# Natural Salicylaldehyde for Fungal and Pre- and Post-Emergent Weed Control
## 3 Results and Discussion
### SA Herbicidal Efficacy during Soil Pasteurization  
Soil pasteurization (solarization) is a method that uses sunlight to control pests (weeds, microbial pathogens, insects) in crop fields. Soil pasteurization requires: (a) keeping a clear plastic cover in place for at least 4 weeks in the hottest part of the summer and (b) providing enough water to generate steam every day, which effectively kills weed seeds [35,36]. However, although the temperature of the top layers of the solarized soil can reach 60 \({}^{\circ}\)C (140 \({}^{\circ}\)F) or higher with sunlight, the heat efficiency of soil pasteurization is practically dependent upon the geographic location or weather condition (namely, the amount/days of solar radiation) [36]. Considering that SA exerted potent fumigant activity, we reasoned that soil covered with a plastic tarp (during soil pasteurization) could improve the herbicidal (also fungicidal) efficacy of SA at a much lower temperature, such as room temperature, during the anti-pest treatment. Therefore, the efficacy of SA was tested by treating weed seeds (_L. ovatus_) with SA in soil trays, which were covered with a plastic tarp and maintained at 22 \({}^{\circ}\)C, thus mimicking the treatment under colder or cloudy geographic/weather conditions. As shown in Figure 6, 1.6 M of SA emitted from the nutshell vehicles completely prevented the germination of the weed seeds, while 0.8 M SA allowed partial germination (6 seeds out of a total of 40 seeds per tray) of the test seeds. Control trays (both w/ or w/o nutshell vehicles; w/o SA) exhibited normal seed germination. It can be concluded that SA could serve as a potent herbicidal agent as a fumigant, especially during soil pasteurization (temperature: 22 \({}^{\circ}\)C or above), which, therefore, could effectively complement current soil pasteurization (solarization) practices requiring high temperature in the fields. Of note, the soil testing required a higher concentration of SA (1.6 M) than that tested in Petri plate/Magenta vessels (0.3 to 0.8 M) to achieve complete inhibition of weed germination. Figure 5: The post-emergent herbicidal activity of SA tested against _L. ovatus._  
In this proof-of-concept study, we have developed a potent anti-pest system, which used the fumigant compound SA. We repurposed the food additive SA as a pre- and post-emergent weed control agent and developed tree nutshell particles as sustainable SA delivery vehicles for use in orchards. The binary function of SA, namely, the potent herbicidal and fungicidal activity of SA, could lower the pesticide burden in crop fields by reducing the doses and frequency of herbicide and fungicide applications required for effective weed/fungal pathogen control. SA is a natural constituent identified in many plant species [37]. According to its vapor pressure value (5.93 \(\times\) 10\({}^{-1}\) mm Hg, 25 \({}^{\circ}\)C), SA exists mainly as a vapor in the atmosphere; thus, when applied in the fields, SA has high mobility in soil, which then volatilizes from dry soil [38]. Worthy of note, SA is a small molecule lacking \"functional groups\" that hydrolyze under environmental conditions (between pH 5 to 9) [38]. Hence, hydrolysis of volatilized SA is not supposed to occur in the environment; instead, safe biodegradation and/or photolysis by sunlight are considered an important environmental fate of SA, thus warranting environmental safety [38]. Moreover, the fumigant characteristics of SA have been determined to be \"specific\" to this molecule compared to other SA derivatives, enabling the effective application of SA, especially during soil pasteurization; other derivatives of SA from the prior study could not be applied as fumigants [39]. While SA modulates antioxidant defense systems in fungi [12,13], the mechanism of herbicidal action of SA has not been investigated thus far.