1. Introduction
Solid Set Canopy Delivery Systems (SSCDS) represent a novel agrochemical application technology for high-density fruit production. SSCDS consist of a series of stationary microsprayers that are distributed throughout the orchard and fed from a common pumping station. This technology promises a rapid and precise method of chemical application, while removing personnel and heavy machinery from the orchard during pesticide applications. Immediate benefits of SSCDS include the minimization of worker pesticide exposure [
1,
2] as well as tree damage and soil compaction from heavy, tractor based sprayers [
3,
4,
5]. Fixed emitter applications have been investigated sporadically for decades, beginning with the use of overhead frost protection sprinklers for agrochemical delivery, but these systems proved inefficient due to their reliance on a relatively small number of high volume emitters [
6]. More recent research by Agnello and Landers [
7] evaluated an SSCDS composed of a large number of low-cost plastic micro-sprinklers and pressure gated valves. This proof of concept has been expanded on by Sharda et al., and Owen-Smith et al., with results suggesting that a SSCDS could replace airblast spray application in high-density orchards [
8,
9,
10].
The transition to high-density fruiting walls has resulted in a tall and narrow, planar tree architecture that is ill suited to applications by axial fan radial airblast sprayers [
11,
12,
13]. This thin canopy profile, in conjunction with the high wind speeds associated with radial airblast sprayer design, commonly results in over-spraying of the lower and middle portion of the canopy and under-spraying of the top canopy portion [
13,
14,
15]. Furthermore, large amounts of chemical applied by airblast sprayers are wasted when they are discharged into the local environment and atmosphere research as non-target drift [
11,
16,
17,
18,
19]. These issues are exacerbated by sprayers that are poorly calibrated or optimized, with many growers applying a ‘one size fits all’ approach to the diverse canopy architectures presented by plantings of different training systems and phenological stage [
13,
20,
21,
22].
There are two main approaches to testing spray systems: observing pathogen and disease suppression and measuring spray deposits and coverage [
23,
24]. Deposition measurements determine the quantity of a chemical sprayed onto a surface, expressed in mass per area. Targets are sprayed with a tracer compound, and then collected and washed to recover the applied material. An effective method to quantify the recovered tracer utilizes absorption spectrophotometry and dye [
25,
26]. Coverage measurements describe the extent of the treated area, expressed as a proportion of the surface that receives treatment or contacts droplets [
23]. Coverage demonstrates the extent of the treated area as well as the uniformity and quality of the spray, but lacks information on the total amount of chemical retained on target surfaces. Target cards that change color when contacted by droplets are placed within the canopy to mimic leaves, and then sprayed in a simulated application. Spray coverage has been used extensively in previous research into solid set canopy delivery systems [
8,
27,
28] and airblast sprayers [
20,
23,
29,
30].
An SSCDS consisting of arrays of microsprinklers placed above and within a high-density apple canopy in Michigan USA provided the same level of pest management as an airblast sprayer. However, coverage evaluations performed as part of this experiment showed reduced coverage on the underside of leaves for the SSCDS compared with the airblast sprayer [
10]. Studies conducted in high density apples in Washington State USA showed reduced coverage and deposition from an SSCDS compared to an airblast sprayer [
31]. Similar experiments in Quebec, Canada and France evaluating SSCDS in high-density apples showed more heterogeneous spray deposits in SSCDS treated trees, yet both airblast and SSCDS had comparable efficacy in suppressing insect pests and diseases [
28,
32,
33].
Based on the available literature it is clear that SSCDS provide more heterogeneous coverage especially on abaxial leaf surfaces compared to airblast sprayers but are capable of providing acceptable levels of insect pest and pathogen suppression. However, none of these studies have combined coverage, deposition and pest management efficacy measurements. Furthermore, the bulk of previous work either evaluated coverage at only one or two time points and in very small SSCDS systems. Thus, the specific objectives of this study were to:
Quantify spray coverage on both the upper-side (adaxial) and under-side (abaxial) of leaf surfaces throughout a high density apple canopy at multiple time points.
Quantify spray deposition on leaf surfaces at different levels within the canopy at multiple time points using a tartrazine tracer dye and absorption spectrophotometry.
Evaluate season-long pest management of the SSCDS and its ability to suppress arthropod pests and plant pathogens compared to an airblast sprayer.
