Characterizing spray deposits from variable- and constant-rate spray technologies: implications for future optimization to target trunk and foliar pests

Nursery producers use multi-row blocks to increase their land area usage and efficiency; however, spray droplets reaching interior rows can be a challenge. An air-blast sprayer (Storm 2000, Tifone) retrofitted with laser-guided, variable-rate technology was used to assess spray characteristics on multi-row blocks of 2.4-m tall red maple trees using a combination of D6-DC25 and D10-DC46 hollow-cone nozzle tips. The sprayer was operated in variable-rate mode (plant characteristics controlled each nozzle output in real-time) and constant-rate mode (all nozzles discharged the maximum output). Two 3-row blocks of trees were separated by an internal driveway and were sprayed either from external driveways only or external and internal driveways. Water sensitive paper (WSP) was wrapped at 3 heights around the trunk and WSP cards were placed in the canopy and on the ground in exterior and interior rows of each block. Another set of WSP cards was placed at 3 heights beyond the block to assess drift. Constant-rate mode applied 2,060 L ha^‑1 and variable-rate mode applied 290 L ha^‑1 (p=0.0047). For each card location, including ground and airborne drift, constant-rate mode coverage exceeded the overspray threshold for targets of 30% and was greater than variable-rate mode (p<0.05). Coverage for canopy cards exceeded 40% when sprayed from both external and internal driveways, exceeding the overspray threshold, and was greater than when sprayed from external driveways only (p<0.0001). For wraps, total coverage was greater in constant-rate mode (69.4%) than variable-rate mode (38.3%) (p<0.0001). For directional portions of WSP wraps, trees had greatest coverage on the side proximal to the sprayer (74.4%) and least coverage on the side distal to the sprayer (43.7%) (p<0.0001). If high coverage is needed around the entire trunk to control trunk-infesting pests, spraying from the internal driveways as well as external driveways appears to be necessary. For foliar applications to shade trees, it may be possible to achieve adequate coverage and reduce volume by spraying from only external driveways or using variable-rate mode. Future research should focus on optimizing recently developed variable-rate technology to achieve greater trunk coverage such as for dormant oil applications for scale and overwintering mites, and applications for trunk-boring insects.