Comparison of low-volume spray application rates using UASS: implications for vine canopy deposition, spray quality, and in-field ground losses

Purpose This study aimed to identify stage-specific UASS operational settings for canopy-targeted one-way band spray mode in a vineyard. To this end, the trade-offs between canopy deposition, in-field ground losses, and spray quality were quantified across three growth stages. Methods Field trials were conducted at three growth stages (early, middle, late) using a canopy-targeted one-way band spray mode at fixed flight height. Five operational settings were tested, corresponding to application rates of 54.2, 72.9, 90.6, 105.8, and 142.2 L ha− 1 under the selected nozzle and pressure configurations. A dye tracer was used to quantify deposits on collectors placed in the canopy and on the ground. Spray quality was assessed with water sensitive papers to derive spray coverage and coverage density on the upper and lower leaf sides. Results Absolute canopy deposition increased with application rate; however, the vertical distribution consistently skewed towards the upper canopy, and this tendency became more pronounced at later growth stages, indicating reduced deposits within the grape band. In-field ground losses increased sharply with application rate (+ 233% from 54.2 to 142.2 L ha− 1). Spray coverage also increased with application rate, whereas coverage density showed a significant growth stage - application rate interaction, indicating stage-dependent optima. Conclusions Under the tested configurations, the most promising stage-specific operational settings were ISO 015 at 0.30 MPa (54.2 L ha− 1), ISO 03 at 0.30 MPa (105.8 L ha− 1), and ISO 04 at 0.25 MPa (142.2 L ha− 1) for the early, middle, and late stages, respectively. These outcomes should be interpreted as configuration-specific operational recommendations under the tested nozzle, pressure, and flight height conditions, rather than as universally applicable phenology-adjusted volume prescriptions. Further improvements in spray delivery to the grape band will likely require adjustments in flight height in addition to application rate selection, although this will depend on the trellis system adopted in the vineyard.