## 4 Discussion  
In comparison with traditional pesticide methods, biocontrol has a low investment cost and eco-friendly characteristics [29]. Actinomycetes are one of the most important resources for biocontrol. Most _Streptomyces_ have broad-spectrum antifungal activities in controlling plant diseases [21]. In our previous studies, _Streptomyces_ sp. WHL7 was isolated from a marine soft coral and significantly inhibited the growth of Foc TR4 [18]. However, the ecological environment of a banana plantation is different from a marine environment, making it less effective to use in banana plantation. In our present study, _Streptomyces_ sp. Y1-14 was isolated from the non-Foc TR4 disease soil in the banana plantation and exhibited a strong antifungal ability against Foc TR4. Considering _Streptomyces_ sp. Y1-14 isolated from banana plantations, it is more conducive to application in the field. Interestingly, the strain also exhibited antifungal ability against different phytopathogenic fungi, suggesting that _Streptomyces_ sp. Y1-14 has great potential in managing plant diseases caused by fungal pathogens. Microbial VOCs play an important role in microbial communities [39; 40]. _Streptomyces_ are known to emit VOCs with various antifungal components [41]. In this study, the antifungal ability of Y1-14 VOCs against Foc TR4 was detected by double plate analysis and it was found that it could effectively inhibit the growth of Foc TR4. Upon further investigation of the antifungal mechanism of strain Y1-14 VOCs against Foc TR4, it was found that the main components were terpenes, alcohols, and hydrocarbons. The highest contents of trans-1,10-Dimethyl-trans-9-decalinol and 2-Methylisobormeol were in VOCs. The trans-1,10-Dimethyl-trans-9-decalinol in _Streptomyces_ _alloflaxus_ TD-1 was the major antifungal, and was proven to be responsible for the inhibitory activity against _F. _milliforme_[34]. 2-Methylisobormeol (2-MIB) in actinomycetes affected sea urchin development and the germination of some Brassicaaceae seeds [35; 36]. The antifungal properties of 2-MIB were reported against _F. _monitoring_ as fumigants [34]. These indicated that VOCs produced by actinomycetes were rich in antifungal substances to inhibit the growth of phytopathogenic fungi. In addition to VOCs, extracts of _Streptomyces_ can also inhibit the growth of pathogenic fungi. In our present study, the inhibition rate of Foc TR4 mycelial growth reached 91.74% under the treatment of 50 mg/L extracts. The treated hyphae became wrinkled and the cytoplasmic membrane was disrupted. Foc TR4 spore germination was also significantly inhibited after being treated with 8 \(\times\) EC\({}_{50}\) of extracts. Previous studies indicated that natural products produced by _Streptomyces_ species were used as antibiotics, antitumor agents, antioxidants, pesticides, and plant-growth-promoting substances, etc. [24; 42]. About 2/3 of clinically useful antibiotics were produced by _Streptomyces_ sp., including many important drugs, such as streptomycin, neomycin, kanamycin, rapamycin, chloramphenicol, vancomycin, etc. [43]. Therefore, the extracts of _Streptomyces_ sp. Y1-14 were expected to be used in other fields for inhibiting the growth of pathogenic fungi. Our previous study indicated that _Streptomyces_ sp. JBS5-6 extracts could break and disintegrate Foc TR4 mycelia [26]. _Streptomyces_ sp. H3-2 suppressed the growth and spore germination of Foc TR4 in vitro by destroying cell membrane integrity and mycelial ultrastructure [22]. However, the antibacterial mechanism is not clear. To further investigate that _Streptomyces_ sp. Y1-14 extracts affected the production of Foc TR4 metabolites, metabolomics was used to identify the critical metabolism pathways. By contrast, high abundance of metabolites mainly clustered into the carbon metabolism pathway (centralcarbohydrate metabolism, CCM) and pentose phosphate pathway. Carbohydrate pathways played an essential role in energy generation and biomass formation in organisms of all life domains.