## 4 Discussion
Similarly, benzoic acid ethyl ester, 3-methyl-butanoic acid, 2-ethyl-1-hexanol, 3-methyl-1-butanol, and 6-methyl-5-hepten-2-one, which had similar contributions in the random forest model, all showed lethality or significant inhibition at 25% concentration, and were definitely lethal at 50% and 100% concentrations (Figures 4 and 5). In the literature, some other VOCs, such as 2-phenylethylanol, isopentyl acetate, naphthalene, methyl 2-methyl pentanoate, 1,3,5-trichloro-2-methoxy benzene, 1-methyl-4-(1-methylethenyl)-cyclohexene, and 4-flavanone (4H-1-benzopyran-4-one, 2,3-dihydro-2-phenyl), were also reported to inhibit the growth of _R. solani_[19; 23; 27]. These VOCs were not detected in the present study, possibly because we isolated bacteria from the rhizosphere of rice, whereas the previous studies used other bacterial sources. Several studies have described antifungal activity of bacterial VOCs, but few have identified a single VOC or a blends of VOCs responsible for the antifungal activity [28]. [PERSON] et al. used commercial VOCs for in vitro verification of their antifungal activity [29], and only a few of them were found effective. This approach is time-consuming, laborious, and costly [30; 31]. In our study a random forest model was constructed to explore the relationship between VOCs and antifungal activity for the first time. This was an easy and effective way to compare the contribution of different VOCs to antifungal activity and identify the most effective VOCs. Tests of pure commercial compound showed that estimation based on the random forest model was reliable. This method saved time, labor, and money. It is very interesting to note that some of the most potent VOCs were easily synthesized and are commercially available, such as benzoic acid ethyl ester, 3-methyl-butanoic acid, 2-ethyl-1-hexanol, 3-methyl-1-butanol, 6-methyl-5-hepten-2-one. In addition, we found that dimethyl trisulfide was lethal to _R. solani_ at 25% concentration. Benzoic acid ethyl ester, 2-ethyl-1-hexanol, and 3-methyl-butanoic acid were also lethal to _R. solani_ under certain conditions. These three compounds could actually inhibit _R. solani_ infection on detached leaves (Figures 5 and 6). However, the previous studies used pure compounds for verification, and no study has ever used smaller concentrations of compounds to verify the antifungal activity. Hence, the VOCs found in this study showed high antifungal activity. However, we assume that the VOCs produced by the rhizosphere bacteria when grown on a beef peptone medium would also be produced when they colonize roots of the rice plant. That may or may not be the case, as nutrition obtained from a complex medium may result in different products vs. those when grown in a natural system. Nonetheless, our findings are interesting. We also identified volatiles produced by the different bacterial strains as well as evaluated the impact those volatiles have on disease suppression. Again, this was done under \"laboratory\" conditions that do not necessarily reflect the real world, but there is enough information presented for us or others to pursue this line of investigation. For example, the potential interaction of production of the VOCs in a complex environment interacting with many potential feedback mechanisms. Therefore, the system presented here is not ready to release as a commercially ready biocontrol, but the data presented here are intriguing and sufficiently represent a starting point to have a decent system for further study.