DEPOSITION AND COVERAGE OF SOYBEAN LEAF SURFACES BY SPRAYS APPLIED USING DIFFERENT ASSISTED BOOM SPRAYER SYSTEMS

Maintaining homogeneity in the distribution of fungicidal sprays applied throughout the plant, minimizing the variation between the upper and lower third of the plant, is one of the main challenges for application technologies with regard to soybean crops. To increase the deposition and leaf coverage of sprayed compounds, especially in the middle and lower thirds of the plant, we developed and tested alternative assisted boom sprayer systems. In this study, three assisted boom sprayer systems (Vortex, Dropleg, and chain curtain) were evaluated, in addition to the conventional system, in terms of the deposition and coverage of the leaf surface they achieved in the upper, middle, and lower thirds of plants using the fluorescent tracer Helios SC 500TM. The assisted boom sprayer systems allowed the sprayed mixture to be better distributed throughout the plant. With the conventional spray system (without assitance), the deposition in the upper third was approximately 26 times higher than that in the lower third, whereas with the use of the assisted boom sprayer systems, the difference in deposition between these thirds was 11 times higher. Among the evaluated systems, the Dropleg system presented the lowest depositional variation, which was only 5.5 times higher in the upper than in the lower third, because of the increased deposition achieved in the lower third of the plant. Leaf coverage varied from 41% to 81% in the upper third, from 24% to 43% in the middle third, and from 4% to 13% in the lower third of the plant. Using the conventional spray system, the leaf surface coverage was approximately 13 times higher in the upper third than in the lower third of the plant, whereas with the use of the assisted boom sprayer systems, especially the Dropleg system, this variation was reduced by 73%, and the coverage of the upper and lower thirds only differed by 3.5 times. Indeed, the Dropleg system reached a leaf coverage of close to 13% in the lower third of the plant. The use of assisted boom sprayer systems improved the distribution of the mixture applied throughout the plant, reducing the differences among the thirds of the plant both in the levels of deposition and coverage of the leaf surfaces. The Dropleg system increased the deposition and leaf coverage in the lower third of the plant the most, facilitating better distribution of the mixture.


INTRODUCTION
Many factors can depreciate the productivity of soybean crops, including pests and diseases, which means that management actions are needed, including the use of pesticides. To provide adequate levels of control, two variables are the most important in the application of pesticides: deposition levels and leaf surface coverage. Application deposition is related to the amount of the active ingredient present in each organ, portion, or stratum of the plant, and this is associated with the percent coverage of the plant or its organs, such as the leaves, achieved by the application, since many agrochemicals have low or no mobility in plant tissues.
In general, conventional spray techniques using hydraulic or centrifugal nozzles, regardless of the application volume, provide low deposition and coverage in the middle and lower thirds of the plant, especially in plants with high leaf areas like soybean, causing the applied products to have low efficiency (Prado et al., 2015; Engenharia Agrícola, Jaboticabal, v.39, n.2, p.198-207, mar./apr. 2019 Graziano et al., 2017;Roehrig et al., 2018;Müller et al., 2018). This has negative impacts on pest and disease management, as the evolution or development of these problems proceeds from the lower portion of the plant towards the apex, between which the variations in the coverage of the leaf surface by applied pesticides can be as high as twenty-three times (Roehrig et al., 2018).
When evaluating the penetration of applied pesticide droplets into the soybean canopy using spray nozzles with differing spectra of droplets (fine, medium, and thick), Wolf & Daggupati (2009) observed that the leaf coverage in the lower third of the plant did not exceed 10%, since the leaf density was the factor that most strongly influenced the penetration of the droplets. In a study conducted by Debortoli et al. (2012), it was observed that, regardless of the applied droplets spectrum, only the upper third of the plant showed sufficient deposition of the active ingredient for the adequate protection of the leaves. Therefore, as the plants grow, the concentration of the droplets increases at the top of the canopy relative to that at the lower and middle parts of the plant.
Because of the variations in deposition and leaf coverage of the applied chemicals in sprays between plant parts, strategies to improve (i.e. reduce) these variations need to be developed, especially for the lower third of the plant (Cunha et al., 2014). In this sense, the use of airassisted boom sprayer systems, such as Vortex ® , Dropleg ® , and curtain of chains, could provide an effective alternative to conventional application methods.
