Comparative Analysis of Antagonistic Activity and Secondary Metabolite of Streptomyces spp. Isolates Against Alternaria solani and Their Effects on Tomato Growth

Streptomyces isolates are particularly well known for producing enormous amounts of bioactive secondary metabolites, among which some exhibit exceptional antimicrobial activity. Twenty five Streptomyces isolates were isolated from tomato field soil and tested for their ability to inhibit Alternaria solani. Two such isolates, M1 (Streptomyces gilvosporeus), which inhibited A. solani 64%, and M5 (Streptomyces sp.), ineffective, were chosen. Secondary metabolites of these isolates were isolated and contrasted by Gas Chromatography-Mass Spectrometry (GC-MS), identifying 40 compounds in isolate M1 and 91 compounds in isolate M5. Among them, 12 bioactive compounds were found in common in both isolates. In the M1 extract, the most dominant detected constituents were 2-[2-(7-Chloro-7-norcaranyl)ethynyl]thiophene (4.88%), n-Hexadecanoic acid (6.88%), Methoxymethyl(triethyl)stannane (23.72%), Ergoline-8-carboxamide, 9,10-didehydro-N,6-dimethyl-, (8.beta.)- (8.68%), 1,2,6b,7,8,9,10,10a-Octahydroindeno[1,2,3,-cd]pyrene (15.91%). A second follow-up pot experiment confirmed that the M1 isolate and its secondary metabolites completely suppressed disease manifestation (100% suppression of early blight disease) and improved tomato plant growth significantly. This study illustrates the potential use of Streptomyces sp., particularly S. gilvosporeus, as superior and environmentally safe biocontrol molecules for the regulation of tomato early blight under agricultural settings.