Evaluation of biocontrol properties of Streptomyces spp. isolates against phytopathogenic fungi Colletotrichum gloeosporioides and Microcyclus ulei

South American Leaf Blight (SALB) of the rubber tree, caused by Microcyclus ulei and foliar anthracnose caused by Colletotrichum gloeosporioides, are diseases that adversely affect rubber cultivation in America. Both diseases have a significant economic impact on this agricultural subsector. The aim of the present study was to evaluate the potential as biological control agents of three Streptomyces species strains, namely A20, 7.1 and 5.1, against M. ulei and C. gloeosporioides. The results of analysis of variance (ANOVA) and Tukey post-hoc test of the in vitro antifungal activity assays evidenced the potential of the three Streptomyces strains to inhibit C. gloeosporioides growth through the production of diffusible (A20 and 5.1) and volatile compounds (7.1). Furthermore, other results indicated that strain 5.1 had a high biocontrol activity against C. gloesporoides, and thus such strain was selected for further evaluations as a possible biocontrol agent against M. ulei. In vitro assays suggested that active compounds produced by 5.1 inhibited M. ulei growth by interfering with conidia germ tube and stroma formation. Bioassay-guided fractionation with organic solvents of 5.1 fermentation broths, suggested that antifungal compounds produced by this strain were nonionic compounds of medium-polarity. Currently, studies are ongoing to elucidate the chemical structure of these antifungal compounds. These approaches aim to generate a biological control agent to provide the Colombian rubber subsector with a preventive measure for controlling M. ulei and C. gloeosporioides.