The Vortex ® air-assisted system consists of a turbine driven by a hydraulic motor with variable rotation, located in the center of the boom, which produces a volume of air that is distributed in the boom by an inflated duct and is released towards the canopy of the plants through orifices that are close together (Bonadio et al., 2015). The chain curtain system includes a set of chains that are arranged on the spray boom, which make the plants incline with the movement of the machine (Moura et al., 2017). The plants are inclined in the direction of the movement of the boom, causing an opening in the canopy and allowing for more deposition of droplets to be achieved in the middle and lower thirds of the crop plants (Witt et al., 2009). The Dropleg ® system consists of a pendant, similar in shape to an inverted cane, at the end of which two spray nozzles are inserted that penetrate the canopy of the crop, directing its sprays from the bottom up with the purpose of reaching different parts inside the canopy of the plants, where it is typically difficulty to deposit agrochemicals. Therefore, this is a system that is applicable for use in several row crops and tends to present greater biological effectiveness against pests and diseases located in parts of the plants that are difficult to access, such as the lower third (Rüegg & Total, 2013).
The objective of the present study was to evaluate the spray deposition, as well as the coverage of the leaf surface, in the lower, middle, and upper third of soybean plants achieved by different assisted boom sprayer systems.

MATERIAL AND METHODS
This study was conducted during the 2015/2016 harvest in the experimental field of the University of Passo Fundo, Passo Fundo, RS, Brazil, a site that has an altitude of 687 m and is located at a latitude of 28°15′41′′S and a longitude of 52°24′45′′W. The experiment was conducted with soybeans of the cultivar BMX Ativa RR, which presents a determined growth habit and belongs to maturation group 5.6, grows erect, has an early growth cycle, and is susceptible to soybean rust. The soybean crop was planted with a spacing of 0.45 m between rows and with a density of 30.7 plants m -2 .
The deposition and leaf surface coverage of an applied mixture achieved by three different assisted boom sprayer systems were evaluated: the Vortex ® , Dropleg ® , and the chain curtain systems; the deposition and coverage achieved by the conventional spray system without the use of assisted mechanisms were also evaluated ( Table 1). The deposition and leaf surface coverage were evaluated at the R5.3 stage (Fehr et al., 1971)   The deposition data obtained from the plant material were expressed in units of ng cm -2 , and the results were calculated as the normalized deposition, (ng cm -2 )/(g ha -1 ), in relation to the initial concentration of the tracer in the standard-base mixture, which allowed comparisons to be made between the different treatments.
To evaluate the spray coverage, 30 leaves were collected from five replicate plants, which were divided into three levels each (upper, middle, and lower), as described above. Both sides of the leaves (abaxial and adaxial) from each portion of the plant were photographed under ultraviolet light because under such light the tracer becomes fluorescent. These images, with a resolution of 7.0 megapixels, were submitted to the FluorSoft™ image analysis software to determine the area of the leaf on which the presence of the tracer was detected. Thus, the coverage on the abaxial and adaxial sides of the leaves was determined, as well as the total leaf coverage based on the sum of both sides' coverage.
The data were compared among treatments using analysis of variance with the F-test, and means were compared based on their 95% confidence intervals (95% CIs).

RESULTS AND DISCUSSION
Based on the comparisons of means and 95% confidence intervals (P < 0.05), the deposition of the mixture was significantly influenced by the spray system used ( Figure 1). In the upper portion of the plant, the conventional spray system and the Vortex ® system behaved similarly, reaching mean (±95% CI) tracer deposition levels close to 4.57 (±0.35) and 4.28 (±0.26) ng cm -2 /g ha -1 , respectively. The chain curtain system resulted in similar deposition in the upper thirds of the plants (3.72 ± 0.35 ng cm -2 /g ha -1 ) to the Vortex ® system, and achieved higher deposition than the Dropleg ® system by up to 34% (Figure 1).
Engenharia Agrícola, Jaboticabal, v.39, n.2, p.198-207, mar./apr. 2019 The deposition achieved with the Dropleg ® spray system was equivalent in the middle and upper thirds of the plant, and thus this system distributed the applied mixture better than others. The reduced deposition in the upper third with the use of the Dropleg ® system may be related to the reduction in the volume of the mixture coming from the upper boom with this system when compared to the other mechanisms.
