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    Published on: 01/31/2024

Testing crop protection performances in a vineyard subjected to variable spray

R. D’Ambrosio et al. | Univ. Cattolica del Sacro Cuore I.CRAST, Piacenza, Italy

R. D’Ambrosio1, M. Furiosi1, T. Caffi1, F. Graziosi1, T. Frioni1, S. Poni1,2, M. Gatti1,2
1Università Cattolica del Sacro Cuore, Via E. Parmense 84, I-29122 Piacenza, Italy 

2Remote Sensing and Spatial Analysis Research Center (CRAST), Via E. Parmense 84 – I-29122 Piacenza, Italy 
*matteo.gatti@unicatt.it 

Introduction

Addressing within-field variability in vineyard represents a challenging opportunity to significantly reduce pesticide dependency. For example, downy mildew is widely considered one of the most dangerous grapevine diseases, and its control is mainly based on preventative copper-based fungicides (Rossi et al., 2013). However, being a heavy metal, copper can accumulate in the soil (Rusjan et al., 2007), and the European Union has limited its use to 4 kg/ha per season.  

The accuracy in plant protection products (PPP) application, by adjusting the spray volume depending on canopy characteristics or by avoiding product application to runoff points, is an essential factor in reducing the environmental impact (Miranda-Fuentes et al., 2016). Indeed, Campos et al. (2020) demonstrated as the application of a higher volume is not related to increased spray efficiency, while a lower spray volume can optimize the canopy coverage. Thus, canopy shape and volume must be considered to set the adequate spray volume. Several models are available to establish the optimal dose depending on canopy characteristics such as tree row volume (TRV) (Ruegg et al., 1999), and seasonal variation of Leaf Area Index (LAI) (Siegfried et al., 2007). In addition, the adoption of novel sensing technologies can improve the management of spatial variability through a more accurate and faster definition of TRV (Anifantis et al., 2019). For instance, LiDAR (Walklate et al., 2002; Arnó et al., 2013) and ultrasonic sensors (Gil et al., 2007) are available technologies that can be used to obtain more efficient spray systems.

Implementation of the so-called “environmentally safe spray” is based on the variable rate spray application depending on the information sensed by proxy, a technique allowing the reduced chemical application, ensuring adequate deposition on target, and limiting spray drift (Llorens et al., 2010; Doruchowski and Holownicki, 2000). Such a condition can be achieved through different systems (Grella et al., 2022). However, despite a few hi-tech commercial solutions are now available on the market, the adaptation of already existing sprayers might represent a challenging perspective for speeding up the conversion process toward precision viticulture. Thus, the objectives of this study were i) testing the performance of a novel on-the-go system for pesticide dose management based on variable volume rate at constant concentration and ii) optimizing copper use in vineyard. 

Materials and methods  

The study was carried out in 2022 in a commercial organic vineyard of Vitis vinifera L. cv. Barbera, established in 2015 at Pianello Val Tidone, Tenuta Villa Tavernago Estate (44°56’N; 9°27’E, 280 m asl), Italy. The 1.31 ha vineyard is northwest facing, with a maximum 30% slope grade. Vines were single-cane pruned and trained to a vertical shoot positioning trellis. Theorical plant density was 4000 plants ha-1.

The treatments corresponded to the combination of different vigor levels (HV, high vigor; LV, low vigor) detected by proxy, and two canopy spray methods: Variable Rate Application (VRA) and Conventional treatment (CT). They were duplicated in two completely randomized blocks and for each combination, four representative vines in each vigor zone were identified, leading to a total of 32 studied vines.  

A commercial “Sting 50” low-volume air-blast sprayer (V.M.A. s.r.l., Santa Maria della Versa, Italy) combining two upper cannon-type spouts and two hand-type spouts was used by a tractor at a speed of 5 km/h. Estate protocol required a fixed volume of 300 l ha-1. On 27th of May (DOY 147), it was reduced (220 l ha-1) according to the actual canopy growth. In addition, the same sprayer was adapted for VRA through an electronic system allowing adjustment of the spray volume depending on canopy vigor. This system combined (i) the MECS-VINE® sensor (Appleby Italiana s.r.l., Cadeo, Italy) describing vigor as Canopy Index (CI) (Gatti et al., 2016), (ii) the electronic controller, and (iii) the commercial sprayer. Spray volume was reduced from 300 to 150 l ha-1, with the maximum volume of 300 l ha-1 corresponding to the maximum CI value. The relation between CI and spray volume was linear and the spray volume was kept constant below 150 l ha-1. Such a set up was adopted since DOY 157 when shoots outgrew the top foliage wire. 

