**1. Introduction**

Due to the progressive increase of the world population, which, according to the Food and Agriculture Organization (FAO), will exceed 9 billion inhabitants in 2050 [1], the agricultural sector will have to satisfy an increasing demand for food. To ensure high productions, a crucial role will be played by pesticides.

According to FAO, pesticides include any substance, or mixture of substances of chemical (natural or synthetic) or biological ingredients for repelling, destroying, or controlling any pest, or regulating plant growth [2]. Indeed, they can help to protect seeds and safeguard crops from unwanted plants, insects, bacteria, fungi and rodents. Pesticides includeawiderangeofherbicides,insecticides,fungicides,rodenticides,andnematicides.

 Usually, pesticide is a more general term that comprises plant protection products (PPPs) which are aimed to protect crops or desirable/useful plants. PPPs contain at least one active substance and have different functions such as to protect plants or plant products

**Citation:** Facchinetti, D.; Santoro, S.; Galli, L.E.; Fontana, G.; Fedeli, L.; Parisi, S.; Bonacchi, L.B.; Šušnjar, S.; Salvai, F.; Coppola, G.; et al. Reduction of Pesticide Use in Fresh-Cut Salad Production through Artificial Intelligence. *Appl. Sci.* **2021**, *11*, 1992. https://doi.org/10.3390/ app11051992

Academic Editor: Giuseppe Manetto

Received: 31 January 2021 Accepted: 19 February 2021 Published: 24 February 2021

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against pests or diseases before or after harvest, to influence the life processes of the plant, to preserve plants products while destroying or to prevent the growth of undesired plants or parts of plants [3]. When pesticides are used irresponsibly, they might involve health risks, can have negative environmental impacts, on the soil and water, can reduce biodiversity, and, in some cases, decrease crop yield.

The total pesticides used in agriculture remained stable in 2018 compared to 2017, with a light decrease from 4.15 Mt to 4.12 Mt, but in the last 30 years, from 1990 to 2018, the global use of pesticides in agriculture has increased from 1.80 to 2.66 kg/ha. The global application of pesticides increased in this period both for herbicides, fungicides, and insecticides [4].

#### *1.1. Pesticide Use in Europe*

Europe increased pesticide use in agriculture in the 2010s compared to the 1990s by just 5%, most likely due to the stringent European Common Agricultural Policy that came into force, which monitors and controls the use of pesticides. Indeed, due to the application of the Directive 2009/128/CE of the European Commission, member states adopted national action plans to set up their quantitative objectives, targets, measures, and timetables to reduce risks and impacts of pesticides use on human health and environment, to encourage the development and introduction of integrated pest managemen<sup>t</sup> and alternative approaches or techniques to reduce dependency on the use of pesticides [5]. Pesticide use per area of cropland was approx. 1.66 kg/ha in 2018 [4]; the average use of pesticides from 1990 to 2018 has been 465,556 tons [6]. Four EU member states, Germany, Spain, France and Italy, represent by themselves over two-thirds of the total EU pesticide sales volume. These countries are also the main agricultural producers in the EU, with collectively 51% of the total EU utilized agricultural area (UAA), and 49% of the total EU arable land. In terms of the categories of pesticides sold, the highest sales volume in 2018 was for fungicides and bactericides (45%), followed by herbicides (32%) and insecticides and acaricides (11%) [7].

#### *1.2. Pesticide Use in Italy*

Italian agriculture has one of the highest uses of PPPs in Europe. In 2016, according to the Italian National Institute of Statistics (ISTAT) [8], the use was 7.22 kg/ha of active substances, corresponding to approximately 124,000 t per year. The value of the Italian market for crop protection products has increased by 43.6% in the last 10 years. The reason for this variation was the constant improvement of the mix of products that, due to a lower dose rate, has led to an increase in unit price. A strong decrease in the quantities used has been observed ( −22%), shifting from 141,200 to 109,860 t from 1990 to 2015. In terms of active substances, the categories most concerned by the introduction of innovative molecules with a low dosage are mainly represented by fungicides and herbicides, which have determined the consistent decrease. A survey, carried out by the European Food Safety Authority (EFSA) in 2016, has highlighted the high qualitative standards of the Italian products, thanks to a control system extremely stringent and efficient that ensure a high safety level to consumers. Only 1.2% of the sample analyzed proved to be irregular, compared to a European average of roughly 2.9% [9].

