Phosphorous acid residues in apples after foliar fertilization: results of field trials.

The levels of phosphorous acid residues in apples after foliar fertilization with P fertilizers and after treatment with a phosphonate fungicide (Fosetyl-Al) were determined and compared. Two field trials and a glasshouse experiment, using different genotypes and plants of different age, were carried out and monitored over a three-year period. Phosphorous acid residues were found in apples after application of foliar P fertilizers. Concentrations of the residues ranged between 0.02 and 14 mg kg(-1) depending on the phosphorous acid content in the fertilizer used and the plant size and yield. The treatments induced an accumulation of the residue in the course of the experiments, which in some cases reached a level exceeding the maximum limit set by EU legislation. Residues were also detected in other plant organs, i.e., roots and buds. Plants treated with Fosetyl-Al contained phosphorous acid residues in their fruits and buds two years after the suspension of the treatment, suggesting a long-term persistence of the substance in plant storage organs. A second experiment, involving treatment of trees with seven foliar fertilizers of different composition, also induced accumulation of phosphorous acid residues in fruits. It is concluded that a wide array of foliar products containing phosphorous acid, even as a minor component, could mimic the residue effect of phosphonate fungicide treatments.