## 4 Discussion
The PCR results suggested that ST-TJ4 contains genes for the biosynthesis of PCA and PRN. Phenazine type of antibiotics, such as PCA, are particularly active against oomycetes [43; 44]. [PERSON] et al. [45] demonstrated that PRN produced by _P. chlororaphis_ O6 plays a pivotal role in inhibiting _Phytophthora infestans_, which causes tomato late blight disease. In addition, genes related to the biosynthesis of HCN were also detected in strain ST-TJ4, and several recent studies reported that HCN may be involved in the effective control of wheat sheath blight by several _Pseudomonas_ spp [46]. The volatile gas produced by biocontrol bacteria can inhibit the spore germination and mycelial growth of plant pathogens, and it has attracted wide attention [47]. Compared with non-volatile antimicrobial substances, volatile fungistatic substances have smaller molecules, are easier to permeate and spread in soil and atmosphere, and can kill pathogens in the environment more comprehensively [16]. Our study analyzed the VOCs produced by strain ST-TJ4 using SPME-GC-MS/MS and detected five compounds. [PERSON] et al. [48] and [PERSON] et al. [49] reported that 1-undecene, which has strong antifungal activities, was the main active compound released by _Pseudomonas fluorescens_. Similarly, in our analysis, 1-undecene was the most abundant volatile in strain ST-TJ4. The antifungal activity of 1-undecene against _R. solani_ AG-1(IA), which causes banded leaf and sheath blight (BLSB) of maize, has been reported [50]. The volatile methylsiloxane octamethylcyclotetrasiloxane (D4) has been reported to have antibacterial and antifungal activities [51] and was found in the present study. To our knowledge, this is the first report on the emission of 1-undecene and octamethylcyclotetrasiloxane by _Pseudomonas_ sp., which has a broad fungistatic spectrum. Interestingly, the diffusible substances produced by ST-TJ4 showed weak antagonistic effects against _C. chrysosperma_, _R. solani_, and _Co. tropicale_, whereas its volatiles almost completely inhibited the growth of these three pathogenic fungi. Because we used LB medium to culture bacteria in the two-sealed-base-plates method, while strain ST-TJ4 was streaked onto PDA plates in the dual-culture experiment. It is possible that the inoculation strategy led to different antagonistic effects; that is, the inhibition rate of spreading small cells may not have been as great as that of the spread cells exposed to larger surfaces of the medium, which would allow the single cells to obtain more nutrients and facilitate their rapid reproduction [52]. The results of [PERSON] et al. [53]. support our speculation. Only when a variety of biocontrol mechanisms are combined and complement each other can microorganisms play the greatest role in biological control. To conclude, our results suggest that _Pseudomonas_ sp. ST-TJ4 is a good biocontrol agent candidate, although it is unclear how effective this antagonist would be under the field conditions. Diffusible and volatile compounds produced by ST-TJ4 have the potential to be used in agriculture and forestry as direct contact biofungicides and biofumigants. The following are available online at [[http://www.mdpi.com/2076-2607/8/4/590/s1](http://www.mdpi.com/2076-2607/8/4/590/s1)]([http://www.mdpi.com/2076-2607/8/4/590/s1](http://www.mdpi.com/2076-2607/8/4/590/s1)). [PERSON] completed the data analysis and the first draft of the paper; [PERSON] and [PERSON] were the finishers of the experimental research; [PERSON] participated in the experimental result analysis; [PERSON] directed the experimental design and result analysis; [PERSON] directed experimental design, data analysis, paper writing and revision. All authors have read and agree to the published version of the manuscript. This work was supported by the National Key Research and Development Program of China (2017 YTD0600104) and the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD).