ReviewDevelopment of alternative plant protection product application techniques in orchards, based on measurement sensing systems: A review
Introduction
Plant protection against disease, insects and weeds is necessary to ensure good quantity and quality of harvest. It can be performed in many ways, but today chemical protection with PPP and a range of spraying techniques (sprayers, airblasters, foggers, etc.) represents the most common method. The purpose of plant protection by various chemicals is to exterminate harmful organisms and to prevent infection, but PPP residues remain in food and find their way into the environment. Pollution of the soil, groundwater, air, plants and animals are important problems for modern agriculture. It is impossible to solve these problems overnight by returning to the traditional way of farming or to the ecological method of cultivation without the use of PPP, because modern agriculture has already harmed biodiversity, biological cycles and soil biological activity. Sustainable agriculture must therefore strive to reduce harmful influences on the living environment. Sustainable agriculture will be the only future alternative, in relation to which issues of nature protection will have to be taken into consideration. The application of chemical substances in amounts as small as possible will need to be ensured; however, it must also to be ensured that plant protection quality will be retained. Among others methods, this can be achieved through selective and precise PPP application. On the contrary, fruit growers nowadays still make extensive use of traditional dose expression models. Owing to the use of traditional dose expression models, the dosage of PPP remains independent from the properties of each tree canopy in the orchard. The result is that potentially excessive dosages of PPP may be used in the orchard, because of the distinct properties (volume, leaf area, height, age, growth stage, etc.) of individual trees in the orchard. For example, researchers (Sutton and Unrath, 1984) have established that it is not appropriate to apply the same dosage of PPP on both small canopy and large canopy trees, without taking into account the common density of the total leaf area in the orchard.
Characterization of canopy trees in orchards is a very complex task where a Tree Row Volume (hereinafter TRV) model was first used to describe the geometric structure of canopy trees. TRV is based on manual measurements of the volume of tree canopies, (Byers et al., 1971). In practice, it was shown that a large TRV estimation uncertainty led to difficulties in determining the dosage of PPP. Later, the canopy height model was introduced, which today is the basic dose expression model for orchards in some European countries. This uses canopy height as the dominant crop parameter (Friessleben et al., 2007). In more advanced models for estimating properties of the tree canopy, some researchers (Ladd and Reichard, 1980, Giles et al., 1989, Balsari and Tamagnone, 1998, Doruchowski et al., 1998, Meron et al., 2000, Walklate et al., 2002, Escolà et al., 2007, Escolà et al., 2011, Escolà et al., 2013, Solanelles et al., 2006, Balsari et al., 2009, Wenneker et al., 2009, Doruchowski et al., 2011, Jejcic et al., 2011, Llorens et al., 2011, Sanz Cortiella et al., 2011, Stajnko et al., 2012, Chen et al., 2012, Osterman et al., 2013) have begun to use precision sensing systems including ultrasonic and spectral principles which assist in control of the PPP process. Instead of using canopy measurements obtained by sensors mounted on sprayers, some researchers (Meron et al., 2000, Shimborsky, 2003) found photogrammetric aerial scanning appropriate as an equivalent method of tree mapping. To link aerial scanning with sprayer position in the orchard, GIS and RTK-DGPS support is required (van de Zande et al., 2001, van de Zande et al., 2003). In recent years most commonly used measurement systems have operated on the principle of Light Detection and Ranging (hereinafter LIDAR) sensors for dose expression rate. The LIDAR measurement system is able to measure the distance between the sensor and objects in its surroundings quickly and precisely. Escolà et al. (2007) found that the spot diameter is clearly smaller for the LIDAR sensor than for the ultrasonic sensor; ultrasonic sensors measure only the foliage in front of them, while LIDAR sensors are able to estimate more precisely the tree foliage cross section. Moreover, LIDAR enables representation of an individual tree at the level of the leaves and branches. The capacity to estimate the three-dimensional spatial structure of canopy trees with LIDAR represents a significant advantage over competing sensing principles, such as ultrasonic, radar or aerial scanning.
Based on information provided by the sensors, the sprayer’s processing system detects trees, and through input–output control units and actuators (electromagnetic valve EMV, hereinafter EMV) controls the dosage of PPP in ON/OFF, discrete or continuous mode. In recent years reports have emerged on the application of PPP, based on the principle of discrete decision and actuation (Moltó et al., 2001). The discrete system recommended by Moltó et al. (2001) operates on the principle of the ultrasonic sensor and can deliver three application dosages: full dosage, reduced dosage and the nil dosage, while in all other cases a fixed nozzle position and orientation were used. In the application of the PPP process, three modifying dosages through the EMV was defined as fixed, and precise dosage control could not therefore be provided. So in future, the possible alternative to precise control would be a continuous dosage control of PPP according to the individual tree canopy in an orchard.