3. Results
3.1. Adaxial Coverage
A repeated measures ANOVA on data from the three months with balanced comparisons (June, July, and August) showed a significant difference in adaxial (upper surface) coverage attributed to the main effect of treatment (F
1,6 = 15.92,
p = 0.0072), and the interaction of treatment and height fixed effect terms (F
2,12 = 5.66,
p = 0.0186). Airblast treated plots had significantly higher coverage overall, and significantly higher coverage at the highest height on each date (
Figure 4). Date was also a significant fixed effect (F
2,44 = 9.13,
p = 0.0005). Multiple comparisons by month using the Tukey-Kramer adjustment show that airblast treated plots displayed significantly higher coverage in one month, August (t
α = 0.05,44 = 3.20, adj.
p = 0.0289), while in June and July they were not significantly different at the alpha = 0.05 level (t
α = 0.05,44 = 1.87, adj.
p = 0.4359; t
α = 0.05,44 = 2.89, adj.
p = 0.0625) (
Figure 4).
Comparisons between the two treatments at the same height showed a significant difference with greater airblast coverage on the highest sampled leaves (tα = 0.05,12 = 5.07, adj. p = 0.0029), but cards at the lower and middle heights did not show any significant differences (tα = 0.05,12 = 0.85, adj. p = 0.9517; tα = 0.05,12 = 2.03, adj. p = 0.3816). Different heights in airblast treated plots were not significantly different from each other: high and low (tα = 0.05,12 = 0.12, adj. p = 0.9046), high and middle (tα = 0.05,12 = −0.59, adj. p = 0.9896), and middle and low (tα = 0.05,12 = −0.71, adj. p = 0.9764). In comparison, different heights in SSCDS treated plots showed significant differences at the high and low heights (tα = 0.05,12 = −3.83, adj. p = 0.0226), but not at the high and middle (tα = 0.05,12 = −3.21, adj. p = 0.0641) or middle and low (tα = 0.05,12 = 0.62, adj. p = 0.9973).
Coefficients of variation for abaxial coverage were calculated for each plot for the June, July and August sampling dates. Application technology yielded a significant difference (F
2,6 = 127.84,
p < 0.0001), as well as height (F
2,12 = 10.36,
p = 0.0024), and date (F
2,36 = 8.28,
p = 0.0011). Coefficient of variation least square means were 0.5127 for the airblast and 0.9889 for the SSCDS, with a shared standard error of ± 0.029. Comparisons between least square means of coefficients of variation using Tukey’s adjustment were significantly different from each other in each of the three months, with a higher SSCDS σ²/µ at each date: June (t
α = 0.05,36 = −3.38, adj.
p = 0.0201), July (t
α = 0.05,36 = −5.86, adj.
p < 0.0001), and August (t
α = 0.05,12 = −5.41, adj.
p < 0.0001). Neither of the treatments displayed any significant differences with themselves across months-except for the SSCDS-where a significant difference occurred in σ/µ between July and August (t
α = 0.05,36 = −3.51, adj.
p < 0.0144) (
Table 3).
3.2. Abaxial Coverage
A repeated measures ANOVA on data from the three months with balanced comparisons (June, July, and August) showed a significant difference in abaxial (lower surface) coverage by treatment (F
1,6 = 200.72,
p < 0.0001) with significantly higher coverage from the airblast. The interaction of treatment and height fixed effect terms (F
2,12 = 12.88,
p = 0.0010) was also significant. Date was also a significant fixed effect (F
2,44 = 14.6,
p < 0.0001). Multiple comparisons by month using the Tukey-Kramer adjustment show that airblast treated plots displayed significantly higher coverage in all three months: June (t
α = 0.05,44 = 7.86, adj.
p < 0.0001), July (t
α = 0.05,44 = 10.18, adj.
p < 0.0001), and August (t
α = 0.05,44 = 10.48, adj.
p < 0.0001) (
Figure 5).
