## 1 Introduction  
Severe crop loss remains inevitable due to plant diseases, particularly those caused by pathogenic fungi, which are responsible for an estimated 70-80% of plant diseases [1]. Phytopathogenic fungi reduce both crop yield and quality. They are major restraints to sustainable agriculture production, especially in intensive cropping systems [2]. Synthetic fungicides have been used extensively to control diseases caused by these pathogens [3]. However, these chemicals may lead to toxic residues in treated products [4; 5]. Synthetic pesticides can also pollute the environment due to their slow biodegradation and can induce resistance or reduce the susceptibility of pathogenic fungi. Furthermore, that the species and numbers of fungal phytopathogens in the rhizosphere change with environmental conditions and evolution increases the difficulty of controlling plant diseases [6]. Facing the severe threat to global crop security caused by plant diseases, it is important to develop environmentally friendly and highly effective fungicides against plant pathogens.  
Compared with application of synthetic chemical fungicides, the use of microorganisms and their metabolites is a promising and environmentally friendly alternative for the prevention and control of plant diseases [7]. Biological control agents, such as _Bacillus_, _Pseudomonas_, _Burkholder_, and _Paenibacillus_ spp. play important roles in inhibiting pathogens [8; 9; 10; 11]. Bacteriocin, lipopeptide and polyketide are active substances produced by _Bacillus_ species against pathogens [12].  
The siderophore-mediated competition for iron gives beneficial microbes a competitive advantage in suppressing the proliferation and root colonization of plant pathogens [13].  
In addition, microbial volatile organic compounds (VOCs) have attracted more attention because they can spread over long distances, mediating indirect contact interactions between organisms. VOCs at low concentrations can be sensed, so they can directly inhibit the growth of pathogenic fungi and induce systemic resistance in plants [14; 15]. The VOCs produced by _Bacillus_ can inhibit mycelial growth of _Alternaria solanti_ and _Botrytis cinerea_, which cause early blight of potatoes and grey mold of a broad range of hosts, respectively [16]. They can also control _Ceratocystis fimbita_ in postharvest sweet potatoes [17]. It is suggested that the application of microbial volatiles has a great potential in plant diseases.  
Current research on biocontrol bacteria is mainly focused on crop applications and rarely on forest diseases. In preliminary analyses prior to this study, we screened a bacterial strain with high siderophore production from the poplar rhizosphere; however, it remains unclear whether this strain has antagonistic effects on fungi causing forest diseases or produces additional antagonistic substances. Therefore, in this study, we identified diffusible substances and VOCs produced by this strain to determine whether they may be applied as antagonistic substances. We also determined the taxonomic placement of this strain via morphological identification and molecular biology. The results of this study may provide a new and effective antagonistic bacterial strain for the biological control of forest fungal diseases, and lay a foundation for molecular analyses of the bacteriostatic mechanism of this strain.