All of the assisted boom sprayer systems showed variations in deposition, in which deposition was higher in the upper third of the plant and reduced in the middle and lower thirds. The conventional spray system resulted in 2.4 times higher deposition in the upper third of the plant when compared to the middle third, and in the middle third of the plant the deposition was 10.6 times higher than that in the lower third. Further, when analyzing the deposition in the upper third in comparison to the lower third of the plant, a difference of 25.9 times was observed. When using the assisted boom sprayer systems, these variations were minimized, in that using when these methods the deposition was 1.9 times higher in the upper than in the middle third, 5.8 times higher in the middle than in the lower third, and 11.1 times higher in the upper than in the lower third. Among the air-assisted boom sprayer systems, the Dropleg ® system presented the lowest variation in deposition among plant parts, with differences only reaching 1.2 times between the upper and middle thirds, 4.4 times between the middle and lower thirds, and 5.5 times between the upper and lower thirds (Figure 1).
The level of coverage of the leaf surface on the adaxial side varied among the assisted boom sprayer systems. In the upper third of the plant (Figure 2), the conventional system (63.2 ± 6.8%) and the Vortex ® system (59.9 ± 6.0%) had the highest percent coverages. The Dropleg ® system (35.7 ± 3.0%) and the chain curtain system (40.6 ± 5.2%) behaved similarly, and actually had reduced leaf surface coverage by up to 43.5% compared to the other methods.
In the middle third of the plant (Figure 2), the highest coverage levels of the adaxial surface were obtained with the conventional spray system (41.4 ± 6.5%), followed by the Vortex ® system (25.4 ± 5.8%) and the chain curtain system (23.5 ± 2.8%). The use of the Dropleg ® system (16.3 ± 2.7%) reduced the percent leaf coverage in the middle third of the plant by up to 60% compared to the conventional spray system. In the lower third of the plant (Figure 2), the Dropleg ® system (10.0 ± 4.1%) and the chain curtain system (6.4 ± 1.5%) had the highest leaf coverage levels, which were up to 3.8 times higher than those achieved by the conventional spray system (2.8 ± 0.4%) and the Vortex ® system (2.4 ± 0.4%).
Engenharia Agrícola, Jaboticabal, v.39, n.2, p.198-207, mar./apr. 2019 On the abaxial side of the leaves (Figure 3) in the upper third of the plants, the coverage attained using the Dropleg ® spray system (6.5 ± 0.9%) was equivalent to that of the conventional spray system (8.1 ± 1.7%), and both of these achieved higher coverage than the chain curtain system (4.2 ± 0.9%). Among the air-assisted boom sprayer systems, the leaf surface coverage achieved by the Vortex ® system (1.6 ± 0.3%) was up to 80.2% of that achieved by the conventional system in the upper third of the plant. In the middle third of the plant (Figure 3), the Dropleg ® system (1.9 ± 0.2%) achieved approximately 58% higher coverage than all the other systems. In the lower third of the plant, the conventional spray system (1.7 ± 0.3%) and the chain curtain system (1.9 ± 0.2%) achieved higher coverage than the Vortex ® system (1.2 ± 0.1%). In addition, in the lower third of the plant, the coverage achieved by the Dropleg ® spray system (1.4 ± 0.7%) was equivalent to that of the other systems tested (Figure 3).
Engenharia Agrícola, Jaboticabal, v.39, n.2, p.198-207, mar./apr. 2019 The total coverage of the leaf surface responded significantly to the different tested assisted boom sprayer systems. In the upper third of the plant (Figure 4), the highest percent coverage was obtained with the conventional spray system (81.5 ± 9.4%), followed by the Vortex ® system (70.0 ± 4.2%). The total coverage with the Dropleg ® system (46.2 ± 3.5%) and the chain curtain system (41.6 ± 4.9%) was up to 49% lower than that obtained with the other evaluated systems.
In the middle third of the plant (Figure 4), the highest total percent leaf coverage was obtained with the conventional spray system, reaching levels of 43.1% (±6.7%) that represented 47% higher coverage than that achieved by the other assisted mechanisms, since the Vortex ® system achieved a total leaf coverage of 24.1% (±4.3%), the chain curtain system a coverage of 24.5% (±2.7%), and the Dropleg ® system a coverage of 20.3% (±2.90%). The three assisted mechanisms did not significantly differ from one another in this parameter. The total percent leaf coverage obtained with the Dropleg ® and the chain curtain systems in the upper third of the plant was equivalent to that obtained with the conventional system in the middle third of the plant, with coverage levels of 42% (Figure 4).