 Starting when shoots were almost 1 m long, nine sprays were applied until DOY 210 using the following PPP: sulfur (2 kg ha-1) and copper (300 g ha-1). The system registered the treated area and the corresponding sprayed volume per plot. 

Per each tagged vine, shoot length was measured weekly from bud burst until trimming on two primary shoots growing from the 4th and the second to last node along the cane. Shoot length was then used to total leaf area estimation.

At BBCH 79 (Lorenz et al., 1995), the canopy structure was assessed through Point Quadrat Analysis (PQA) and the percent of canopy gaps, interior leaves, and the leaf layer number were then calculated.  

Downy mildew infections in all the treatment x vigor combinations were assessed at BBCH 79. Following the EPPO guidelines for disease monitoring, incidence and severity were assessed on leaf and clusters samples using a diagrammatic scale for downy mildew evaluation (Caffi et al., 2010). 

The spray distribution quality was assessed per block through the evaluation of coverage quantification (% CQ) and spray deposit (µg cm-²) using Water Sensitive Paper (WSP) (Syngenta, Basel, Switzerland). At BBCH 79, they were placed on two representative vines, considering geometric structure of canopies. 64 WSP were used for each block.

Off-target losses to the ground and to adjacent rows were assessed by placing the same WSP left and right in the four adjacent rows to the one where spray distribution quality was assessed. Concerning the off-target deposit to adjacent rows, WSP were placed at two different canopy heights per each row only on the upper leaf side facing the sprayer. Likewise, losses to the ground were quantified by placing the WSP in the inter-row of the four adjacent rows and even in the inter-row where the spray distribution quality was evaluated (inter-row “0”). To assess off-target losses, WSP was removed immediately after the first passage of the tractor through the inter-row “0”. All WSP were processed with ImageJ software to quantify the percentage of spray coverage and the results were used to assess copper deposit (µg cm-²) by regression. According to official methods, copper concentration was analyzed on a subset of 20 representatives WSP.

At harvest, all clusters per vine were counted, and their total mass was weighted using a portable scale. From each tagged vine, three representative clusters were collected and brought to the laboratory for assessing the following parameters: total soluble solids, titratable acidity, must pH, malate, anthocyanins, and phenolics concentration. 

Statistical analysis was conducted on the SPSS software (IBM SPSS Statistics 27.0). A one-way analysis of variance (ANOVA) was conducted for both disease incidence and severity. The treatments (given by the combination of treatment x vigor) were compared according to the Tukey test. Data concerning canopy architecture, spray deposit, yield components and grape composition were subjected to a two-way ANOVA assuming Vigor and Treatment as main factors. Treatments were compared according to the SNK test at P ≤ 0.05.  

Results 

The first VRA treatment was carried out at BBCH 59 (DOY 147) using a reduced spray volume to compensate for the incomplete canopy growth. So, according to the estate protocol, a maximum volume of 220 l ha-1 ensured canopy coverage during this phase. Contrariwise, the spray volume was fixed at 300 l ha -1 throughout the season in CT plots, while an average volume of 255 l ha -1 was applied in VRA. Therefore, VRA allowed saving 15.4 % of the applied volume. 

Before shoot trimming, on DOY 188, HV shoots were longer than LV resulting in a more than double TLA (5.70 m2 in HV) as compared to LV vines (2.51 m2). At BBCH 79, HV vines had a reduced proportion of CG compared LV (16 vs. 29.5 %, respectively). Thus, the canopy density, expressed in terms of leaf layer number (LLN) and percentage of IL, was significantly higher in HV than in LV.

