Antifungal Potential of Streptomyces aquilus against Soil-Borne Phytopathogen Sclerotium rolfsii and Alternaria solani Using Qualitative HPLC Profiling Bioactive Metabolites

Fungal soil pathogens like Sclerotium rolfsii and Alternaria solani trigger massive losses in the yields of various crops grown in agriculture. Their high usage to control them has resulted in ecotoxicity and development of resistant strains, thus the need to develop greener alternatives. The current research has assessed an actinomycete isolate that was characterized as Streptomyces aquilus to determine its antifungal activity against Sclerotium rolfsii and Alternaria solani. Molecular identification through 16S rRNA gene sequencing revealed 99% sequence similarity with Streptomyces aquilus, confirming its taxonomic identity these phytopathogens. The strain was inoculated in ISP2 medium and the secondary metabolites were extracted after 15 days of fermentation with the help of ethyl acetate. The crude extract was concentrated, resettled in dimethyl sulfoxide (DMSO), and the antifungal activity was screened by using primary and secondary screening tests. The perpendicular streak method in the primary screening showed that the fungal growth was highly inhibited whereas the secondary screening with the agar well diffusion method showed a high antifungal zone against the Sclerotium rolfsii and Alternaria solani. Distilled water and clotrimazole were used as negative and positive control, respectively. Moreover, the metabolic diversity of S. aquilus was confirmed by the possibility of identifying several peaks with different retention times in qualitative HPLC profiling of the crude extract. The proposed research shows that S. aquilus has the potential of emerging as a valuable biocontrol agent against phytopathogens that affect the soil. The availability of bioactive secondary metabolites with high antifungal properties indicates that this strain can be used as an efficient alternative to fungicides to protect crops in a sustainable manner.