Effect of phosphonate application timing and -strategy on phosphite fruit and root residues of avocado

Abstract

In South Africa, phosphonate fungicides are widely used in avocado in a preventative management strategy against Phytophthora root rot (PRR), caused by Phytophthora cinnamomi. Trunk injections, curative and preventative (0.3-g a.i./m2 and 0.5-g a.i./m2 of canopy area respectively), done as two annual applications (fall and summer) are widely used to manage PRR. However, these applications have become problematic due to frequent exceedances of the European Union (EU) maximum residue level (MRL) of phosphonic acid (synonym phosphite) in fruit. Our study showed that fruit residues can be reduced by the use of specific phosphonate application methods (foliar versus trunk injections), and application timing (fall versus summer). In the 2016/17 season, trunk injections applied only in fall (1-g a.i./m2 of canopy area), or applied at the registered curative split dosage in fall (0.5-g a.i./m2) and summer (0.5-g a.i./m2), resulted in exceedances or near exceedances of the MRL for some replicates among treatments. In comparison, 3% a.i. ammonium- or potassium phosphonate foliar sprays applied only in fall had fruit residues that were consistently well below the EU MRL. The 2% a.i. foliar sprays, specifically the ammonium phosphonate formulation, done as a split application in fall (1.5% a.i) and summer (0.5% a.i.) sometimes resulted in significantly higher fruit residues than fall only applications. Some replicates of this treatment nearly exceeded the MRL. In the 2017/18 season, 2% and 2.5% a.i. foliar sprays (ammonium- and potassium phosphonate) applied only in fall yielded fruit residues that were consistently below the MRL. In contrast, the registered split dosage trunk injections (curative and preventative) sometimes resulted in exceedances of the MRL or near exceedances. In both seasons, foliar sprays at the start of the season yielded phosphite concentrations in roots that did not differ significantly from trunk injections, but towards the end of the season, concentrations were sometimes significantly lower. Foliar sprays applied only in fall have the potential to reduce fruit residues.