The two spray techniques provided a similar leaf coverage, being slightly lower than 25%. There were no variation in leaf coverage between vigor areas and spray techniques assessed by WSP processing. Both VRA and CT provided similar coverage on both adaxial and abaxial leaf sides. Moreover, no significant difference was found between VRA and CT in the copper deposit (µg cm-²) on both upper and lower leaf sides.  

Losses to adjacent rows was higher in the first row due to the sprayer configuration; however, no differences between CT and VRA were observed. Cu ground deposit was higher in the first inter-row and decreased as the distance from the sprayer increased.

Downy mildew infections were evaluated in terms of incidence and severity on both leaves and clusters (BBCH 79). Traditional (CT) and innovative (VRA) spray presented similar infection rates on both clusters and leaves. Contrariwise, vigour level shown a significant effect: infection rates were higher in high vigour zones.   

Vigor significantly affected yield ranging between 1.07 and 3.2 kg vine-1 in LV and HV, respectively. Even though the number of clusters did not vary between vigor areas, cluster, and berry weight was higher in HV. Vigor also affected fruit composition, being positively correlated with malate and negatively correlated with anthocyanins and phenols. The two spray techniques did not affect yield and grape composition showing similar performances for all the assessed variables. 

Discussion 

Disease control is key in viticulture to reach optimal yield and fruit quality. Addressing stringent restrictions on PPP use, the increased costs of agricultural chemicals, and the need of for more sustainable agriculture, a variable rate spray system was tested in the vineyard for real-time pesticide dose adjustment depending on vigor variations. 

The two treatment techniques ensured a comparable downy mildew control; the only difference was related to vigor variation. Indeed, downy mildew incidence and severity on leaves and clusters were higher in HV than in LV. This condition is explained by vineyard topography and the higher density of HV canopies was assessed during the growing season (Gessler et al., 2011). Considering spray techniques, there were no differences in canopy coverage and copper deposit. In this trial, the seasonal sprayed volume was reduced by 15.4%. It demonstrates the effectiveness of the tested VR approach in reducing the environmental impacts of viticulture and it’s crucial as part of repeated applications. Indeed, other studies frequently assessed the effectiveness of VRA spray as part of a single application performed at full canopy (i.e., at BBCH 79), describing a spray volume reduction of up to 40-60% (Llorens et al., 2010). In addition, despite the lower spray volumes allowed by the VR system, leaf copper deposit (ug cm-2) was effective in controlling downy mildew in both CT and VRA as reported by Mian et al. (2021). Indeed, as part of the current study, no differences in downy mildew incidence and severity were described between the two spray systems. This result agrees with Llorens et al. (2010) and Campos et al. (2020) when testing variable rate spray in vineyards. Furthermore, both VRA and CT provided an average leaf coverage of 24 %, that is considered as an adequate rate to control diseases and minimize overspray applications and spray product waste, as already proved by Chen et al., 2013.  

Finally, no differences in losses to the ground and adjacent rows were described when comparing the two spray systems. In particular, the losses to the ground showed a lower copper deposit in the rows adjacent to the treated row, except for the first row. This result can be due to the use of an air-blast low-volume sprayer that kept the drift at low levels compared to other commercial sprayers based on different technologies (Costas et al., 2020). 

Conclusion 

Variable rate spray allowed an overall 15.4% saving of pesticide solution compared to traditional protocols, maintaining similar application efficiency in terms of canopy coverage, spray penetration, and off-target losses. VRA did not affect downy mildew incidence and severity on both leaves and bunches. Yield and fruit composition did not vary among treatments, while expected differences among vigor zones were confirmed. The study demonstrated the effectiveness of variable volume rate at constant concentration in reducing Cu use in organic vineyards and allowed adaptation of a traditional sprayer for real-time adjustment of canopy spray in response to data sensed by proxy at high resolution. Consequently, the tested system may actively contribute to speeding up the conversion process to precision viticulture in relatively small farms. 

Acknowledgements 

The authors want to thank Tenuta Villa Tavernago Estate for hosting the trial, Studio di Ingegneria “Terradat” and Appleby Italiana for technical support, and Marco Galbignani for agronomical assistance. Research carried out within the Ripreso project (id. 5149719) funded by the Emilia-Romagna Region. 

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Published on 10/16/2023
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