This study was conducted in two orchards with different canopy sizes: young and small trees and adult and large trees.
2.1. Experimental Site
Field tests were performed in an experimental orchard growing ‘Clemenvilla’ mandarins (Citrus clementina Hort. × (Citrus paradisi Macf. × Citrus tangerina Hort.)) in El Puig de Santa María (Valencia, Spain) (39°37′7″ N, 0°21′57″ W). This orchard had an area with young trees, established in 2013, and another with adult trees, established in 1990, which were considered as the ‘young plot’ and the ‘adult plot’ for the trial, respectively.
Firstly, the vegetation of both plots was characterized (
Table 2) by measuring the planting pattern (row spacing (
Rd, m) and tree spacing (
Td, m)), the height of the raised bed, the size of the canopy, and the leaf area density. Canopy size was determined by measuring the heights of the skirts (from the ground to the bottom of the canopy, without taking into account unusual extreme shoots, m), total tree height (from the ground to the top, m), diameter along the row (
Dl, m), and diameter across the row (
Dc, m) (
Figure 1) in ten representative random trees. After, canopy height (
Hc, m) was calculated by subtracting the height of the skirts from the total tree height. The canopy volume
Ve (m
3 tree
−1) of each tree was calculated considering the citrus canopy as an ellipsoid with
Hc,
Dl, and
Dc as the axes.
Likewise, the LWA (m
2 ha
−1) and TRV (m
3 ha
−1) in the two plots were calculated using Equations (1) and (2), respectively.
where
Hc (m) is the canopy height,
Dc (m) is the diameter of the canopy across the row, and
Rd (m) is the row spacing.
The average leaf area density
LADt (m
2 of leaves m
−3 of canopy) was estimated in three trees per plot by calculating the LAD in 18 sections of the canopy. These sections were the result of dividing the canopy into three heights (bottom, middle, and top), three widths (W1, W2, and W3) with respect to the diameter along the row, and two depths (inside and outside) (
Figure 2).
In each section, a 0.125 m
3 cube was defoliated and the total amount of leaves was weighed. From each zone, a random sample of 30 leaves was weighed with a precision analytical balance and digitized by scanning at 600 dpi resolution; the resulting image was analyzed through image analysis with the ImageJ software [
12,
13] to calculate the leaf area (considering only one side of leaves). In this way, the weight/leaf area ratio of each section
i was determined, and the density of vegetation in each canopy section
LADi (m
2 of leaves m
−3 of canopy) was calculated (
Figure 3). Averaging the value of each section, the mean density of vegetation
LADt was obtained.
The highest LAD of the canopy was found in the outer part and middle height, while the lowest LAD was found in the interior area of Width 2, that is to say, the central zone of the canopy, especially at the bottom, both in the adult plot and in the young plot (
Figure 3). The mean LAD of the adult plot was 11.61 ± 0.52 m
2 of leaves m
−3 of canopy and that of the young plot was 11.42 ± 0.48 m
2 of leaves m
−3 of canopy.
2.2. Treatments and Spray Applications
In each plot (adult and young), the application rate for each dose expression was calculated according to Equations (3) and (4) based on application rate indexes (
Ii) for each dose expression.
where
ALWA (L ha
−1) and
ATRV (L ha
−1) are the application rates based on LWA and TRV dose expressions, respectively, and
ILWA (L m
−2 of
LWA) and
ITRV (L m
−3 of
TRV) are the application rate indexes for LWA and TRV, respectively.
The
ILWA was assigned the value of 0.25 L m
−2 for LWA by Bayer. For
ITRV, Bayer initially proposed a value of 0.21 L m
−3 of TRV, but in the adult plot, this meant an application rate much higher than 3000 L ha
−1, which is the maximum established by Bayer for many products due to ecotoxicology limits. Furthermore, in the young plot, with this
ITRV, the spray volumes
ALWA and
ATRV estimated were very similar (1832 and 1892 L ha
−1, respectively). Therefore, it was decided to calculate a new
ITRV index based on the volume rate recommended by CitrusVol, a decision support tool designed and validated to adjust the pesticide treatments carried out with air-blast sprayers in citrus crops [
14,
15]. The CitrusVol tool recommends an application volume rate (L ha
−1) based on canopy size, planting pattern, and pruning and density levels of the orchard, together with the pest to be controlled and the active ingredient to be applied. For this study, the application volume rate recommended by CitrusVol (
ACV) was obtained for a model treatment with the following characteristics: (a) canopy size and planting pattern of the adult plot; (b) normal pruning level; (c) medium density level; (d) control of two-spotted spider mite
Tetranychus urticae Koch (Acari: Tetranychidae), taken as a model pest in citrus; and (e) application of spirodiclofen, taken as a model PPP against this pest. The corresponding
ITRV was calculated with Equation (5), obtaining a value of 0.18 L m
−3 of TRV.
