2019 ASHS Annual Conference

Variable-rate spray technology that applies pesticides based on real-time scanning laser rangefinder measurements of plant presence, size, and density was developed and retrofitted to existing sprayers. The objective of this experiment was to characterize spray application at four rates when applied to Malus domestica ‘Golden Delicious’ apple trees using this technology.

Four pairs of uniform trees were selected, and four clips for water sensitive cards (WSCs) were placed equidistant from one another within each canopy at increasing distances from the driveway and approximately 175 cm from the ground. A fifth card was placed below card 1 to detect non-target spray to the orchard floor. One of four rates, 0.03, 0.05, 0.07, or 0.09 fl oz/ft³, was randomly assigned to each pair of trees within the row so that all four rates were tested on each run. WSCs were placed in the clips, and the trees were sprayed with water. Rates were randomly re-assigned to trees, and a WSC was placed in each clip prior to each of the three subsequent runs. Cards were collected and analyzed using DepositScan software. Then, the sprayer was operated at each rate in the same random order as previously to spray cards, and volume was recorded. Each rate was repeated three times. There was no year effect so data were pooled.

Coverage and deposit density were affected by rate (P-value <0.0004, or lower) and card position (P-value <0.0001). Coverage decreased with increasing distance from the sprayer. Coverage decreased between cards 1 and 2, 2 and 3, and 3 and 4, 12%, 14%, and 22%, respectively. Non-target coverage on card 5 (Southern row) exceeded 20% at the two highest rates.

Deposit density was lowest for the cards closest to the driveway (cards 1 and 2) at the two highest rates due to coalescence. Densities at each rate met or exceeded the recommended thresholds (insecticides 20-30 droplets/cm²; fungicides 50-70 droplet/cm²).

Spray volume correlated with rate (R²=0.9999). Volume applied from each spray rate differed from one another (P-value <0.0001 for each comparison) and ranged from 5.9 gallons (0.03 fl oz/ft³) to 14.4 gallons (0.09 fl oz/ft³).

On average, the lowest rate reduced non-target, orchard floor pesticide coverage by 81% compared to the highest rate in the study. Applying 0.03 fl oz/ft³ with a dynamic laser-based sprayer achieved or exceeded deposit density efficacy levels and would reduce spray volume 84% per acre compared with the tree row volume method and 72% compared to the grower’s standard application of 125 gal/acre.