Although many technicians and producers consider them as synonymous, there are important differences in concept between the application and spraying of agrochemicals. Therefore, spraying can be considered as a mechanical process that aims to break the volume of spray solution into a large number of particles, called drops. While application is the placement of a chemical on a certain target, to protect the crop. Therefore, for fungicides to deliver their maximum effectiveness in disease control, it is necessary to apply more and spray less.
After the droplet is produced by the spray tip, it will have to face a climatic barrier and then a plant barrier to reach the target inside the canopy. The great challenge of Application Technology is to ensure that the drops cross the upper layer of plant leaves and arrive in conditions to be distributed, deposited and adequately cover the crop canopy. To achieve this, it is necessary to study strategies that increase the deposition of drops sprayed with phytosanitary products in the lower part of the canopy.
The penetration of droplets into the canopy of a crop is a fundamental factor in the chemical control of diseases, especially those that initiate the infectious process in the lower leaves, as is the case with Asian soybean rust and leaf spots. Product penetration into the canopy is related to the stage of development the crop is in and its leaf area index, and the more closed the canopy is, the greater the difficulty for the drops to reach the basal leaves of the plant.
The application of the fungicide before closing between rows (vegetative stage - Vn) aims to increase the penetration of the fungicide spray droplets, improving leaf coverage in the middle and lower parts of the canopy, consequently protecting the leaves from diseases that occur at the beginning of the cycle (Figure 1). Once the inter-row is closed, the sprayed syrup drops have difficulty reaching the lower canopy of the crop, due to the excess foliage, forming a kind of physical barrier for the penetration of the fungicide. This way, the application will not reach the target, and the evolution of the disease will compromise the entire control program and, consequently, productivity.
Although there are several works in the area of application technology that quantify the deposition of spray solution in the plant canopy, few consider the barriers imposed by differences in architecture between cultivars. Therefore, plant architecture can be cited as a factor that limits or facilitates the entry and distribution of the application in the plant profile. For Huyghe (1998), architecture includes several characters, such as number of stems and branches, structure of stems or branches (number and length of internodes) and structure, size and orientation of leaves. Each cultivar presents peculiar characteristics with regard to its own architecture, which vary according to the sowing time and environmental growing conditions, which influence the droplet coverage throughout the plant canopy due to fungicide applications.
The efficiency of fungicide application depends on uniform coverage of the leaves (target), whether in the upper, middle or lower thirds, which is provided by the different droplet spectra. Figure 2 shows how a volume of liquid contained in a 400µm drop can be reduced to eight 200µm drops, providing greater coverage and distribution of the product on the leaf area. In general, larger spectrum drops tend to be deposited at the top of the plant. Medium to fine spectrum droplets tend to be deposited in the middle and lower thirds of plants, the main sites of pathogen infection. Therefore, the application of fungicides with fine to medium drops is recommended to obtain greater effectiveness in disease control.
Fine droplets provide better coverage, whereas very small droplets may suffer from drift or evaporation. This occurs because they have little weight and inertia, remaining suspended in the air for longer, becoming susceptible to drift and evaporation during application. However, under suitable weather conditions, penetration and target coverage of fine droplets are often greater. The ideal conditions to maximize applications are relative humidity above 55%, temperature below 30°C and wind speed of 3km/h to 10km/h.
Typically, finer droplets are a result of applications with fine orifice spray tips and reduced application volumes under high pressures. Conversely, larger orifice tips, high volumes and low pressures tend to generate larger droplets (Santos, 2003). The choice of spray tip allows the size of the drops produced to be adapted to the application conditions, ensuring, at the same time, biological efficacy and environmental safety.
Droplet coverage can be defined as the percentage of the target that was covered by the spray (Baesso et al., 2014). Therefore, it can be said that it is the area occupied by the drops after application, the number of drops/cm² of the target. According to Christofoletti (1999), for good effectiveness in disease control, coverage of 30 drops/cm² to 40 drops/cm² for systemic fungicides, and between 50 drops/cm² and 70 drops/cm² for protective fungicides is necessary. (Table 1). Working with spray volumes of 120L/ha and 160L/ha, Ugalde (2005) observed a minimum efficient drop coverage for systemic and protective fungicides of 45 drops/cm² and 60 drops/cm², respectively. However, the main objective is to produce drops with the maximum coverage possible with an appropriate volume of spray, following the recommendations contained in agronomic recipes.
The coverage of the leaf by the drops can be positively added by the addition of adjuvants in the spray solution, which, in addition to contributing to the adhesion of the drop, will help in its spreading over the leaf and its absorption into the tissue (Figure 3). As a result, the product's lifespan will be adequate, ensuring the health of the crop and the efficient control of diseases.
It is clear that application technology plays a fundamental role in increasing the coverage of the plant by drops, with the aim of depositing the amount of active ingredient necessary to control diseases. Optimizing disease control effectiveness not only depends on the choice of efficient fungicides, but also on factors related to droplet penetration and coverage throughout the plant canopy.
Marlon Tagliapietra Stefanello
With each new edition, Cultivar Grandes Culturas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in the cultivation of large crops, the Magazine helps farmers in the search for management solutions that increase their profitability.
Receive the latest agriculture news by email
Receive the latest agriculture news by email