Comparisons between the two treatments at the same height yielded significant differences between at all three heights, with consistently higher coverage from the airblast sprayer: high (t
α = 0.05,12 = 12.57, adj.
p < 0.0001), middle (t
α = 0.05,12 = 6.07, adj.
p < 0.0001), and low (t
α = 0.05,12 = 9.89, adj.
p < 0.0001). Different heights in airblast treated plots were not significantly different from each other: high and low (t
α = 0.05,12 = 3.1, adj.
p = 0.0774), high and middle (t
α = 0.05,12 = 1.82, adj.
p = 0.4899), and middle and low (t
α = 0.05,12 = 1.27, adj.
p = 0.7942. In contrast, different heights in SSCDS treated plots yielded significant differences between the middle and low heights (t
α = 0.05,12 = −4.99, adj.
p = 0.0048) as well as between the middle and high heights (t
α = 0.05,12 = −4.99, adj.
p = 0.0033), but not between the high and low heights (t
α = 0.05,12 = −0.23, adj.
p = 0.9999) (
Figure 5).
Coefficients of variation for abaxial coverage were calculated for each plot for the June, July and August sampling dates. The fixed effect of application technology was significant (F
2,6 = 197.67,
p < 0.0001), with higher coefficients of variation for the SSCDS treatments. Interaction of date*treatment and treatment*height were also significant (F
2,36 = 5.45,
p = 0.0085; F
2,12 = 16.12,
p = 0.0004). Back-transformed σ/µ least square means were 0.563 for the airblast and 1.345 for the SSCDS. Comparisons between least square means of the coefficient of variation using Tukey’s adjustment were significantly different from each other in each of the three months, with a higher SSCDS σ/µ at each date: June (t
α = 0.05,36 = −8.97, adj.
p < 0.0001), July (t
α = 0.05,36 = −11.06, adj.
p < 0.0001), and August (t
α = 0.05,36 = −12.58, adj.
p < 0.0001). SSCDS σ/µ displayed significant differences between July and August (t
α = 0.05,36 = −4.57, adj.
p = 0.0007) as well as June and August (t
α = 0.05,36 = −4.68, adj.
p = 0.0005), while airblast plots did not. (
Table 4)
3.3. Deposition
Analysis using data from the June, July, and August trials showed significantly higher chemical deposition in SSCDS treated plots than in airblast treated plots (F
1,6 = 15.84,
p = 0.0073). The interaction of treatment and height fixed effect terms was also significant (F
2,12 = 7.41,
p = 0.0080). Date was also a significant fixed effect (F
2,36 = 16.41,
p < 0.0001). Multiple comparisons by month using the Tukey-Kramer adjustment show that SSCDS treated plots displayed significantly higher deposition in all three months: June (t
α = 0.05,36 = −3.09, adj.
p = 0.0413), July (t
α = 0.05,36 = −3.69, adj.
p = 0.0088), and August (t
α = 0.05,36 = −3.10, adj.
p = 0.0402) (
Figure 6).
Comparisons between treatments at the same height showed no significant differences between the deposition on the highest sampled leaves (tα = 0.05,12 = −1.16, adj. p = 0.8450), but leaves at the lower and middle heights showed significantly higher deposition from the SSCDS (tα = 0.05,12 = −4.58, adj. p = 0.0064; tα = 0.05,12 = −4.08, adj. p = 0.0149). Different heights in airblast treated plots were not significantly different from each other: high and low (tα = 0.05,12 = 2.40, adj. p = 0.2296), high and middle (tα = 0.05,12 = 1.23, adj. p = 0.8132), and middle and low (tα = 0.05,12 = −1.21, adj. p = 0.8221). Differences in deposition due to height in SSCDS treated plots were also non-significant: high and low (tα = 0.05,12 = −2.68, adj. p = 0.1519), high and middle (tα = 0.05,12 = −281, adj. p = 0.1239), and middle and low (tα = 0.05,12 = −0.18, adj. p = 0.8600).
Coefficients of variation (σ/µ) calculated for each height in each plot for the three dates yielded treatment as a significant effect (F
2,6 = 28.99,
p = 0.0017), as well as date (F
2,36 = 7.84,
p = 0.0015), but not height (F
2,12 = 2.16,
p = 0.1579). There were no significant interactions. Coefficient of variation least square means were 0.7185 ± 0.028 for the airblast and 0.9343 ± 0.028 for the SSCDS. Comparisons between least square means of the coefficient of variation using Tukey’s adjustment were significantly different from each other in June (t
α = 0.05,44 = −3.79, adj.
p = 0.0068) and August (t
α = 0.05,44 = −3.26, adj.
p = 0.0271), with a higher SSCDS σ/µ, but not in July (t
α = 0.05,44 = −2.28, adj.
