# Natural Salicylaldehyde for Fungal and Pre- and Post-Emergent Weed Control
## 3 Results and Discussion
### SA Herbicidal Efficacy during Soil Pasteurization
It is speculated that, in nature, the allelopathic activity of SA (namely, SA released from plants into the environment function as inhibitors towards the germination or growth of the neighboring plants) might be one mechanism of herbicidal action. Precise determination of the SA mechanism of action (cellular targets) as a herbicide warrants future investigation. The derivatives of SA or salicylic acid have previously been developed as herbicides and bioregulators [40]. It was speculated that SA/salicylic acid derivatives might affect plant metabolism or inhibit vegetative and/or generative plant growth, but the mechanisms of herbicidal action of the derivatives were not comprehensively investigated or discussed in the study [40]. Meanwhile, a fungal pathogen of _Elytrigia repens_ (couch grass; perennial weed) _Ascochyta agoryprina_ var. _nana_ has been shown to produce natural toxins [41]. One of the toxins characterized is agoryprenol, a substituted SA that causes phytotoxicity in several  
Figure 6: Pre-emergent herbicidal activity of SA (pilot-scale soil pasteurization test): (**a**) covering of trays (No SA w/o nutshells (Control), No SA w/ nutshells (Control), SA 0.8 M w/ nutshells as SA delivery vehicles, SA 1.6 M w/ nutshells as SA delivery vehicles) after sowing _L. ovatus_ seeds; (**b**) determination of germination/growth of _L. ovatus_ seeds after 11 days. weeds, including _Mercurialis annua_ (French mercury), _Chenopodium album_ (goose foot), and _Setaria viridis_ (wild foxtail millet), resulting in necrotic lesions in plants [41]. Additionally, SA is one of the compounds identified in the cattail species _Typha domingensis_[42]. The aqueous extracts of cattail plants, which also contain two phenolic agents (namely, 2-chlorophenol, SA), inhibited the growth of the watermross _Salvinia_, suggesting SA could function as one of the allelopathic substances released from _T. domingensis_[42]. Recently, the sugar crop \"sugar beet\" has been determined to be vulnerable to continuous cropping, where allelopathy was one of the crucial factors triggering cropping disorder [43]. The transcriptomics and metabolomics study identified that SA was one of the allelochemicals continuously accumulated in sugar beet root exudates with continuous cropping; it functioned as an autotoxic substance [43]. It was hypothesized in the study that continuous cropping changes the metabolome of sugar beet, which interferes further with the diversity of the rhizosphere microbial community, thus negatively affecting sugar beet growth and quality [43]. However, none of these studies explored the mechanisms of the phytotoxic action of SA or its structural derivatives. In contrast, SA and its structural derivatives have recently been shown to possess metal chelating activities [44; 45; 46]. Of note, certain SA derivatives, such as SA isonicotinoyl hydrazone, exhibited pro-oxidant activity as an iron-chelator, thus increasing the levels of reactive oxygen species (ROS) and modulating the anti-/pro-oxidant balance [44]. It was surmised that, in addition to the intrinsic redox-active nature of SA (see above) [47; 48], the pro-oxidant activity exerted by SA or SA derivatives (as metal-chelators) would contribute to the phytotoxicity in the treated plants, thus resulting in disruption of normal germination/growth of weeds. This pro-oxidant effect, along with the modulation of the cellular antioxidant system, as determined in other organisms [12; 13], would disrupt the integrity of normal cellular components (such as cellular membranes, lipids, and proteins). Elucidation of the precise mechanisms of SA herbicidal action warrants future in-depth investigation.