Controle de doenças em soja com diferentes manejos e momentos de aplicação de fungicidas baseados no índice de área foliar

Abstract

The annual losses caused by diseases in the soybean crop are estimated in around 15% to 20%, as they directly affect the leaf area index (LAI) of the plants, the main component of the production due to photosynthesis. The lower sensitivity of the pathogens to the site-specific fungicides significantly alters the management adopted in the country’s crops, thus, the objective of this work was to propose a new recommendation methodology for the beginning of fungicide applications, based on the leaf area index of the crop and a more efficient management system. The work was conducted in two locations within the municipality of Uberlândia-MG, in a randomized complete block with four replications, in a triple factorial scheme with additional, 4x2x2+1, beign four leaf area indexes (LAI) for the beginning of fungicide applications (0.9; 1.8; 3,8 and 6.1), two sprays volumes (70 and 120 L ha-1), presence or absence of multisite fungicide (mancozeb - 1,125 g ai ha-1) and one additional treatment corresponding to the control, without any application. The LAI was calculated during the development of the culture for the beginning of the applications, in total three fungicide applications were carried out for each treatment; six severity assessments of each disease (Asian rust, soybean powdery mildew and brown spot) and an evaluation of the deposition of spray in the lower and upper parts of the culture in each LAI studied. At the end of the crop cycle the area under the disease progress curve and crop productivity were calculated. The results obtained for the deposition of spray showed that for the LAIs of 1,8 and 3,6 there were no differences between the types of managements adopted for the two thirds studied, although the trend was of lower deposition in the lower third with the greater development of culture. In the control of diseases, all the treatments differed statistically from the control in the two areas, with emphasis on treatments with mancozeb and application volume of 120 L ha-1, which presented better controls of the diseases, which in turn required the most appropriate LAI, soybean powder and brown spot (0.9); and Asian rust (3,6), for the three applications of fungicides in the 14-day interval. The best yields were obtained when the applications were started with LAI of 3.6 for the two applications volumes and in the presence or not of mancozeb. It can be concluded that for three applications of fungicides, the best relationship between control and yield was obtained with applications starting with LAI close to 3.6 in the presence of mancozeb for the two applications volumes (70 and 120 L ha-1).