## *1.3. Fresh-Cut Products*

According to the International Fresh-Cut Produce Association (IFPA), fresh-cut products are fruits or vegetables that have been trimmed and/or peeled and/or cut into 100% usable products. Fresh-cut products are cut, washed, packaged, and maintained with refrigeration. They are in a raw state and even though minimally processed, they remain in a fresh state, ready to eat or cook. Since their origin in Europe in the early 1980s, they have become more and more common in consumer baskets. The innovation represented by this sector involves the technologies adopted in growing, processing and marketing. Fresh-cut products such as ready-to-eat salads require substantial capital investment in plants and

machinery [10]. This sector is characterized by intensive cultivation that requires large use of chemical products, such as fertilizers and pesticides.

#### *1.4. Fresh-Cut Salad Sector in Italy*

The agricultural area devoted to the production of fresh-cut vegetables in Italy is about 6500 ha. Production takes place mainly in plastic tunnels (in northern Italy), which are called "Bergamasca", or multitunnels (in southern Italy) [11]. The most common structure adopted is plastic tunnels of semicircular shape, with a width of 8–10 m, a length of 50–100 m and a height of 4 m. Usually, the greenhouse is divided into four rows of crops, with a typical inter-row of 1.5 m, separated by small ruts of about 0.2 m of width, being the paths for the tractor wheels.

In 2015, Italy produced 110,000 t of fresh-cut vegetables, with a value of € 744 million. Of the total value, lettuce comprised 75.4%, followed by wild rocket (9.5%), spinach (4.5%), and Swiss chard (1.3%) [10].

Normally, the Italian ready-to-eat salad producers cultivate various types of salads, such as lettuce (*Lactuca sativa* L.), wild rocket (*Diplotaxis tenuifolia* L.), spinach (*Spinacia oleracea* L.), lamb's lettuce (*Valerianella olitoria* L.), some subspecies of chicory (*Cichorium intybus* L. subspecies), and many others. These salads are grown in five to six cycles per year in the same greenhouse, with a duration of the production cycle that may vary from 20 to 90 days, according to the season. These plants grow under high crop density, with a lack of adequate crop rotation, and require a high number of pesticides treatments to avoid severe product losses.

Pesticide treatments are typically performed two times during each life cycle of the plants; therefore, the number of treatments is estimated at 10 to 12 every 12 months, and the quantity of volume sprayed currently used is 1000 L/ha. For the treatment, machines designed for open fields are typically used, instead of machine optimized for greenhouses. These are constituted of a tank (usually from 300 to 1100 L capacity) where the pesticide mixed in water is contained, a hydraulic circuit which distributes the liquid up to the set of nozzles in charge of the spraying, and, sometimes, a fan that uses air to move the vegetation and partially avoid the drift effect of the droplets (movement of spray particles away from the target area). Usually, the spraying is performed through a simple boom, with no particular setup parameters, attached to the tank, and driven by a small tractor.

Plant protection products are sold in liquid form or in soluble powder; to be used, they must be carefully mixed with water in the doses reported on the product label. Doses describe the amount of product per quantity of water (g/L) and the recommended quantity of PPPs per hectare (kg/ha); the last item of data, which is redundant, shows separately the water amount to be added in the mixture per unit area (L/ha). Regulators require that the quantity prescribed on the product label be used, so farmers cannot define an optimal amount of spray in different ways, as it is legally forbidden in Italy.

It has been established that for the pesticides acting by contact, the correct dose to be used should not be referred to the field area, but rather to the overall surface of the leaves. However, during the growing period, the overall surface of a crop changes remarkably. Taking into account these dynamics, the scope of this work is to develop an artificial vision system supported by artificial intelligence, able to define optimal levels of mixture (PPP + water) related to different growth stages, to assure the highest protection effect, minimizing waste at the same time.

#### **2. Materials and Methods**

In greenhouses, the salad is picked approximately after one or two months from when it is planted, so five to six cycles per year are possible, resulting in 10 to 12 pesticide treatments every twelve months. This number of treatments is excessive, and it can create some problems regarding the environmental impact. Specifically, the spraying modes could be significantly improved, since no dedicated machines for greenhouses have been developed ye<sup>t</sup> and decisions about when and what to treat are often based on personal experience. This leads often to an overestimation of the PPPs amount to be sprayed, to guarantee that also the lower layer of leaves is reached.

