## 3 Discussion  
In the current study, the most effective strain of actinobacteria, _S. albulus_, was isolated and identified based on morphology, a biochemistry test, and the nucleotide sequences of 16S rDNA. _Streptomyces albulus_ displayed a strong antibiosis mechanism both in vitro and in vivo against _S. rolfsii_, the pathogen causing mungbean stem rot. According to our consistent results from both assessments, it was revealed that _S. albulus_ has the potential to be a candidate for BCA in restricting stem rot in mungbeans due to its capacity for the production of antifungal compounds, CWDEs, and VOCs. One of the most effective abilities of _Streptomyces_ is the production of extracellular metabolites that were produced and released into a medium to restrict fungal growth in PDA plates [10; 26]. The antifungal metabolites released by _Streptomyces_ species may consist of extracellular CWDEs, chitinase, and \(\beta\)-1,3-glucanase [27], as well as heat stabilizing antifungal metabolites [10; 28] and VOCs [29; 30]. As observed in this study, using a dual culture assay, _S. albulus_ Z1-04-02 effectively inhibited the growth of _S. rolfsii_ in dual culture plates. This result suggests that _S. albulus_ Z1-04-02 produced antifungal metabolites in GYMA media, which caused the restriction of fungal growth. Figure 6: Morphology of _Streptomyces albulus_ Z1-04-02 in GYMA (**A**), morphology of spore shape observed by SEM (**B**), and maximum likelihood tree of the 16s rDNA of _S. albulus_ Z1-04-02 strain and related species with 1000 bootstrap replications (**C**). Bold letters indicate the sample used in this study. Cell wall degrading enzymes composed of chitinase and \(\beta\)-1,3-glucanase are responsible for degrading the cell wall components, chitin and \(\beta\)-glucan, into small molecules respectively; they are also involved in antifungal activities [31; 32; 33]. Hydrolytic enzymes produced by _Streptomyces_ caused abnormalities in fungal morphology, as observed by scanning electron microscope [10]. Some strains of _Streptomyces_ can produce and release CWDEs into media, as observed with the enzyme assay [27; 28]. For instance, _S. exourensis_ SY224 produces chitinase and \(\beta\)-1,3-glucanase, thus inhibiting the growth of _Colletotrichum gloeosporioides_, and causing anthracose in pepper [28]. In this study, we found that _S. albulus_ Z1-04-02 also produces chitinase and \(\beta\)-1,3-glucanase, as observed with the enzyme assay. This ability may be due to an antagonistic activity in _S. albulus_ Z1-04-02 that works against fungal pathogens. The current study shows that the inhibitory effect of _S. albulus_ Z1-04-02 against the mycelial growth of _S. rolfsii_ may be due to its capacity to produce and release heat stabilizing antifungal compounds. A similar result was observed in _S. exourensis_, which showed a heat stabilizing metabolite that worked against _C. gloeosporioides_[28]. Furthermore, [PERSON] et al. [10] found that _S. angustmycticus_ NR8-2 produced a heat stabilizing antifungal compound that is responsible for suppressing the growth of _Colletotrichum_ sp. and _Curvularis lunata_. In the present study, heat-treated cell-free CF showed a lower inhibition percentage than that of untreated cell-free CF, and this may be due to the heat treatment deactivating some antifungal compounds in the cell-free CF; however, we did not observe metabolite components in this study other than the observed CWDEs and VOCs for the biological control of stem rot in mung beans. Among the four major VOCs found in this study, a bicyclic monoterpene myrtrenal has been observed in several plants with antimicrobial activity [34]. Myrtrenal and its derivatives have been widely studied via bioassay. A novel myrtrenal-based compound has been shown to have strong antifungal abilities against plant disease pathogens, including _F. oxysporum_ f. sp. _c. _cucumerium_, _Physalospora piricola_, _Alternaria solani_, _Cerospora nachidicola_, and _Gibberella zeae_[35].