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

What was it all about?

This investment, which ran from 2019 to 2023, improved the avocado industry’s understanding of how the fungicide phosphite is metabolised by trees infected with Phytophthora root rot, so that applications of phosphite can become more targeted and effective, resulting in healthier and more productive orchards.

Effects of phosphite on growth and inherent plant defence responses were studied in glasshouse and laboratory trials, while links with the carbohydrates starch and soluble sugars were investigated by comparing various tissues at different times from phosphonate sprayed versus unsprayed trees. The results show there are no growth stimulation effects of phosphite, in the absence of the pathogen. That is, phosphite is not acting as a fertiliser.

Quantitative PCR analysis demonstrated that prior to and during infection by Phytophthora cinnamomi, phosphite primes and activates genes associated with the host defence response in roots, which has previously been shown in model plant species, but not in avocado.

Phosphite clearly impacts carbohydrate availability and/or metabolism in avocado. At the autumn sampling, there was a trend for starch (and phosphite) to be increased in expanding flush leaves, corresponding to reductions in soluble sugars in those tissues. The reverse occurred in mesocarp (flesh) of maturing fruit. This trend was not observed so clearly at other sampling times. Overall, a positive correlation between phosphite and the soluble sugars glucose, fructose and sucrose was demonstrated.

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 southeast Queensland. Shifted timing of applications in southeast 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.