acid. In most regions, leaf flush is almost continual from spring through to early autumn, making sprays as per the current recommendation very difficult to apply. The soil drench applications of phosphonate failed to achieve acceptable and sustained root phosphite levels, suggesting that this method of application may be suboptimal.

There have been several communication and extension activities in April-June 2022. Two dedicated disease management workshops have been delivered to growers and industry stakeholders in Bundaberg (QLD, Manjimup (WA) and Tolga (QLD), which included an outline of project activities and a brief update on progress. Two abstracts have been submitted and accepted for oral presentation at the Australasian Soilborne Disease Symposium, to be held in Cairns in August 2022. The PhD student successfully completed the first candidature milestone, contributing to the training and capacity-building component of the project.