Advancing sustainability in tree crop pest management: refining spray application rate with a laser-guided variable-rate sprayer in apple orchards

Authors: FESSLER, LAUREN - University Of Tennessee; FULCHER, AMY - University Of Tennessee; LOCKWOOD, DAVE - University Of Tennessee; WRIGHT, WESLEY - University Of Tennessee; Zhu, Heping

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2020
Publication Date: 10/20/2020
Citation: Fessler, L., Fulcher, A., Lockwood, D., Wright, W., Zhu, H. 2020. Advancing sustainability in tree crop pest management: refining spray application rate with a laser-guided variable-rate sprayer in apple orchards. HortScience. 55(9):1522–1530. https://doi.org/10.21273/HORTSCI15056-20.
DOI: https://doi.org/10.21273/HORTSCI15056-20

Interpretive Summary: is a common practice that apple growers apply pesticides liberally, spraying to the point of runoff to ensure sufficient control of pests. Air-assisted sprayers are typically used to spray an entire farm at a constant rate regardless of variations in tree canopy characteristics. This practice has caused considerable pesticide waste due to off-target loss and spray drift. In this research, a newly developed laser-guided variable-rate intelligent sprayer was introduced into apple orchards for improving pesticide application efficiency. The sprayer applied spray volume based on canopy foliage volume instead of conventional gallons per acre. Spray applications at four spray rates per foliage volume were characterized in a commercial apple orchard to refine spray rate recommendations for tree crops. The new intelligent spray technology at all four rates decreased total spray volume outputs while maintaining adequate coverage and deposit density throughout the canopy. The lowest rate applied, 0.03 liter of spray volume per cubic meter of foliage volume, reduced spray output by 83% compared to the grower’s standard application practice. These results provided a baseline for further investigations of pest control levels achieved with the combination of variable-rate technology and low application rates, and also enhanced the understanding of new pesticide spray equipment operated in commercial scale orchards.

Technical Abstract: Advanced variable-rate spray technology, which applies pesticides based on real-time scanning laser rangefinder measurements of plant presence, size, and density, was developed and retrofitted to existing sprayers. Experiments were conducted to characterize the application of four programmed spray rates (0.03, 0.05, 0.07, or 0.09 L·m-3 of crop geometric volume) when applied to Malus domestica Borkh. ‘Golden Delicious’ apple trees using this crop sensing technology. Water sensitive cards (WSC) were used as samplers to quantify spray coverage, deposits, and deposit density in the target areas. Spray volume discharged ranged from 22.3 L at the programmed spray rate of 0.03 L·m-3 to 54.5 L at the rate of 0.09 L·m-3. For a given WSC position, spray coverage and deposits increased as the spray rate increased. The effect of spray rate on deposit density varied with WSC positions, with high densities achieved by low spray rates for WSC closest to the sprayer but by high spray rates for WSC positioned either deeper within or under the canopy. When coalescing deposits were accounted for, deposit densities met or exceeded the recommended pesticide application thresholds (insecticides 20-30 droplets·cm-2; fungicides 50-70 droplets·cm-2) at all WSC positions for each spray rate tested. The lowest spray rate reduced off-target loss to the orchard floor by 81% compared to the highest rate, dramatically reducing potential exposure to non-target organisms, such as foraging pollinators, to come into contact with pesticide residues. Applying the lowest rate met deposit density efficacy levels while reducing spray volume by 83% compared to the orchard standard application of 1540 L·ha-1 and by 87% compared to the 1950 L·ha-1 application rate recommended when using the tree row volume method. Thus, there is potential for growers to refine pesticide application rates to further achieve significant pesticide cost savings. Producers of other woody crops, such as nursery, citrus, and grapes, who use air-assisted sprayers may be able to achieve similar savings by refining pesticide applications through the use of laser rangefinder-based spray application technology.