Adjusting sprayers for effective application

Factors point to a high level of risk in the incidence of Asian rust in Brazilian crops. Among the many measures that need to be adopted, the correct choice of equipment for applying phytosanitary products is fundamental for their control and depends on several aspects.

Chemical control of Asian soybean rust is a very common cultural practice in Brazilian crops. In the absence of this control, rust can cause major losses in productivity. In this process, greater importance is often given to the product to be used in the control, and less to the way in which it is used. Accumulated losses in Brazil reach US$21 billion considering the use of fungicides and direct losses due to frustration and harvest.

Read also

In Brazil, the disease was initially described in Lavras, Minas Gerais, in 1979 by Deslandes. It later occurred in São Gotardo, Minas Gerais, in 1986. Large-scale epidemics began in the 2000/01 harvest, where the disease was found in the State of Paraná, simultaneously by Dr. Davi Jaccoud Sousa Filho from UEPG (Campos Gerais – Ponta Grossa) and Dr. José Tadashi Yorinori – EMBRAPA-Soja and then quickly spread to other states in Brazil. In the 2002 harvest, the disease was reported in the states of Goiás, Mato Grosso, Mato Grosso do Sul, Minas Gerais, Rio Grande do Sul, Santa Catarina and São Paulo, and in the 2003/04 harvest it occurred in a widespread manner, in almost the entire country, causing great losses. From then on, an intense search for effective active ingredients began, resulting in several products with good control of the fungus, mainly in studies under controlled conditions. However, in the field, control difficulties remained, largely due to the way in which the products were applied.

Figure 1 shows the gradual decline, year by year, in the effectiveness of fungicides, whether due to inadequate management or application technology with failures in deposition on the target.

The coverage provided by the application of fungicide over the soybean canopy is generally not uniform, especially in the lower part, resulting in inefficient control, even with systemic products. These products generally have limited translocation in soybean plants and also require good distribution throughout the canopy. Therefore, for the application to be successful, it is necessary to master the appropriate form of application, in order to ensure that the product reaches the target efficiently, minimizing losses and reducing environmental contamination. Often, part of the applied product is lost, mainly due to the poor quality of the application, whether terrestrial or aerial, which is one of the biggest problems in modern agriculture to be overcome. Droplet size and application uniformity are two very important parameters for effective pest and disease control. Therefore, application technology is a very important tool. The chemical and agricultural machinery and components industry has launched new products onto the market with the aim of improving application, including new designs of spray tips, auxiliary bars and adjuvants.

A similar problem also occurs with aerial application, which is a reality in most grain-producing regions in Brazil. Its use has grown a lot, however little scientific information exists regarding its effectiveness, especially in comparison to terrestrial application using boom sprayers, which causes distrust among farmers regarding its technical feasibility, especially regarding the spray's penetration capacity. sprayed on the crop canopy.

Therefore, given the importance of efficient and safe control of Asian soybean rust, this article summarizes results found in studies carried out in Goiás and Minas Gerais with the aim of evaluating and relating the various factors involved in the technology for applying fungicides to soybean crops.

Spray Tips

When carrying out a combined analysis of the tests, it is noted that in general the spray tips had a more pronounced effect on the deposition of spray on the target and a lower effect on productivity. Some trials did not show productivity differentiation in relation to the use of different tips, as this is linked to the occurrence and aggressiveness of the fungus in the area or is still dependent on the tolerance or partial resistance of the cultivar or even on the interaction of the cultivar with the management used. Therefore, management and control depend on the cultivar and the moment or time of appearance of the disease. The experimental variability itself can also make it difficult for deposit differences to be significantly felt in the productivity analysis. Furthermore, in the lower part of the canopy, a critical location for rust control, the differences in deposition between tips are usually smaller, with drop density below 60 drops/cm.

There are currently many tip models on the market. Although generalization must be analyzed carefully, it was noted that tips with medium drops appear to be more suitable for controlling Asian soybean rust, given the better results found in deposition and rust control. Medium drops suffer less intensely from the phenomenon of drift and evaporation, having a longer lifetime than fine drops. On the other hand, they have a greater target coverage capacity than thick drops.

