Correct use of adjuvant against Asian rust

In the fight against Asian rust, the correct use of the adjuvant recommended by the fungicide manufacturer is an important tool so that the product can be properly absorbed by the leaf. This effect is mainly highlighted by the performance of strobilurins and carboxamides, which are highly dependent on foliar absorption to ensure greater control residuals.

The planning of plant protection actions is basically aimed at managing Asian soybean rust, which is among the main factors that limit the achievement of high soybean yields. The establishment of the disease includes climatic conditions suitable for pathogen infection, such as prolonged leaf wetness associated with daily temperatures below 28ºC.

It is generally known that Asian rust damage is most effective from the full flowering phase until the end of the crop, a period that corresponds to the greatest mobilization of photoassimilates to the grains and no longer to the effective activation of defenses. Therefore, the plant is physiologically compromised with grain filling due to the large energy expenditure, unable to avoid the accelerated progression of Phakopsora pachyrhizi.

In order to manage the crop, it is necessary to use alternatives that protect the leaf area or reduce the evolution of the disease. In this sense, this includes the use of early cultivars with some level of tolerance, planting at the beginning of the recommended season and increasing the spacing between rows. These strategies end up improving the penetration of fungicides, especially in the middle and lower third, in addition to favoring a microclimate with greater air circulation, higher temperature and, consequently, lower humidity and less leaf wetness. Furthermore, the elimination of voluntary soybean plants, the absence of soybean cultivation in the off-season through sanitary vacuum and the monitoring of the crop from the beginning of the crop's development are essential measures to provide greater health to the crop.

As an integrated complement to management, the use of a mixture of fungicides from the triazole + strobilurin group is essential for foliar protection and damage reduction. Furthermore, recently, new molecules from the carboxamide group (fluxapyroxad and benzovindiflupyr) were launched to assist in control, as Brazil has been working for 14 years with the same two groups already mentioned. Unfortunately, correct use and positioning is still an impasse, whether due to a delay in the arrival of the pathogen, excessive interval between applications or failure to follow the company's recommendation related to the adjuvant in the mixture.

In this sense, the use of foliar fertilizers associated with fungicides can be an application alternative, taking advantage of the sprayer's entry into the area. However, this management does not preclude the use of the adjuvant recommended by the company that owns the fungicide. If this occurs, in addition to not being recommended by the manufacturer, the efficiency of the product is compromised, as the use of the adjuvant is essential to accelerate the absorption of the product by the leaf. This effect is mainly highlighted by the performance of strobilurins and now carboxamides, as they are highly dependent on foliar absorption to guarantee greater control residuals (Figures 1 and 2). This fact is due to the non-polarity of mineral or vegetable oils, having a lipophilic characteristic and interacting strongly with the components of the cuticle and cell membranes, eliminating the barriers that reduce the absorption of products by plants.

The association of different products in the tank can generate chemical or physical changes in the syrup, resulting in antagonistic, additive or synergistic effects. In this situation, as a consequence, there may be less coverage of the leaf area by the active ingredient and less penetration into the leaf tissues, reducing its effectiveness (Figures 3, 4 and 5), accumulating a greater final amount of disease (Area Below the Disease Progress Curve – AACPD) (Figure 6). If the energy expenditure that the plant mobilizes for the production and activation of compounds to defend its own metabolism, which would be directed to the emission of new leaves or grain filling, was not enough, it also compromises the photosynthetically active leaf area. In this way, the expansion of lesions ends up taking over the leaf area, accelerating the process of early defoliation (Figures 7 and 8). Consequently, the premature loss of leaves removes the main source of carbohydrate production that would be directed to the drains (grains), thus reducing final productivity and grain mass (Figure 9).

Simone Gripa Minuzzi, Felipe Frigo Pinto, Ricardo Balardin, Federal University of Santa Maria; Marcelo Gripa Madalosso, Phytus Institute/Univ. Integral Reg. - Santiago

Article published in issue 191 of Cultivar Grandes Culturas.