## 4 Discussion
To investigate the precise contribution of 26 commercially available compounds identified in the headspace of strain CT32 to the previously observed volatile-mediated inhibition of mycelial growth, we assessed their antifungal activity against _V. dahliae_ and FOF. The majority of the tested compounds showed low to moderate fungistatic effects, while benzothiazole exhibited 100% inhibition against the two tested pathogens. Benzothiazole is widely found in the volatiles produced by _Paenibacillus polymyna_, _Bacillus_ spp., _Ensifer alhaerens_, _Stentrophomonas maltoophilia_, _Sporosarcina ginsengli_ and _Arthobacter nitroguajacolicus_[37; 38]. This sulphur-containing heterocyclic compound has been shown to display a wide range of pharmacological and biological properties, such as antifungal, anticancer, antidiabetic and antimicrobial activities [39]. A previous study based ontranscriptomic and proteomic analyses has suggested that benzothiazole has antimicrobial activity against _Phtyophthora capsici_ by suppressing detoxification and stress responses as well as by inducing apoptosis [40]. In the present study, 2,4-dimethyl-6-tert-butylphenol showed excellent growth inhibition towards all of the fungi tested in vitro (reaching \(>\) 90% inhibition), and thus this phenolic antioxidant can be considered a potential inhibitor of phytopathogenic fungi. Dodecanenitrile is an important fragrance ingredient widely used in consumer products such as detergents and cosmetics, but its effect on fungal growth has so far not been reported. In the present study, this nitrogen-containing compound greatly hindered the mycelial growth of _V. dahliae_ and FOF. According to the mVOC 2.0 database, almost 70% of the recorded VOCs produced by _Bacillus_ spp. are fatty acid derivatives, for example, alkenes, alcohols, ketones, and aldehydes, which make them the most important group of volatile metabolites [41]. Alcohols have long been known to display broad antimicrobial activity and are utilized as preservatives or disinfectants [42]. All five alcohols assayed in this study showed antifungal activity against both pathogens. In addition, we found that the antifungal activity of alcohols was negatively correlated with the number of carbon atoms in the alcohols, and thus, 2-undecanol showed the highest level of inhibition. As proposed by [PERSON] et al. [43], the antifungal activities of ketones towards _F. oxysporum_ f. sp. _cubense_ decreased in the following order: 2-undecanone \(>\) 2-dodecanone \(>\) 2-tridecanone. In this work, when individual compounds were tested on _V. dahliae_ and FOF, this order was also observed. In addition, 3-undecanone was proven to be effective against the two target pathogens. The antimicrobial properties of aldehydes have been demonstrated in previous studies, for example, the in vitro inhibition of _Sclerotinia sclerootomy_ by decanal and nonanal [44] and the in vitro inhibition of _P. infestants_ by undecanal and tridecanal [45]. Similarly, decanal and undecanal showed high antifungal activity against the mycelial growth of _V. dahliae_ and FOF, but they were produced in low amounts. In contrast, dodecanal was the most abundant compound in the headspace of strain CT32, but it had a weak effect on the mycelial growth of the two target pathogens. The results presented here indicate that the inhibition of _V. dahliae_ and FOF observed with the natural emissions of strain CT32 is probably not caused by one or a few of these compounds but is most likely the result of synergistic or additive actions of various components of the complex mixture. However, our study was performed in vitro, which differs from that under natural conditions as most of the volatiles evaporate easily under greenhouse and field conditions. Nevertheless, field applications of bacterial VOCs have been reported recently, and some showed promising results. For example, drench application of 2-butanone emitted from _Bacillus_ spp.