An important item to be considered when recommending the most suitable tips is the size and foliage of the crop. When analyzing deposition in the lower canopy of heavily leafed plants, it is clear that conventional hydraulic spraying, regardless of the tip, is not capable of promoting high coverage. Therefore, applications need to start when the leaf area index (LAI) is not yet very high.

This is even more important if we take into account that most systemic fungicides used in soybeans have limited translocation. The active ingredient needs to be placed on the target, otherwise the disease will not be controlled.

Syrup volumes

In general, no improvement in rust control or increase in productivity was detected with the use of larger application volumes. For example, in a study carried out it was shown that a volume of 160 L/ha allowed greater uniformity of spray deposition throughout the canopy than 130 L/ha, but this did not reflect on productivity.

Given these results and the known gain in operational capacity of the mechanized set, it appears that there is feasibility in using reduced spray volumes, close to 130 L/ha. However, this reduction requires an increase in application technology and constant monitoring of meteorological conditions, which can make target coverage difficult.

Adjuvants

The addition of adjuvants to spray mixtures can be important when applying different phytosanitary products, but it should not be a general practice, given their different behavior in each type of application. The study of the effect of adjuvants in the spray solution is specific to each product and formulation used. There is a big difference between the different commercial adjuvants. Even products considered similar, such as vegetable oils, can vary greatly in terms of their form of action. Therefore, consultation with independent experts is essential.

The physical properties of the spray can interact with the type of tip used, promoting the formation of the jet in a unique way for each situation. This may explain the existence of different results between the tips, in relation to the use of the adjuvant.

Final Words

There is no clear definition regarding the selection of the ideal spray tip. In many cases, the variability of data intrinsic to this type of comparative trial between different application techniques makes it difficult to differentiate treatments. In general, tips that produce medium drops seem to be more suitable for controlling Asian soybean rust, also avoiding high risks of drift. There is technical feasibility in using reduced spray volumes, close to 130 L/ha.

Aerial application proved to be viable, in terms of spray deposition, in comparison to ground treatments.

The use of adjuvants proved to be a good tool to help control rust, however, their effect is related to the type of tip used and the syrup, which prevents generalizations.

The uniformity of syrup distribution, provided by ground and aerial applications, evaluated throughout the soybean canopy, is generally low. It is necessary to seek strategies that increase deposition, especially in the lower part of the crop.

In the sprayer evaluation and inspection program (IPP) carried out in recent years in Brazil, Triângulo Mineiro and Alto Paranaíba (MG) have demonstrated lower deposition on target (40-60%) or in some cases over dosage (30%) in pneumatic and hydraulic sprayers. Despite other mechanical problems that were detected such as tips with variation in deposition (3-10%), leaks and defects in the circuit and pressure gauges (5-15%), the biggest concern can be discussed for the lower deposition on the target (underdosing). This deficiency increases the risk of field selection of biotypes or pathotypes in the pathogen population and reduce the effectiveness of the fungicide in greater use or greater adoption by farmers. Hence the importance of using protective fungicides associated with systemic fungicides with efficient and quality application technology.

Aerial application

In comparative studies between aerial and terrestrial application, it was found that both methods presented difficulty in covering the lower part of the canopy, which suggests that, regardless of the application method, priority should be given to preventive applications in which there is not yet a total closure of the culture, allowing better coverage of the target. In this sense, both methods proved to be viable.

In this type of comparative study, it is common to use water-sensitive papers, however, their use must be done with great care when using different volumes of syrup. As the marking of the drop on the paper is caused by water, high volumes can lead to a false impression of superiority of the treatment compared to low volumes, if it is not jointly correlated with the concentration of the active ingredient in the syrup. Simply counting the number of impacts per area, without taking into account the coverage area, can also lead to a false idea of ​​superiority with high volumes. In these cases, the use of tracers compared to the use of this paper, or even the active ingredient, appears to be more reliable, since the same dose is used, although with different volumes of syrup. Another point to be highlighted is that the spread of drops on sensitive paper is heterogeneous and dependent on several factors, making it difficult to accurately measure droplet size.

João Paulo Rodrigues da Cunha, Fernando Cezar Juliatti, Federal University of Uberlândia

Article published in issue 157 of Cultivar Máquinas.