## 4 Discussion  
In this study we found that nine bacterial strains isolated from the rice rhizosphere inhibited the growth of _R. solani_ without direct contact (a double Petri dish assay), indicating that these bacteria released VOCs that were inhibitory to the growth of _R. solani_ (Figure 1). Although there are many related reports in the literature on bacteria that could inhibit the growth of pathogens by releasing VOCs [21; 22], no report can be found on the VOCs released by rice rhizosphere bacteria that can inhibit the growth of _R. solani_, specifically. Here we discovered nine bacterial strains that can release VOCs, which can then elicit inhibitory effects on _R. solani_. Based on sequencing and comparison in the NCBI database, we found these bacterial strains belong to _Pseudomonas_ sp. (No. 3), _Enterobacter_ sp. (No. 26), _Enterobacter_ sp. (No. 34), _Pseudomonas_ sp. (No. 35), _Ralstonia_ sp. (No. 50), _Bacillus_ sp. (No. 62), _Arthobacter_ sp. (No. 146), _Breviobacillus_ sp. (No. 2-18), and _Paenisporosarcina_ sp. (No. 2-60), respectively. As expected, different strains exhibited different inhibitory effects on _R. solani_, ranging from 7.84% to 100%. Surprisingly, strain No. 50 of _Ralstonia_ sp. showed 100% inhibition of _R. solani_ growth (Figure 1). In comparison, [PERSON] et al. found that the endophyte _Geotrichum candidium_ utilized different carbon sources to varying extents [23], and the antifungal activity of its VOCs against phytopathogen _R. solani_ changed with each carbon source, with the maximum mean growth inhibition of about 62% observed with glucose. [PERSON] et al. (2015) found 25 VOCs producing strains inhibited (by 27-50%) the growth of _R. solani_[24]. However, no previous study has found a strain that would kill _R. solani_. Hence, the present study is the first one to document complete (100%) inhibition of _R. solani_ growth by bacterial VOCs produced by _Ralstonia_ sp. No. 50. The profiles of VOCs released by the nine bacterial strains were also analyzed in our study. We found that some VOCs were commonly released by different bacterial strains, whales others were unique to each strain (Table 1, Figure 2). In addition to the differences of VOC profiles, the abundance of different VOCs was also different. These findings are consistent with those reported previously by [PERSON] et al. [25], who tested VOCs released by eight _Bacillus_ strains, which released a variety of VOCs in different amounts. [PERSON] et al. also found that four _Pseudomonas fluorescens_ UM16, UM240, UM256, and UM270 produced different VOCs in different concentrations [26]. The different bacterial strains characterized in the different studies are the likely reason for the differences in the type and content of VOCs observed. In the random forest variable importance plot of different VOCs, benzoic acid ethyl ester and 3-methyl-butanoic acid contributed more to _R. solani_ inhibition than 2-ethyl-1-hexanol, 3-methyl-1-butanol, and 6-methyl-5-hepten-2-one. Benzoic acid ethyl ester and 3-methyl-butanoic acid were found lethal to _R. solani_ at 25% concentration. The VOCs 3-methyl-1-butanol, and 6-methyl-5-hepten-2-one were able to significantly inhibit the growth of _R. solani_ at 25% concentration, but their contribution was less than that of benzoic acid ethyl ester and 3-methyl-butanoic acid. Therefore, it may be reasonable to speculate that 2-methyl-2-(methylthio)-butane and 6,10,14-trimethyl-5,9,13-pentadecatri-2-one can also totally inhibit _R. solani_ under different conditions. These two VOCs have not been reported hitherto to have antifungal activity previously and their antifungal activity remains to be investigated more extensively and vigorously. 2-Ethyl-1-hexanol was also lethal to _R. solani_ at 25% concentration.