Impact of Spray Concentration and Application Frequency to Modulate Phosphonic Acid Residues in Container-Grown Grapevines

This study investigated the translocation and persistence of inorganic phosphonate in container-grown vines of <i>Vitis vinifera</i> L. cv. Riesling after foliar and soil applications over two consecutive years. Phosphonate concentrations were monitored in leaves, petioles, grape canes, shoot tips, inflorescences, and berries during the season, applying an identical total amount of 3 or 4 sprays of 0.54 or 0.4% (w/v, aq) phosphonate, respectively. The overall uptake of inorganic phosphonate into the leaves was either identical (year 1) or substantially lower (year 2) when spraying 3 times (0.54%) instead of 4 times (0.4%) as expressed by the area under the concentration vs time curve. Residues found in leaves at the end of the vegetation period were also lower when spraying 3 times. Across both years, residues in berries were also significantly lower when applying the 0.54% phosphonate solution (20.2-30.9 mg/kg) 3 times as compared with the 4× application of 0.4% phosphonate (38.5-40.6 mg/kg). Soil applications resulted in a comparably low overall uptake but still yielding measurable residues in berries (6.0 ± 1.2 mg/kg). Further data on grape cane, shoot tips, and inflorescences supported the hypothesis that phosphonate residues in the plant and, ultimately, in the berries and the resulting products might be significantly reduced when spraying 3 times (0.54%) instead of 4 times (0.4%).