Impactos de tratamentos foliares na abundância de bactérias e fungos do filoplano e seu potencial no controle de Fusarium verticillioides em milho

Abstract

In the tropical and subtropical regions where maize is produced there are quantitative and qualitative losses of the grains associated to the fungus Fusarium verticillioides (Fv). Among management strategies, chemical control is the most used. However, studies show several products registered for the fungus control, showing no effect on the incidence of the pathogen and reduction of fumonisins. The use of biocontrol agents (BCAs) can soften or reduce this problem, acting with different mechanisms that is still little studied. The goals of this work was to evaluate the impact of different foliar treatments on fungi and bacteria of the phylloplane and their potential to control Fv in maize. The microbial collection was obtained from maize leaves and spike inculated with Fv and previously treated in two different phenological stages (V9 and R1) with fungicide - Azoxystrobin + Ciproconazole (Az), BCAs - Bacillus subtilis BIOUFLA2 (Bs) and control treatments (water and bacterial culture medium YPD). It was found that when two applications of Bs are made, the percentage of native antagonistic bacteria and fungi of the phylloplane against Fv increased 25 and 27.3% respectively. However, the two applications of the fungicide at different phenological stages in maize reduced the communities of antagonists. From the field assays, 100 fungi and 140 bacteria were selected and preserved. Under controlled conditions the incidence and severity of Fv were evaluated in maize grains inoculated with the selected antagonists seven days before and simultaneously with the pathogen. Thirty-four bacteria and thirteen fungi were selected for having inhibitory activity against Fv, the majority coming from the exclusive treatment with Bs. These isolates were evaluated concerning their mechanisms of action. Antibiosys and niche overlap index (NOI) tests were conducted to select the best antagonists. For eletronic microscopy analisys, 5 mm discs were used from the zone of interaction between the antagonists and the pathogen and these were fixed in Karnovsky until standard processing. Two bacteria (Bacillus amyloliquefaciens B45,2) and (Burkholderia gladioli B100) were obtained and identified as antibiotics producers. Three fungi (Phomopsis sp F42, Epiococcum sp. F46 and Trichoderma sp F8) showed evidence of mycoparasitism and production of metabolites against Fv. Among these isolates, F8, F46 and B100 competed for space and nutrients in maize against Fv. The results confirmed the potential use of Bs in the integrated management of Fv in the field, acting in the selection, conservation and proliferation of the antagonistic microorganisms native to the maize phyllosphere. Thus, the potential of these isolates in the field, in the treatment of seeds or in foliar applications in the management of rotten grains should be investigated.