Quantification of root phosphite concentrations for evaluating the potential of foliar phosphonate sprays for the management of avocado root rot

In South Africa, phosphonate trunk injections are widely used in a preventative management strategy against avocado root rot caused by Phytophthora cinnamomi. Due to increasing costs, alternative application methods must be investigated. The efficacy of different phosphonate foliar spray treatments was evaluated in two trials that were each situated in a climatically different region. Efficacy was evaluated through quantification of root phosphite (breakdown product of phosphonates) concentrations at different time points, following fall and summer applications. Since no high-throughput cost-effective analytical methods are available for phosphite quantification from avocado roots, a phosphite extraction and purification method was first developed, from which phosphite was quantified using a publically available liquid chromatography-mass spectrometry (LC-MS/MS) method. Foliar potassium phosphonate sprays, applied as three weekly sprays (full- and ¾ volume sprays) in fall, did not result in significantly lower root phosphite concentrations (8, 12 and 23 weeks after application) than the trunk injection. This was also true for two potassium phosphonate foliar sprays applied in summer (8 and 14 weeks after application) in the one trial. However, in the other trial, the summer applied potassium phosphonate foliar sprays had significantly lower root phosphite concentrations than the trunk injection. Ammonium phosphonate foliar sprays, three sprays applied in fall and two in summer, consistently yielded higher or similar root phosphite concentrations than the trunk injection. The ammonium phosphonate foliar sprays furthermore yielded significantly higher root phosphite concentrations than the corresponding potassium phosphonate foliar spray treatment. This was true for almost all time points, except 8-weeks after the summer application in one trial. Phosphite fruit residues were significantly higher for the foliar spray treatments than for the trunk injection in the one trial, but in the other trial it was similar or lower.