The has soybean production capacity can be linearly in recent years. However, there are problems that can exert productivity up to 90%, such as a pachy Asian rust (Phakopsora pachy) (Sinclair; Hartmann, 1999).
According to Fiallos (2011), one of the most relevant methods for controlling the disease is through the use of fungicides, minimizing the damage and the appropriate uses. According to Freitas et al. (2016), the best control of Asian rust was obtained when three chemical groups of fungicides were mixed in four applications, associated with mancozeb. This is a multisite fungicide that can help to delay the resistance of fungi and increase the efficiency of site-specific fungicides, explains Camargos (2017).
It is important that it stands out that it is a compound of interaction by temperature and can be put from environmental factors, such as still water. In addition, it is also possible that there are changes in the physical-chemical characteristics of the mixture and in the quality of spraying, says Scariot (2016).
However, the magnitude and interaction of the temperature of the mixture with the components present in it are unknown (CUNHA et al., 2010). According to Araújo (2006), water is the most used diluent in the applications of phytosanitary products, and its condition in the spray arrangement has been much debated.
According to Reis and Casa (2007), the effectiveness of chemical control is linked to the application technology, the moment or criterion for starting the fungicide application and the spraying quality.
For fungicides with difficulties to stabilize, there are two ways to increase the solubility of a solvent: through intense agitation with surfactants or heating. The first point is already performed by those who work with these products, but the second is not yet.
Thus, the objective of the study was to verify if the variation of the water temperature for the preparation of the syrup with mancozeb (Unizeb Gold® WG) exerts any synergistic or antagonistic effect on the preparation of the syrup and on the effectiveness of soybean rust control.
The preparation of the grout treatments for the water temperature variation was carried out directly in the immediate application field, to avoid temperature fluctuations. For this, hot water (80°C) was transported in a thermos and cold water (5°C) in another bottle. By mixing both, it was possible to achieve the desired treatments (T1: 10°C, T2: 20°C, T3: 30°C, T4: 40°C and T5: 50°C) and all were measured with a digital thermometer (Minipa MV-363) (Figure 1), in addition to a control (T6) without application.
By changing the chemical properties of the spray solution, heating can promote a loosening of the hydrogen bonds between the water molecules, decreasing the cohesive forces and, consequently, reducing the surface tension of the water. Sundaram (1987) explains that the lower surface tension of water can help solubilize very complex solutes at high doses, such as mancozeb. Thus, according to Hansen (2007), with increasing temperature, the solubilized amount of a solute can be increased and the action of solvents improved.
However, it was possible to observe some limits for this technique with mancozeb. Of all the temperatures studied, as shown in Figures 2 and 3, the 10°C and 50°C limits presented more problems for rust control performance and productivity, respectively. The treatment at 20°C was the second most productive, standing out from the treatment at 40°C. The most productive was when mancozeb was mixed with water at 30°C, standing out from all the others. Therefore, this spray temperature appears to have caused less stability and performance problems in the water mixture plus the mancozeb formulation.
For Vidal (2014), the influence of temperature on plant absorption is small, and increases above a maximum, instead of accelerating, end up delaying absorption. It is believed that the inhibitory effect of high temperatures is linked to the denaturation of enzymes and proteins, which will be reflected in absorption.
Thus, the decrease in temperature will delay the processes of free diffusion, as well as the biochemical reactions that intervene in the active absorption, says Rodrigues (2003). The intermediate temperatures (20°C, 30°C and 40°C) did not differ statistically in the control levels, despite the variation in the raw data.
According to Azevedo (2006), temperatures below 15 °C reduce the physiological activity of plants, reducing the absorption of products that present physical or chemical instability, such as systemic or translaminar action. The ideal temperature must be below 32°C, so that a greater translocation of the product occurs in the plant and, consequently, greater efficiency at the time of spraying.
Thus, as highlighted by Vieira (2016), there are studies on herbicides that report on the spray temperature, which can influence practically all physical-chemical processes and their components, consequently improving the effectiveness of the application process. According to Vidal et al. (2014), the use of herbicides at room temperature has a great impact on several physiological processes of plants, influencing biochemical reactions, as well as penetration, absorption (diffusion through the leaf cuticle) and translocation of herbicides in the phloem. In this way, there is a greater effect on weeds.
When the mixture was chemically analyzed in the laboratory, significant changes were observed, corroborating the field data (Figures 4, 5 and 6). It was possible to infer that the fact that the mancozeb particle is a metallic complex can promote the agglomeration of the particles due to the presence of electrostatic charges. In water at low temperatures, these bonds are difficult to break, which makes it difficult for the mancozeb particle to disperse. Likewise, in water with higher temperatures, there is a greater movement of mancozeb molecules in the medium, which provides greater collision and, consequently, greater possibility of agglomeration. In both cases, mancozeb agglomerates form in the spray solution and this reduces the contact area of the active and leaf cover, which will cause a decrease in the effectiveness of the mancozeb.
This water heating technique to facilitate mixing with mancozeb has already been used in the field at the company Sementes Costa Beber (Figures 8 and 9). In this case, the 350 liter heated water tank is located above the pre-mix tanks, maintaining the temperature between 35 and 40°C. When emptied, it is replenished with water and maintains temperature with an electric thermostat.
When premixing, a hose brings the warm water by gravity to the 250 liter premix tank. This tank is used only for mixing with mancozeb.
For the dilution of mancozeb, 80 kg are used per 200 liters pre-mix, slowly diluting with warm water (around 30°C), then stirring until the dilution is complete. After pre-mixing, the solution is transported to the tank and the other dilutions are carried out. After everything is sent to the tank, the 3,000 liter PV 4730 JD tank is filled with water at room temperature. Only the pre-mix is made with heated water.
When carried out in the field in more distant fields, a 200-liter thermally protected tank with warm water is taken (Figure 10).
Thus, according to studies for making the syrup with only mancozeb, water at 30°C proved to be the best option. Waters with temperatures around 10°C and around 50°C were harmful to disease control and productivity.
More studies are needed on the temperature of grouts, with the combination of other products mixed in the tank.
