Using Laser-guided Variable-rate Spray Technology with Reduced Rates to Control Pests and Decrease Off-target Movement in a Tall Spindle-trained Apple Orchard

Apple orchardists in the United States protect their crops from numerous insects and diseases using foliar pesticide applications to produce blemish-free produce; however, doing so incurs significant labor and input costs. Variable-rate spray application technology used at the default spray rate has successfully reduced pesticide volume and, consequently, input costs in orchards and nurseries while effectively controlling several diseases and insects. Our objectives were to evaluate spray characteristics and the pest control efficacy of a sprayer operated in the variable-rate mode at a lower rate than the default rate and compare them with those associated with the conventional constant-rate spray mode. Through preliminary experiments, a reduced spray rate of 0.05 L·m −3 of crop volume, which equated to 321.1 L·ha −1 , was selected and compared with a conventional constant-rate of 808 L·ha −1 using a sprayer retrofitted with variable-rate spray technology and operated in the two different modes throughout two growing seasons. Foliage was scouted biweekly for diseases and two arthropods, and apples were scouted weekly for disease. The variable-rate mode reduced the pesticide volume applied by 58% and maintained pesticide coverage at or above the overspray threshold at all canopy locations except for one. Nontarget ground applications were greatly reduced using the variable-rate mode. Foliar disease measured as leaf spot incidence and leaf spot count severity was unaffected by the spray mode. Fruit rot incidence, fruit rot severity, and disease index for fruit were also unaffected by the spray mode. Fruit disease index, which incorporates both incidence and severity measurements, remained low throughout both seasons. Disease progression was assessed by calculating the area under the disease progress curve (AUDPC). The foliar AUDPC was significantly higher in the variable-rate treatment at the middle of hill location, but not at the bottom or the top of the hill, compared with that of the constant-rate treatment. The fruit AUDPC was unaffected by treatment. Although normally common in apple orchards, neither of the two arthropods was detected on any date. The utility and limitations of existing spray characterization metrics as well as the need for yet-to-be-developed metrics are discussed.