Screening, Identification, and Fermentation Optimization of the Antagonistic Actinomycete Strain TCS21-117 Against Botrytis cinerea.

Biological control is considered one of the most important methods for preventing and controlling the worldwide fungal disease gray mold, caused by Botrytis cinerea. Among the various agents used in biological control, actinomycetes represent a significant group of microorganisms that offer valuable resources for biocontrol strategies. In this study, a total of 132 actinomycetes, belonging to four genera (Streptomyces, Kitasatospora, Amycolatopsis, and Nocardia), were isolated from soil. Among the five media tested, ISP-2 and GS NO.1 media were found to be highly suitable for isolating actinomycetes. It is worth mentioning that the strain TCS21-117 displayed significant inhibitory effects against Botrytis cinerea and nine other pathogenic fungi. The strain TCS21-117 was identified as Streptomyces roietensis by its morphological characteristics and phylogenetic analysis of the 16S rRNA gene. The optimum culture conditions for the strain TCS21-117 were a potato dextrose broth medium at an initial pH of 8.0, a liquid volume of 125 mL in a 250 mL flask, 180 r·min(-1) at 28 °C, and an inoculum size of 1% for 7 days. Under these conditions, the inhibition rate against Botrytis cinerea was 93.31%, a significant increase (31.98%) as compared to the control. Notably, the antifungal compounds produced by the strain TCS21-117 exhibited strong stability across a range of temperatures, pH levels, and durations of storage and UV irradiation. This study showed that the Streptomyces roietensis strain TCS21-117 had strong inhibitory activity against Botrytis cinerea under optimized fermentation conditions, enriching the microbial resources for gray mold control.