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

Effects on growth and inherent plant defence responses were studied in glasshouse and laboratory trials, while links with the carbohydrate cycle, starch and soluble sugars, were investigated by comparing various tissues at different times from phosphonate sprayed versus unsprayed trees. Accumulation of phosphite in fruit pulp was studied after sprays at different times in three field trials.   

The mode of action studies have shown that there are no growth stimulation effects of phosphite, in the absence of the pathogen. That is, phosphite is not acting as a fertiliser. At low concentrations phosphite primes the host defence response in avocado roots, which has previously been shown in model plant species, but not in avocado. A positive correlation between phosphite and carbohydrates, specifically with soluble sugars glucose, fructose and sucrose, in avocado has been demonstrated, and more detailed analyses are currently underway. 

Extensive field trials have helped to understand translocation patterns of phosphite, and the differences amongst climatic regions and tree age.  Phosphite accumulates in fruit when applied in the window preceding commercial harvest, whether it be the summer application window to Shepard trees in north Queensland, or autumn/winter applications to Hass in south east Queensland. Shifted timing of applications in SE Queensland shows promise for fewer applications at more favourable times of the year for more sustained translocation of phosphite to roots, where it is required. 

Industry support, education and extension activities have been key components of the project. Project outputs will facilitate more targeted and effective applications of phosphite, contributing to healthier and more productive orchards.

Glasshouse and laboratory trials investigating the modes of action of phosphite, and their relative contribution to suppression of Phytophthora are progressing. Glasshouse trials have demonstrated that at relatively low concentrations of phosphite in roots, 30-60 mg/kg, there is an activation of the plant defence pathway, correlating with a reduction of P. cinnamomi infecting roots. This has not previously been reported in avocado, and suggests that under low disease pressure, healthy trees are able to defend themselves against infection at relatively low concentrations of phosphite.

Analyses of starch, soluble sugars and phosphite in different tissues collected between April and October 2022 are continuing. Results of starch accumulation in phosphite sprayed vs untreated trees from April 2022 tissues were inconsistent with those obtained from similar analyses in April 2021, where phosphite treatment had significant effects on starch accumulation in most tissue types, and indicates the complexity of the interactions.

The field trials at three locations are in final stages of tissue collection and analyses. In the trial at Capel, WA, with young trees, phosphite concentrations in all tissues remained very low throughout the trial period, confirming previous results and reports from growers that it is difficult to achieve elevated (recommended) concentrations in young (2-4 y.o.) trees. Despite the low root levels, the trees are still looking very vigorous and healthy. It is likely that younger trees with smaller canopies don’t capture and “store” as much phosphite as older trees. If trees are healthy, and other methods of integrated management are being adopted, e.g. good drainage, mulching, etc., then these apparently low concentrations should not be concerning. 

Results from the Shepard trial at Walkamin (QLD) indicated that sprays during both autumn and summer application windows are necessary to achieve good root phosphite concentrations in this high rainfall, high risk and vigorous growing region. The field trial at Ravensbourne, QLD, has been the most comprehensive of the three. Soil drench treatments were included, as well as sprays to leaf flush before hardening. Trial results have shown that soil drench applications of a particular phosphonate product, not registered with APVMA for management of Phytophthora root rot, was not effective at sustaining adequate concentrations in roots, even when applied at 2-3+ times the recommended label rate. Sprays of registered product, as per label rates, applied February to April (leaf flush) and May to June (traditional autumn/winter timing) were effective at enhancing concentrations in roots. At both sites, sprays resulted in phosphite residues in pulp at commercial harvest, and, while still under the maximum residue limit (MRL) set for domestic market, exporters will need to be mindful not to exceed MRLs for key international markets. Another consideration is timing of pruning operations, with respect to phosphonate applications to ensure adequate canopy is maintained for efficiency of translocation of phosphite to the roots.

There have been several communication and extension activities in the June to November 2022 reporting period. Two dedicated disease management workshops have been delivered to growers and industry stakeholders in Port Macquarie, NSW and Walkamin, QLD, which included an outline of project activities and brief update on progress. There was also a presentation at the Tamborine/Northern Rivers regional forum, focusing on management of Phytophthora root rot, including use of phosphonates. There were two presentations on project outputs at the Australasian Soilborne Disease Symposium, Cairns, August 2022. Two abstracts have been submitted for presentation at the World Avocado Congress, Auckland, NZ, April 2023.

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.