In the lower third of the plant (Figure 4), the Dropleg ® system (13.2 ± 5.8%) and the chain curtain system (7.6 ± 1.5%) achieved higher percent leaf surface coverages than the other systems; however, the coverage obtained with the chain curtain system was similar to that obtained with the conventional spray system (6.2 ± 1.1%). The Vortex ® system reached levels of 3.8% (±0.4%) leaf surface coverage, which were lower than those obtained with the other spraying methods evaluated by up to 71% (in comparison with that of the conventional spray system).
In all four spraying systems evaluated, the percent leaf coverage obtained was reduced from the upper to the lower third of the plant, with variations among treatments of approximately 41% to 81% in the upper third, 43% to 24% in the middle third, and 13% to 4% in the lower third ( Figure 4). The conventional spray system presented 1.9 times higher coverage in the upper than in the middle third, 6.9 times higher coverage in the middle than in the lower third, and 13.1 times higher coverage in the upper than in the lower third.
Among the evaluated assisted boom sprayer systems, the Dropleg ® system had the lowest variation among plant parts, with this reaching 2.3 times between the upper and middle thirds, 1.5 times between the middle and lower thirds, and 3.5 times between the upper and lower thirds using this system. The chain curtain system had reduced coverage variations compared to those obtained with the conventional system, reaching variations of 1.7 times between the upper and middle thirds, 3.2 times between the middle and lower thirds, and 5.5 times between the upper and lower thirds (Figure 4). Among the mechanisms evaluated, the Vortex ® system had the highest levels of variation, with variations of 2.9 times between the upper and middle thirds, 6.3 times between the middle and lower thirds, and 18.4 times between the upper and lower thirds (Figure 4).  Assisted boom sprayer systems have the potential to increase the deposition and leaf surface coverage of sprayapplied chemicals throughout the plant, and may be especially useful in minimizing the variations among the different thirds, thus achieving greater homogeneity in the distribution of applied agrochemicals. Thus, it is possible to use these to maximize pest and disease control levels, as well as to minimize problems caused by the excess accumulation of the active ingredient in the spray at only one part of the plant, which can lead to phytotoxicity. The Vortex ® system is an important tool that permits adjustments in the control of spray drift (Bauer & Raetano, 2000); however, there is no consistency in its performance. In previous studies, the Vortex ® system was shown to increase the deposition of a spray mixture inside the canopy of a soybean crop (Baesso et al., 2014), as well as to improve the control of soybean rust, increasing the yield and the mass per thousand soybean grains Prado et al., 2010). On the other hand, cases have been reported wherein such assisted methods did not influence spray deposition levels (Aguiar Júnior et al., 2011), and in some studies, air-assisted boom spraying had no effect on the performance of agrochemicals, in that it did not potentiate nor did it compromise the control levels of phytopathogenic fungi (Oliveira et al., 2016).
The variation in the results obtained with the Vortex ® system may be related to plant-associated factors, especially the opening of the canopy at the time of the application, which can be influenced by the arrangement of the plants (Holtz et al., 2014), characteristics of the cultivar Tormen et al., 2012), and the time of day at which spraying occurs since the angles of soybean leaves relative to the ground vary throughout the day (Graziano et al., 2017;Moura et al., 2017). Therefore, when the canopy is more open, there may be no influence of the air curtain, which is mainly useful only with a closed canopy at the more advanced stages of crop development. In addition, the performance of the Vortex ® system is influenced by the features of the application technology used, such as the air curtain speed (Aguiar Júnior et al., 2011), uniform distribution of air along the boom, angle of the spray nozzles, airflow in relation to the spray direction , and the size spectrum of the droplets applied (Matthews et al., 2014).
Engenharia Agrícola, Jaboticabal, v.39, n.2, p.198-207, mar./apr. 2019 The Dropleg ® system is built on the principle of optimizing deposition in places that are hard to reach, such as the lower third of the plant, since the system operates inside of the canopy. In addition, it allows for greater control of the spray drift by the wind, which permits the use of smaller droplets (Roten et al., 2013;Rüegg & Total, 2013). The use of the Dropleg ® system had negative impacts on the deposition and leaf coverage in the upper third of the crop in the present study. Application of 65 L ha -1 from the top of the boom and 65 L ha -1 from the flexible rod contributed to reductions in the analyzed variables, especially the leaf coverage, which were related to the volume of the mixture (Roehrig et al., 2018). Thus, it is necessary to vary the ratio between the amounts sprayed at different levels with the Dropleg ® system in relation to those of the conventional spray-boom system.
