Molecular characterization and functional properties of deep-soil-inhabiting actinobacteria for combating Fusarium dieback disease in tea crop

Dieback, caused by Fusarium solani, severely damages tea foliage and decreases both the quality and quantity of harvestable shoots. The management of dieback disease through integrated approach has been suggested and the use of microbial biocontrol agents is one of the alternative methods. The search for efficient microbial biocontrol agents is also essential for overcoming the drawbacks of synthetic fungicides. Among the microbial biocontrol agents, actinobacteria have promising role in disease management. In the present study, 18 native isolates of actinobacteria were isolated from tea rhizosphere soil collected from 12 tea gardens. Evaluation of 18 isolates of actinobacteria against F. solani by dual culture technique revealed that isolates MBK05, MBK21, MBK31, and MBK34 were potential antagonists. Among them, MBK05 and MBK21 were fast growing isolates, hence selected for detailed study like morpho-molecular characterization, for antifungal traits as well as their efficacy in disease control under field conditions. Based on the 16S rRNA, the isolates MBK05 and MBK21 were putatively identified as Kitasatospora putterlickiae and Streptomyces bungoensis, respectively. Both isolates possessed multifarious physiological activities and were found to produce cellulase, protease, chitinase (except MBK05), and β-glucanase. The liquid formulations of both isolates in 10 and 20 % concentrations (1 × 10⁸ CFU/mL) were evaluated in two seasons against Fusarium dieback under field conditions. Results revealed that 20 % concentration of both isolates gave maximum percent disease reduction over control, i.e., 72.25 % with MBK05 and 70.3 % with MBK21. Among the synthetic fungicides evaluated, copper oxychloride 50 %WP (1 %) showed maximum disease reduction (75.46 %). These results support that K. putterlickiae and S. bungoensis are promising microbial biocontrol agents against Fusarium dieback in tea crop.