## 1 Introduction  
Rhizosphere soil is an important source of beneficial microorganisms, especially rhizosphere microorganisms, which have been studied for their beneficial effects on plant growth and their roles as biocontrol agents [1, 2, 3]. Actinobacteria are fungal-like microorganisms of the order Actinomycetesales. The structure of actinobacteria consists filaments in a network, and actinobacteria can produce spores or conidia. Actinobacteria are Grampositive and they mostly inhabit soil, utilizing soil substrates and the decaying organic matter in soil. _Streptomyces_ is a genus of the family Streptomycesaceae [4], which has been widely studied for its contributions as a biological control agent (BCA) that works against plant diseases [5; 6]. Different strains of _Streptomyces_ can produce cell wall degrading enzymes (CWDEs), such as chitinase and \(\beta\)-1,3-glucanase, volatile organic compounds (VOCs), and antimicrobial metabolites, which are responsible for the suppression of plant pathogens [7; 8; 9; 10; 11; 12]. Furthermore, some _Streptomyces_ strains can induce disease resistance [13; 14].  
Several strains of _Streptomyces_ have been widely applied in agriculture to control plant fungal diseases, and they have diverse antimicrobial abilities. For instance, _Streptomyces alfalfa_ produced various extracellular hydrolytic enzymes and diffusible antifungal metabolites that work against _Fusarium oxysporum_ f.sp. _vasinfectum_, the pathogen of fusarium wilt in cotton [15]. _Streptomyces griserupbiginous_ regulated plant defense enzyme activities and induced sugarcane smart resistance by regulating stress-related enzyme activities in sugarcane [16]. _Streptomyces sichuanensis_ produced siderophore and antifungal metabolites that work against _F. oxysporum_ f.sp. _cubense_ tropical race 4, the causal agent of fusarium wilt in banana [17]. _Streptomyces_ sp. produced cellulolytic, chitinolytic, and proteolytic extracellular enzymes that inhibit _Macrophomea phaseolina_ and _Rhizoctonia solanti_ in _Phaseolus vulgaris_[18]. Furthermore, _Streptomyces_ sp. AN090126 produced and secreted antimicrobial metabolites into culture filtrates, and they produced volatile antifungal compounds that work against plant pathogens [19].  
The mungbean (_Vigma radiata_) is a plant species of the legume family, and it is mainly grown in the tropical and subtropical areas of East Asia and Southeast Asia, including China, India, Pakistan, Thailand, Indonesia, and the Philippines [20]. As with most economic crops, global mungbean production often encounters serious problems that can cause a reduction in yield. The major problem is that it is susceptible to southern blight or sclerotium rot caused by _Sclerobium rolfsii_[21]. In Thailand, the cultivation of mungbean is distributed throughout the country, and most cultivation areas are located in northeastern Thailand (according to the Office of Agricultural Economics, Thailand).  
In Thailand, mungbeans also suffer from sclerotium rot, even though the use of synthetic fungicides effectively controls sclerotium rot [22; 23]; however, the use of high doses of fungicides might cause the development of chemical resistance [24], and it may also cause harmful side effects that impact human health [25]. _Streptomyces_ strains have exhibited strong antibiosis against plant diseases, and therefore, the use of actinobacteria from the rhizosphere to control soil-borne pathogens is an alternative method to control this disease. This study aimed to find the effective strains of the rhizobacterium _Streptomyces_, which have diverse fungicidal abilities, to suppress _S. rolfsii_ and to reduce disease severity in mungbean crops.