Distribution characteristics of droplets generated by air-assisted tower orchard sprayers with different numbers of nozzles

Abstract：【Objective】Apples (Malus domestica ) are perennial crops, and pests and diseases occurring on it are complicated. It is necessary to adopt suitable application methods to control them. Air-assisted sprayer is the dominating equipment used in large-scale apple orchards in China. In this study, the application of air-assisted sprayers with different numbers of nozzles in apple orchards were evaluated by investigating the droplet characteristics, pesticide utilization ratio and distribution.【Methods】The experiments of this study were conducted in a dwarf- rootstock apple orchard with an intensive planting pattern in Weihai, Shandong. The five-year-old apple variety (M. domestica‘Yanfu 3’) was used as the experimental materials. The regeneration rate of different concentrations of allura red on apple leaves and culture dishes were investigated to determine the applicability of allura red. In addition, the standard working curves of allura red were constructed. The experimental plot was 50 m in length, and three separated apple trees were randomly selected for sampling. To measure the deposition of droplets, 20 g allura red was dissolved in 100 L of water, and the solution was added into a 3WF-1000 air-assisted sprayer. Then the spraying was implemented along both sides of the trees using a 16-nozzle sprayer or a 32-nozzle sprayer with uniform pressure and wind speed. Water consumption was determined by recording the initial amount and remaining amount of water. The volume median diameter (VMD), droplet density and coverage were measured using water-sensitive paper at different canopy positions (2.0 m, 1.5 m and 1.0 m) in five directions (east, south, west, north and middle). The utilization ratio was investigated by detecting the concentration of allura red deposited on leaves in different canopy positions (2.0 m, 1.5 m and 1.0 m) of the apple trees. The loss index was determined by detecting the concentration of allura red deposited on petri dishes in four directions (east, south, west and north).【Results】The water consumption of the 3WF-1000 air-assisted tower sprayer with 32 nozzles (284.63 L per 667 m² ) was about twice that of the sprayer with 16 nozzles (148.25 L per 667 m² ). The VMD (154.80 μm) and coverage (72.00%) of the sprayer with 32 nozzles were higher than that with 16 nozzles (142.69 μm and 62.20%, respectively), while the droplet density generated by the 32 nozzle sprayer (123.01 points·cm^{- 2} ) was lower than that generated by the 16 nozzle sprayer (141.72 points·cm^-2 ). The droplet density in the east, south, west, north and middle of the canopy was higher in 16 nozzle treatment than in 32 nozzle treatment, and the difference in droplet density in the west and north sides of the canopy was significant (p ＜ 0.05) between the two treatments. The VMD of 16 nozzle sprayer was lower than that of 32 nozzle sprayer in all directions except for the east, where VMD in the 16 nozzle treatment (152.44 μm) was slightly higher than in the 32 nozzle treatment (146.33 μm). The coverage of the 16 nozzle treatment was lower than that of the 32 nozzle treatment, the difference being significant (p ＜ 0.05) in the east, west and north of the canopy. The average foliar deposition in the 32 nozzle treatment (0.24 μg · cm^{- 2} ) was slightly higher than that of the 16 nozzle treatment (0.22 μg · cm^{- 2} ). The foliar deposition in the 32 nozzle treatment in the inner canopy was higher than that in the outer canopy, with a droplet deposition ratio of 1.58 and 1.14 (inner∶outer) for the middle and lower canopy, respectively; the foliar deposition in the 16 nozzle treatment in the outer canopy was higher than that in the inner canopy, with a droplet deposition ratio of 0.74 and 0.62 (inner∶outer) for the middle and lower canopy, respectively. The ground deposition in different treatment was significantly different (p ＜ 0.05). The ground deposition and ground loss index in the 32 nozzle treatment, 0.64 μg · cm^{- 2} and 25.1%, respectively, were significantly (p ＜ 0.05) higher than those in the 16 nozzle treatment, i.e., 0.43 μg·cm^-2 and 19.4%, respectively. The utilization rate in the 32 nozzle treatment (36.9% ) was lower than in the 16 nozzle treatment (43.7% ). This might be due to the accumulation of liquid on the leaves resulting in the increase in ground loss index, which in turn reduced the utilization rate of pesticides.【Conclusion】In summary, with the increase in nozzle number, the VMD, coverage, foliar deposition, ground deposition and loss index increased, but droplet density and utilization rate decreased. The droplet density and droplet diameter generated by the 3WF- 1000 air- assisted tower sprayers with both 16 and 32 nozzles satisfy the need for pest and disease control.