In addition, the volume applied per tree in each plot with each dose expression was calculated with Equation (6) in order to calculate the application time per tree needed with the knapsack sprayer.
where
Atree,i (L tree
−1) is the amount per tree of spray liquid with each dose expression,
Ai (L ha
−1) is the application rate corresponding to each dose expression (
ALWA or
ATRV),
Rd (m) is the row spacing, and
Td (m) is the tree spacing.
Table 3 and
Table 4 shows the treatments conducted and their corresponding application rates. It is important to highlight that, due to the method of calculation, in the young plot, the volume applied based on LWA was higher than the one based on TRV, while in the adult plot, the contrary happened, that is, the volume applied based on LWA was lower than the one based on TRV.
Treatments were made between 28 September and 10 October 2018. Hand-held applications were performed with a portable manual hydraulic knapsack MARUYAMA MS073D sprayer (Maruyama US Inc., Auburn, WA, USA), selected because it is widely used by pesticide manufacturers to conduct efficacy trials. Mechanized applications were carried out with an axial fan air-blast sprayer GBV Citfruit (GBV Agrícola S.L., Alginet, Valencia, Spain), powered by a New Holland TN95NA tractor (New Holland Corporation, New Holland, PA, USA), selected because it is widely used by citrus growers. Before each application, the corresponding sprayer was configured for the specific conditions of each treatment.
Each application consisted of spraying the half canopy of two adjacent tree rows facing each other and three consecutive trees from each row. Each application was made on different trees to avoid contaminating the spray collectors. Three replications per treatment were performed. A water-based solution of fluorescent tracer Brilliant Sulfoflavine (BSF) (Biovalley, Marne-la-Vallée, France) was used in the applications with a concentration of 1 g L−1.
During each application, weather conditions (temperature, relative humidity, wind speed, and direction) were monitored at 1 Hz by means of a thermohygrometer LOG32 Data Logger (Dostmann electronic GmbH, Wertheim, Germany) and a 3D ultrasonic anemometer WindMaster 1590-PK-020 (Gill Instruments Ltd., Hampshire, UK). The sensors were placed at 5.5 m height, which was 2.5 m above the canopies in the adult plot and 3 m in the young plot.
Wind data direction is expressed with respect to both the geographic north and the spray pass because off-target losses generated during the treatments depend to a great extent on the external air currents, perpendicular to the target tree rows. For this reason, the inclination of the tree rows of the plot was estimated with respect to the geographic north, obtaining an approximate mean deviation of 66.3° (
Figure 4). Furthermore, the 25th, 50th (median), and 75th percentiles of wind direction during every application were also calculated.
Table 5 shows the mean data collected by the sensors for each treatment. The
Figure 5 shows the average wind conditions during the application of each treatment to better explain the results.
It is important to highlight that all the applications were performed under appropriate temperature and relative humidity conditions, and with an average wind speed lower than 3 m s
−1, which is the maximum established in the Spanish legislation to minimize drift-associated risks [
16]. Regarding average wind direction, in only one treatment, Ad-Air-TRV, wind came from the right side of the sprayer; meanwhile, in all the other cases, wind came mainly from the left side. On the other hand, in the treatments applied with the knapsack sprayer in the young plot, based on both TRV and LWA, wind came almost parallel to the tree rows, while in the others, its direction was more transversal (
Figure 5).
2.5. Data Analysis
The effects of the sprayer and the dose expression on the distribution of the spray volume in the canopy were studied separately in each plot because the volume rate calculated with the different dose expressions, LWA and TRV, depends on the size of the canopy; therefore, both parameters are correlated and cannot be included as independent factors. A multifactor analysis of variance (multifactor ANOVA) was performed on the dependent variable ‘coverage’ and another on the dependent variable ‘leaf deposition’. In addition to the main factors studied (sprayer and dose expression), factors related to the locations of the collectors for analyzing the distribution in the canopy were included: the depth and height in the canopy and the side of the leaf (in the case of the spray coverage). Two-way interactions were included in the study, and they were explained only when at least one of the main factors was involved in the interaction. An iterative process was followed in which all the factors and their interactions were included. Next, the effect with the highest non-significant p-value (α > 0.05) was removed and the model was recalculated. This was repeated until all the effects present were significant.
For analyzing the effects of the sprayer and the dose expression on the off-target losses in each plot separately, a multifactor ANOVA was performed for: (1) losses due to potential airborne drift, (2) losses generated by potential sedimenting drift, and (3) ground losses. For these analyses, losses related with potential airborne drift were considered as the accumulated deposition in all the collectors above the trees. Losses due to potential sedimenting drift were considered as the accumulated deposition in all the collectors behind the trees, both to the left and to the right of the sprayer. Ground losses were assumed as the accumulated deposition in all the collectors placed on the ground.
In every multifactor ANOVA, it was verified that the assumptions of homoscedasticity (through the Levene test) and normality (normal probability plot of the residuals) were fulfilled. When significant differences were found, the least significant difference (LSD) test was applied for the separation of the means. The confidence level used for all the analyses was 95%. Analyses were done using Statgraphics Centurion XVI (Manugistics Inc., Rockville, MD, USA).