In this scenario, a structured solution to tackle the main criticalities of treatments in the fresh-cut salad cultivation is proposed: a real-time advanced monitoring machine, which aims at improving the treatment by deploying robotics and artificial intelligence (AI) techniques to define how to properly spray, given the specific characteristics of the plant. At the base of this principle, we have an autonomous rover adapting the controllable characteristics of the treatment (e.g., the pressure of spraying, volume of air used, or height of nozzles from the ground) was designed and built. The rover was equipped with an advanced boom, according to what is detected in real-time from sensors, about the current state of the plant. The rover is completely autonomous and, ideally, it does not require any human assistance, saving the cost of labor or at least significantly reducing it.

As for the motion, the most effective solution is to adopt a small size rover, ideally performing the treatment on one row at a time. In this way, the prototype design is simplified, and the manufacturing costs are minimized. The rover is coupled to an advanced boom, to guarantee large control possibilities, leading to more improvements in treatment efficiency. The advanced boom can control the treatment by adjusting its height, the air distribution flow, the nozzle distance and angle, the pressure control and other operation parameters.

#### *2.1. Main Subsystems of the Machine*

The main subsystems of the designed machine are: data gathering, spraying circuit, boom structure and actuation, rover structure and motion, and navigation computing/electronics. The aim of the data gathering subsystem is to acquire information regarding the plant and the treatment, carried out thanks to a 3D camera, a pressure sensor and a flow sensor. The 3D camera is by far the most important component, both in terms of information output and cost; its core function is to determine the type and growth stage of the plant, while allowing a visual odometer to measure the speed of the rover, a fundamental parameter for the treatment. The water pressure and flow sensors instead allow a closed-loop control of the droplet size and product amount. While in theory the pressure-flow curve of the nozzles is known, and therefore only one of these sensors would be needed, the presence of both allows to detect faults (clogged nozzles, disconnected pipes, air in the pump, etc.) and consequently stop the treatment, thus allowing the user to solve the problem immediately.

The spraying circuit is the subsystem with the more traditional architecture. The water line starts from the plastic tank and, through a high-pressure (up to 8 bar) variable speed membrane pump, delivers the water to the nozzles. To allow different spacing, two rows of nozzles were installed, driven by an electrovalve to allow using only one row, so resulting in a 50 cm distance between active nozzles, or both, reducing this distance to 25 cm. The 50 cm distance was considered because it is the standard generally adopted on sprayer booms, while the addition of the second row allows multiangle treatment and lower dispersion of the droplets when reducing the boom height from the ground. A lowpressure (0.2 bar) agitation pump recirculates the water in the tank, to avoid sedimentation when a powder-based of PPP is used. The air line is composed only of a variable speed fan and a flexible plastic air sleeve to direct the flow over the treatment area.

The boom structure is an aluminum body-on-frame. The selection of aluminum as the main material, despite the higher cost, is due to the necessity to keep this component as light as possible since the overhang outside the track and the height from the ground have to be adjustable. To change the height, within a range of 30 cm, a linear electric actuator was used, to cover the optimal settings for both the defined nozzle distances.

The rover structure is a mild steel body-on-frame, coated with high-temperature powder paint to increase durability. Due to the low working speed, no suspension system was implemented. The rear wheels have independent in-hub motors, while the front wheels are free-pivoting. This configuration allows controlling the motion without a steering system, by imposing the speed of the right and left wheel. The motors are driven from two independent power drives that implement a closed-loop speed control and are directly connected to the battery. The battery is a 48 V Li-ion pack with a waterproof casing. For a suitable safety of the operations, an emergency switch is put on the battery for the disconnection.

The navigation system is based on a differential real-time kinematic GPS. This system consists of three sensors and antennas, two on the rover and one in a fixed reference position on the ground. Thanks to the real-time kinematic algorithm the relative position of the antennas can be known with precision in the subcentimetric range. The two antennas on the rover allows to determine differentially its spatial orientation, with a typical uncertainty of 0.2◦. The data coming from the GPS are fused with those obtained from the inertial measurement unit (IMU) and the 3D camera, to allow excellent accuracy and to stop safely the system in case of malfunctions. The data taken from all the navigation sensors, as well as the data gathering subsystem, are then collected and elaborated from the main computing unit, a Linux-based x86 Personal Computer (PC), to determine the movement and treatment actuator response. The PC is paired with a microcontroller and a power unit to electrically interface with the sensors and actuators. A separate power supply is used for the PC, while the microcontroller board is powered directly through the PC. To evaluate the potential of this solution, a prototype has been designed and built to run on-field tests to define the optimal way of spraying, given the characteristics of the plants.
