Foliar deposition and coverage on young apple trees with PWM-controlled spray systems

Pulse width modulation (PWM) technology is recently introduced into orchard air-blast sprayers to achieve precision applications of plant protection products and reduce risks to the environment and ecosystems. Field tests in a two-year old, multi-row apple orchard were conducted to study the foliar spray deposition generated by three systems attached to an air-blast orchard sprayer: 1) a manually-PWM-controlled spray system producing a constant-rate application (Manual-PWM), 2) a laser-guided PWM-controlled intelligent spray system producing a variable-rate application (Laser-PWM), and 3) the same laser-guided spray system with disabled PWM control producing a constant-rate application (Disabled-PWM). The capability of applying sprays on multiple tree rows beyond the first target row was also quantitatively evaluated for the three systems. Test results showed that the Manual-PWM and Disabled-PWM produced greater spray deposits and coverage with over-sprayed potentials on the first target row trees than Laser-PWM. However, spray coverage results indicated that Laser-PWM provided higher uniformity across the tree canopy and a stronger relationship between spray deposits and coverage for all the rows than other two systems. Moreover, Laser-PWM used 67% and 76% less spray volume than Manual-PWM and Disabled-PWM, respectively. Test results also demonstrated that spray deposit densities on trees discharged from all three spray systems exceeded thresholds for effective fungicide and insecticide applications (proposed by pesticide manufacturers), not only in the first target row but also second and further downstream rows. A single spray pass from each of the three systems could provide adequate spray deposits and coverage on the two-year old young trees in the first two rows, thereby offering opportunities to save pesticides and reduce labor costs. Future field biological tests to verify the efficacy of these spray systems are necessary to confirm these hypotheses.