p = 0.2299). (
Table 5)
A separate ANOVA model fitted to just SSCDS deposition data from all four dates (May, June, July, and August) resulted in a significant F-test for the main effect of date (F3,27 = 4.91, p = 0.0075). Deposition in July and August were both significantly lower than the deposition in June (tα = 0.05,27 = −3.28, adj. p = 0.0144; tα = 0.05,27 = −2.78, adj. p = 0.0454), but other date combinations did not display significant differences. Another ANOVA model was fitted to airblast deposition from June, July, and August, and showed the same pattern, with deposition in July and August significantly lower than the deposition in June (tα = 0.05,18 = −5.45, adj. p < 0.0001; tα = 0.05,18 = −3.84, adj. p = 0.0033), but July and August were not significantly different from each other (tα = 0.05,18 = −1.51, adj. p = 0.3087).
3.4. Pest Management
Apple scab damage evaluations did not yield any scores higher than 5% on leaves, terminals, or fruit in both treatments (
Figure 7). A single sign of leafroller damage was observed in airblast plots. Collected fruit with entries and frass did not yield any live larvae. Wilcoxon rank sum tests between the non-normal airblast and SSCDS plots did not show any significant differences except for the incidence of apple scab on clusters (
Figure 8 and
Table 6). No apple scab was found on fruit in airblast treated plots, while two incidences of 5% scab damage and 12 incidences of 2% scab damage were found on SSCDS treated fruit. Proportions of damaged fruit and leaves are displayed in
Figure 7;
Figure 8, along with an untreated comparison.
4. Discussion
Coverage evaluations collected in this experiment showed that the prototype SSCDS provides comparable levels of coverage on the adaxial leaf surface to an airblast sprayer (
Figure 4). However, SSCDS coverage on the adaxial surface is far higher than the SSCDS coverage on the abaxial surface, which confirms previous observations [
8,
28,
31]. Additionally, coverage on the underside of leaf surfaces is far lower when sprayed with the SSCDS than the coverage obtained with airblast spraying, and significantly lower in almost all cases (
Figure 5). Despite the low abaxial coverage, SSCDS sprayed plots exhibited equal or greater levels of chemical deposition on sampled leaves (
Figure 6), which implies less chemical was lost from the SSCDS sprays in the form of off target drift. Both systems demonstrated near identical levels of pest control, which is ultimately the most important characteristic of any spray application.
The adaxial coverage measurements only showed an overall significant difference between the two spray types in August, with similar levels of coverage in June and July. Most major sprays in Michigan are applied between mid-April and July, with only three sprays in the test orchard in August (
Table 2). This suggests that despite the lower adaxial coverage in August, the SSCDS can provide similar levels of coverage in the major portion of the growing season. However, SSCDS adaxial coverage was significantly lower at the highest sampled height (2.1 m) than the coverage it provided in the lower and middle portions of the canopy, and significantly lower than the adaxial coverage from an airblast at the same height. This sampling height was approximately 2/3 of the height of typical high density apple trees (2.5–3.3 m), and just under the height the highest set of sprinklers sprayed from. Coverage there could potentially be improved by changes in the height, arrangement, or number of the highest set of emitters to attain levels of coverage seen in the lower and middle portion of the canopy. Mean adaxial coverage was still well within the recommended range of coverage found in the literature, from 15% to 30% [
23,
36], falling within or above this range at all heights and dates.
Coefficients of variation of the adaxial coverage were around two-fold greater in SSCDS sprayed plots, indicating greater heterogeneity in coverage than the airblast. The magnitude of this difference was highest at the top of the canopy, where coverage was the lowest. Coefficients of variation were significantly higher in the SSCDS treatment compared to the airblast treatment at each date, with the biggest differences in August, when the SSCDS also showed significantly lower coverage than the airblast sprayer. This could be attributed to the full canopy development and foliar growth that occurs throughout the summer, blocking spray from the fixed emitters and resulting in lower and uneven coverage.