At the current state of the art, the sprayers, positions and orientation of nozzles used for PPP application are fixed at a given height and are only partly adapted to the non-linear shape of the canopy tree over the entire height. A further development was reported by Osterman et al. (2013), who use an adapted prototype sprayer with adjustable hydraulic manipulator arms. They used three manipulator arms, with installed aerodynamic airflow support and PPP nozzles featuring 8 degrees of freedom in a plane perpendicular to the row, with the intention of spraying targets perpendicular to the canopy contour at a selected distance. However, in the process of PPP application, it is important to measure the geometrical characteristics of each tree canopy precisely by using an electronic measurement system, because this contributes significantly to delivery of an appropriate amount of PPP to the selected canopy segment. The majority of current state-of-the-art sprayers are not equipped with precision electronic measuring systems for defining the geometric properties of the tree canopy, and for this reason they are unable to deliver PPP precisely to selected targets; this means that a new generation of high precision sprayers is required. Sprayers that are currently available for orchards use axial, centrifugal and tangential fans for airflow support. They differ according to the direction and amount of air flow they use for spraying support. The ideal sprayer for PPP application would ensure accurate assessment of each tree’s properties and continuous control of PPP dosage for each tree canopy. Additionally, it is desirable that a sprayer be able to locally detect diseases and pests in the orchard and to determine the biological effectiveness of suppression of pests and diseases with organically prepared PPP.
This literature review does not deal with diseases and pests in the orchard, which might form interesting topics for the future development of precision spraying, but it presents an overview of the development of PPP techniques as used in apple orchards. Thus, this review paper focuses on providing up-to-date information on studies that have so far been carried out on the use of empirical dose expression models in the orchard and their further development, which includes decision-making models for PPP dose rate control based on electronic sensing systems. In addition, we also present visions for the future development of PPP application techniques in the orchard and potential directions for further PPP technological innovation.
Section snippets
Expression of the PPP dosage
Establishment of an appropriate PPP dose expression is one of the key issues in crop protection. PPP dose expression relates to the selection of a variable through which the dose and its quantity is expressed. Across the world, different variables are used: PPP mass per orchard area, PPP mass per row length, PPP mass per crown height and surface, PPP mass adjusted with respect to growth stage, PPP mass per leaf wall area, PPP mass per tree row volume, or PPP volume per area adjusted with
Sensors and systems for electronic canopy detection and characterization
In the PPP process, apple growers operate with various empirical models, which serve to calculate the PPP dosage rate and which have been described in the previous subsections. However, it should be pointed out that the majority of growers do not employ detection of the presence/absence of a tree canopy and its characteristic properties in the PPP application process, because of the economic and environmental costs incurred from the use of electronic sensing systems (Escolà et al., 2013). The
Decision-making models for controlling the dosage of PPP
In the process of PPP application, existing conventional axial, tangential and radial hydraulic sprayers do not control the PPP dosage according to the structure of the tree canopy. For this reason, it is necessary to include a decision-making model for controlling the dosage of PPP with the use of smart algorithms, based on the sensing information (already discussed in Sections 3.1 Ultrasonic measurement sensing systems, 3.2 Aerial photogrammetry measurement sensing systems, 3.3 Optical
Advanced PPP application technique and future directions
Further development of new, environmentally friendly alternative PPP application techniques only began in the last two decades. Its objective has been to use PPP dosage rates that are as low as possible and to apply PPP only to places where this was necessary, with minimum losses transferred to the environment. Innovative techniques of PPP application represent an important contribution to the improvement of PPP application. As explained in the Sections (3.1 Ultrasonic measurement sensing
Conclusions
Alternative PPP application techniques in orchards based on measurement sensing systems constitute one of the most important processes in meeting environmental, economic and safety criteria for good agricultural practice. For about three decades, various procedures and methods for tree canopy detection have been suggested and developed by both computer and agricultural scientists. The detailed review indicates that the establishment of an appropriate PPP dose expression is still one of the key
Acknowledgments
This article was partly created as a result of the applied EUREKA project (No. 3211-10-000040). The authors also acknowledge Professor Michelle Gadpaille and Nataša Belšak for help in editing the manuscript.
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2022, Crop ProtectionCitation Excerpt :In recent years, efforts have focused on improving the design of pesticide application equipment. Nevertheless, although many developments have focused on sensor-based precision spraying to maximize treatment efficacy and minimize the risks of pesticide off-target losses (Gil et al., 2007; Doruchowski et al., 2009; Llorens et al., 2010; Berk et al., 2016; Hołownicki et al., 2017; Li et al., 2018; Campos et al., 2019; Comba et al., 2020; Deng et al., 2020; Mammarella et al., 2020; Salcedo et al., 2020b), the gap between these novel high-end crop protection solutions and everyday European agricultural practices remains significant (Gil et al., 2020). Indeed, a recent inventory conducted among vine growers in Italy underlines that the sprayers used by farmers are characterized by a low-technology level (Marucco et al., 2019).