In previous studies conducted on bean crops in France and Switzerland, the use of the Dropleg ® system improved the biological efficacy of fungicides applied against Sclerotinia sp., which increased from 40% with a conventional spray system to 80% with the use of the Dropleg ® system (Rüegg & Total, 2013). Some studies have also been carried out with soybean crops, in which it was observed that productivity, the coverage of the lower leaves, and the density of droplets deposited on the plants were not changed by the use of the Dropleg ® system compared to the conventional spray system (Alves & Cunha, 2011;Cunha et al., 2014).
In the present study, the variations in deposition and leaf coverage with the Dropleg ® system may have been associated with the adjustment of the system for use in the soybean canopy. The flexible rod used has a length of 0.9 m, so with the spray boom operating 0.5 m above the canopy of the plants, the penetration of the rod was approximately 0.4 m inside the canopy, and thus considering that the plants had a height of 1.0 m the spray did not reach the entire lower third of the plants. This system could be improved with the use of a larger rod. A crop of plants with a height of 1.0 m would require a rod of 1.4 m to 1.5 m in length. In this case, it would also be necessary to use nozzles in the intermediate parts of the rod, not only at the lower end. Whether the maintenance of the total volume of the mixture at both spray nozzles without dividing it between the thirds would improve the performance of this system should also be explored, since the two-way division of the spray volume caused less coverage and less deposition to be achieved on the upper parts of the plants in the present study.
When working with the chain curtain system, Moura et al. (2017) observed that the greatest benefits were obtained if the spray applications were performed in the early morning, as this is when there is dew present and when the upper thirds of the plants assume a horizontal position in relation to the soil, making it more difficult for the spray droplets to be deposited in conventional systems. Thus, interactions between the application system and its application time can occur, as the results of the application of fungicides for the management of soybean rust at 12:00 and 15:00 did not differ depending on whether or not airassisted boom sprayer systems were used in this study. In addition, the use of the chain curtain improved the deposition of spray droplets in the lower thirds of the plants regardless of the time of application (Moura et al., 2017). The chain curtain system can optimize the deposition of droplets on the lower part of the plant, resulting in greater control of soybean rust, and thus optimizing the grain yield. However, it is necessary to take into account the application time and the architecture of the cultivar, since the best results are obtained in cultivars with a high leaf area index, especially if applications are performed in the early morning (Moura et al., 2017).
The time of day at which the application occurs can influence the deposition as well as the percent coverage of the leaf surface achieved due to the movement of the leaves (heliotropism). Thus, a strategy to maximize the effectiveness of agrochemicals should include the means to improve their distribution throughout the plant, especially at times close to 13:00. It should also be noted that the application schedule interacts with the spectrum of the droplets and the volume of the mixture, in addition to the use of air-assisted boom sprayer systems (Graziano et al., 2017;Moura et al., 2017).
Because of the reduced sensitivity of the plant pathogen Phakopsora pachyrhizi to triazole, strobilurin, and carboxamide fungicides, multisite fungicides (mancozeb, chlorothalonil, and copper sulfate, among others) have begun to be used in soybean cultivation to manage soybean rust. This is done with the goal of increasing disease control efficiency, as well as minimizing the risks of pathogens developing resistance (Juliatti et al., 2017;Godoy et al., 2016, Silva et al., 2015. Multisite fungicides form a superficial protective barrier against infection, which is therefore non-penetrating and immobile. When applied to the aerial organs of plants, these fungicides are not absorbed, so they do not move and remain on the surface of the plant where they were deposited (Reis et al., 2010). Given these characteristics, it is very important to search for strategies to maximize the deposition of fungicides, as well as their coverage of the leaf surface throughout the plant, to optimize disease control and thus reduce losses and damages to crops caused by diseases. Therefore, the air-assisted boom sprayer systems examined in this study have the potential to be used, with some adjustments, to achieve these goals. They may also be used in the essential adjustment of fungicide application technologies, taking into account all the aspects that make it possible to distribute such protective agrochemicals throughout the plant.

CONCLUSIONS
The use of assisted boom sprayer systems improved the distribution of the applied mixture throughout the plant by reducing the variations among the upper, middle, and lower thirds of plants in both the levels of deposition and leaf surface coverage. The Dropleg ® system increased the deposition and leaf coverage in the lower third of the plant the most, and thus provided a better distribution of the applied mixture than the other methods tested.