Abaxial coverage was significantly lower for the SSCDS at every height and on every date overall when compared to the airblast sprayer. It was lowest in the bottom portion of the canopy and the highest portion of the canopy. This is likely because the solid set lacks the moving air front from an axial fan, which both lifts and turns leaves [
37]. This action spreads fine droplets within a turbulent airstream so they either intercept the underside of leaves or are sprayed directly onto the upturned abaxial surface. The droplets delivered through the SSCDS are far less likely to reach the underside of the leaf unless it is located near the emitter and received direct spray or natural air movement carries droplets through the canopy. Additionally, SSCDS abaxial coverage exhibited significantly higher coefficients of variation than the airblast (
Table 4). In some cases the coefficients of variation was nearly four times greater than the corresponding coefficients of variation seen in airblast treated plots. This was greater than the disparity between coefficients of variation in adaxial coverage as well, showing abaxial surfaces not only receive less coverage, but have far more variable coverage when treated by the SSCDS.
It is important to note that the heterogeneous coverage and deposition referred to here is at the macroscale level of the tree or plot, rather than on the scale of individual droplets. The SSCDS not only exhibits variable coverage at the plot level, but also has a characteristically coarser distribution of droplets intercepting the spray cards. The coarse coverage at the plot level and the droplet level are likely related to some degree, the large splatters or light dusting of droplets on cards lead to much more variable overall percent coverage. However, the plot level heterogeneity of coverage can also be attributed to the static nature of the sprinklers.
Coverage variability on both surfaces and the low coverage on abaxial surfaces is likely caused by inherent properties of fixed spray systems. They may exhibit greater heterogeneity than airblast sprayers since spray interception is much more likely to occur closer to where it is emitted from nozzles. Leaves further from the nozzle are less likely to intercept droplets, especially if they are distant vertically. Larger droplets are subject to gravity rather than air currents, and are pulled downward once they lose momentum [
38]. This means adaxial surfaces receive a shower from above, but abaxial surfaces only receive droplets sprayed directly up onto them or the finest droplets that travel through the canopy environment on air currents. Literature has also been published on the local cooling effect provided by microsprinklers, which has been used for sunburn protection in apples [
31]. Data collected in this orchard has shown a 2–3 °C drop in temperature immediately following spray applications (Owen-Smith, unpublished). The cool air produced by this effect may also pull spray droplets downward as it sinks, contributing to the lower abaxial coverage and the lower coverage levels seen at the highest sampled portion of the canopy.
Deposition showed a very different profile than coverage: mean SSCDS deposition was greater at every height and date compared to the airblast sprayer. In fact, the lower and middle canopy heights had significantly higher deposition in the SSCDS plot compared with the airblast plots. Coefficients of variation for deposition, while higher in the SSCDS plots compared to the airblast sprayer plots were less pronounced compared to coverage, however, significant differences were detected in June and July. Overall higher levels of deposition in the SSCDS plots suggest that more chemical was retained on the leaf surfaces in SSCDS plots, and less may have been lost to drift.
This hypothesis is consistent with general observations on off-target applications from radial airblast sprayers. Mass balance experiments in dwarf apple trees have shown 10–12% of the spray volume was lost to the ground, and 37–59% lost to the air [
13], with 4–17% lost to the air and 10–22% lost to the ground in semi-dwarf trees. A separate study in Italian apple orchards has shown a loss of 37% of the spray to the ground and 7% to the air [
39]. Without the moving front of air pushing small droplets above the canopy or into the ground, it is likely that more droplets intercept leaves and are deposited. If 40–50% of the airblast spray was lost to the ground and air, the difference between the mean deposition for each treatment would have been negligible if it had not been off target. This suggest that the SSCDS has the potential to reduce off target deposits, and may serve to reduce drift and soil contamination compared to an orchard airblast sprayer. And in fact, a later study that directly compared vertical and horizontal off target drift for this SSCDS prototype and airblast sprayer which showed a nearly two order of magnitude reduction in off target drift for the SSCDS system compared to the airblast sprayer [
40].
Environmental conditions have a great deal of impact on the outcome of spray coverage and deposition. Average wind speed remained under 5 m s
−1 for each of the spray events, but wind direction was different on each date. However, wind was always directed at an angle across the row, and never completely north/south (
Table 1). This may have helped pull spray from row to row in SSCDS treated blocks increasing deposition. Visually, the fixed emitters throw little spray above the row compared to an airblast, and droplets from the airblast may have been caught by these winds and pulled away from the target environment.
