## 4 Discussion  
It is an urgent problem to develop environment-friendly products instead of chemicals to control plant diseases [28]. Plant pathologists and microbiologists believe that the use of beneficial microorganisms and their metabolites in the future crop production practice is safe, reasonable and one of the most promising methods [29]. Therefore, we tried to isolate biocontrol bacteria with strong spectral antagonistic activities against plant pathogenic fungi. We found that strain ST-TJ4 had strong inhibitory effect on 11 pathogenic fungi and oomycetes from branches, leaves and roots of forest trees. This suggested that strain ST-TJ4 has a wide fungicide spectrum and could produce antifungals. Through morphological observation, physiological and biochemical experiments as well as molecular biological identification, the strain ST-TJ4 was identified as _Pseudomonas_ sp. Iron is one of the indispensable nutrients in all organisms. Although total iron is rich in the most soils, most iron exists either in chemical forms that cannot be used by microorganisms or imbedded deeply in mineral particles. Pathogens need to compete with other members of a microbial community for scarcely available iron before the host plant is infected. Therefore, iron acquisition is the basis of the complete virulence of many plant pathogens [30]. Many PGPR can produce siderophores that can specifically chelate Fe\({}^{3+}\) under the condition of iron deficiency. Studies have shown that these siderophores can inhibit the growth of pathogens by depriving iron, so as to achieve the purpose of controlling plant diseases. [PERSON] et al. [31] have achieved good results in using  
Figure 6: Morphological identification: colony morphology (**A**), oxygen demand test results (**B**), and Gram staining (**C**) of _Pseudomonas_ sp. ST-TJ4. Figure 7: Phylogenetic tree derived from the 16S rRNA-encoding gene sequences of strain ST-TJ4 and related taxa of _Pseudomonas_. The tree was constructed using the neighbor-joining method using MEGA 7.0. The bootstrap test was conducted with 1000 replicates. Bootstrap values >50% were indicated at the nodes. The scale bar represents substitutions per nucleotide position. catechol iron carriers produced by _Azospirillum brasiliensis_ to control strawberry anthracnose caused by _Colletotrichum cuspidatum_. The siderophores produced by _Pseudomonas clove_ BAF1 has a great effect on the spore germination and mycelial morphology of _Fusarium oxysporum_[32]. Strain ST-TJ4 was screened by siderophile production, and its strong chelating ability of Fe\({}^{3+}\) is also expected to be one of the reasons for its inhibition of pathogen growth. Biocontrol bacteria can inhibit the growth of other microorganisms by producing cell wall degrading enzymes and secondary metabolites. It is well known that fungal cell walls play an extremely important role in cell division, maintaining the normal growth of hyphae and coping with environmental stress [33; 34]. Moreover, maintaining the integrity of fungal cell wall is the key to host infection [35]. The cell wall of true fungi is mainly composed of chitin, glucan and protein [36; 37; 38]. Once these components are destroyed, the mycelial morphology and biological function of fungi will be threatened. In this study, ST-TJ4 showed strong cellulase and protease hydrolysis activity and could degrade the cell wall of oomycetes. Antibiotics produced by beneficial microorganisms have direct contact killing effect on pathogenic bacteria [39; 40]. Therefore, antibiotics are not only one of the most studied mechanisms of biological control, but also a character to be considered when screening potential biocontrol agents (BCAs) [41; 42]. Strain ST-TJ4 could change the color of the whole culture medium to orange after three days in LB culture, and the production of pigment suggested that the strain might produce metabolites. In order to test our hypothesis, we examined the antibiotic-related genes in ST-TJ4 with reference to the results of previous studies on _Pseudomonas_ antibiotics.