Phytophthora root rot, caused by Phytophthora cactorum, is an economically important disease on young apple trees. Limited information is available on the effect of different phosphonate application methods and dosages on disease control, fruit and root phosphite concentrations, and soil and root pathogen inoculum levels. Evaluation of phosphonate treatments in three apple orchard trials (two in the Grabouw and one in the Koue Bokkeveld region) showed that foliar sprays (ammonium or potassium phosphonate), trunk sprays and trunk paints, were equally effective at increasing trunk diameter in one trial and yield in a second trial over a 25-month period. Foliar ammonium and potassium phosphonate sprays (12 g of phosphorous acid/tree), and two different dosages of the ammonium phosphonate sprays (∼4.8 g or 12 g of phosphorous acid/tree) were all equally effective at improving tree growth. The addition of a bark penetrant (polyether-polymethylsiloxane-copolymer) to trunk sprays did not improve the activity of trunk sprays. The low dosage ammonium phosphonate foliar spray (∼4.8 g a.i./tree) was the only treatment that, in general, yielded significantly lower root phosphite concentrations than the other phosphonate treatments. Root phosphite concentrations were significantly positively correlated (P < 0.0001) with an increase in trunk diameter and negatively (P < 0.0001) with P. cactorum root DNA quantities. Phosphite fruit residues were <31 ppm for all treatments, with the trunk paint treatment (80 g of phosphorous acid/tree applied annually) yielding significantly lower residues than the higher dosage foliar sprays (∼12 g a.i./tree). Twenty-one months posttreatment, most of the phosphonate treatments in all of the trials similarly significantly reduced P. cactorum DNA quantities estimated directly from roots, but not from soil based on soil baiting DNA analysis. Pathogen quantities in fine feeder roots did not differ significantly from those in higher-order roots (<5 mm diameter). P. cactorum DNA quantities estimated using DNA quantification directly from roots were significantly correlated (P < 0.0001) with those obtained through root leaf baiting DNA analysis and, to a lesser extent, with soil leaf baiting DNA quantities (P = 0.025).

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.