Though the SSCDS demonstrated higher deposition, it was distributed less homogenously. Typically, a uniform dispersion of droplets with similar levels of coverage and deposition throughout the canopy is considered ideal and coarse droplet patterns with variable coverage and deposition as something to be avoided as wasteful or inefficient [
41,
42]. Coarse sprays may result in less optimal coverage than fine sprays [
43]. However, Doruchowski et al. also found that air induction nozzles, known for their coarse droplets size and low drift potential, had a similar biological efficacy when compared to fine spray hollow cone nozzles [
44]. A concern raised by the heterogeneous distribution of droplets on the micro scale, rather than the macro or plot scale, is phytotoxicity from concentrated agrochemicals. Localized overexposure on leaves that are in close proximity to microsprayers is a potential problem, and would necessitate the selection of compounds where this is a low risk. There was no observed issues with leaf discoloration, malformation, or dropping—but it was not looked for.
Despite what might be considered inferior spray coverage, SSCDS plots exhibited near identical levels of disease and pest management to the airblast sprayer plots, with the only significant difference being a very low incidence of light (<5%) apple scab on fruit. Near identical levels of coverage and insect and disease control were reported over a two year study on the same orchard block for a previous prototype of the SSCDS tested in this study. The tested areas in that study were confined to narrow two row plots, the present study demonstrates that equivalent control can be achieved in larger square plots. In addition, the previous study did not evaluate deposition, relying solely on water sensitive cards to estimate coverage [
10]. This highlights the importance of not relying solely on coverage estimates when evaluating spray application technology and suggests that deposition estimates may be a better predictor of pest management efficacy. Another recent study evaluating coverage and deposition provided by a prototype SSCDS in vineyards showed a similar pattern of lower coverage but higher deposition, but did not evaluate pest management [
45]. There are several possible explanations for the consistent disparities between coverage and deposition provided by SSCDS. One potential explanation is that fine droplets below the detectable threshold of either the WSP or scanner contributed to coverage. Verpont et al. hypothesized these tiny droplets may still provide enough chemical residue for biological activity [
28]. A second explanation is that the higher deposition provided by the SSCDS was redistributed during rewetting periods (e.g., dew and rainfall events) leading to enhanced coverage over time. Prior studies have also reported there is often minimal correlation between the observed deposition profile and the actual biological efficacy of the spray [
44], and that coverage and pest control do not necessarily correspond with each other [
46]. Pest management is the true goal of any application, and these results support the solid set canopy delivery system’s potential as an alternative to airblast sprayers.
Despite its proven ability for pest management, there are still some concerns raised by heterogeneous coverage. Potential issues may arise with pests or pathogens that reside on the underside of the leaf-where the SSCDS has inferior coverage. Reservoirs of fungal bodies or spores may also avoid treatment if they are sheltered from treatment by dense foliage occluding spray. For example, Viret et al. showed powdery mildew control in vines was best when both sides of the leaf received near equal treatment [
46]. Research has demonstrated that suitable coverage patterns are partly dependent on the mode of action of the compound [
24,
47]. Many of the modern pesticides used in apple IPM programs hold plant penetrative properties that allow translaminar movement of compounds from adaxial to abaxial leaf surfaces [
48]. For these materials, coverage is less important than deposition to provide the expected plant protection. Arthropods such as European Red Mite (
Panonychus ulmi, Koch) require near complete coverage for control since they lay eggs in crevices and spend much of their time on the underside of the leaf [
49]. The SSCDS could conceivably have issues controlling pests that have concealed life stages or aren’t motile, or that manage to avoid areas that receive spray. On the other hand, pests such as apple maggot (
Rhagoletis pomonella, Walsh) adults are very active and don’t require high levels of coverage to receive lethal exposure to toxicant. Thus, the efficacy of SSCDS is likely determined by coverage and deposition, the chemistry used, pest targeted, and environmental conditions.
Further work is needed to evaluate the management potential of SSCDS for other growing environments, such as in the near desert conditions of Washington State USA. Studies on the drift profile of SSCDS systems are also needed to evaluate whether these systems do in fact reduce off target loss of product. Probably the most pressing need for developing this technology is the development of specialized microsprayers optimized for agrochemical delivery and their arrangements in the canopy. Singha et al. (2019) showed that the incorporation of hollow cone nozzles as well as positioning microsprinklers so that they spray upward from the base of the canopy improved both the amount and uniformity of coverage, particulary abaxial coverage, in vineyards [
45]. Finally, further research in different perennial crops (e.g., blueberries, cherries, stone fruit, etc.) are needed so that we can better understand canopy architectures that are compatible with this exciting, potentially disruptive agrochemical delivery technology.