Biocontrol potential of Streptomyces sp. M4 and salvianolic acid B produced by it against Alternaria black leaf spot.

The proposed study was designed to evaluate the biocontrol potential of Streptomyces sp. M4 and its bioactive metabolites (culture cells/cell free culture supernatant/EtOAc extract/salvianolic acid B) against Alternaria black leaf spot disease. The antifungal metabolite salvianolic acid B attacked A. brassicicola caused fungal abnormalities viz. loss of pigmentation, lysed spores, distorted hyphae and leakage of cellular contents were observed under bright field and scanning electron microscope. The bioactive metabolites in the culture supernatant were found to be thermostable (up to 70 °C), photostable and also stable under acidic and basic conditions. In in vitro experiment, treatment of fungal infected seeds with M4 antagonists (20% of culture supernatant/cells/EtOAc extract/salvianolic acid B) resulted in 82-96% seed germination, 60.71-86.20% healthy seedlings and 1470-2067 seedling vigour as compared to pathogen infected seeds. In vivo pot experiment, in which fungal spores (A. brassicicola) infected Raphanus sativus seeds were treated with Streptomyces sp. M4 culture cells/culture supernatant/EtOAc extract/salvianolic acid B (100 μg/ml), showed increase in various agronomic traits and decrease in disease incidence rate (4-16%) as compared to pathogen treated seeds. In vitro and in vivo studies unveiled that the Streptomyces sp. M4 and its bioactive metabolites could be used as potent biocontrol agents against A. brassicicola infected plants and therefore, to increase the resistance power of plants against pathogens.