Structure-activity relationships of cinnamaldehyde and eugenol derivatives against plant pathogenic fungi

Rhizoctonia solani and Fusarium oxysporum are very well known phytopathogens with worldwide spread that cause one of the most destructive diseases, resulting significant yield losses in crop production. In order to find effective fungicides for controlling phytopathogen, the structure-activity relationships of cinnamaldehyde and eugenol derivatives against R. solani and F. oxysporum were evaluated. Additionally, the composition and fungicidal activity of Cinnamomum cassia and Syzygium aromatica essential oils was assessed. Cinnamaldehyde (49.75%) is the major compound in cinnamon bark oil, and eugenol (89.17%) is most abundant in clove bud oil. The clove bud oil exhibited good antifungal activities against R. solani and F. oxysporum (IC50 = 106.5 and 149.9 μg/mL, respectively). Cinnamon bark oil showed a lower activity than clove bud oil. The fungicidal activity of cinnamaldehyde (IC50 = 75.4 and 156.9 μg/mL, respectively) and eugenol (IC50 = 58.9 and 52.9 μg/mL, respectively) against R. solani and F. oxysporum was also evalutated. Comparisons of the antifungal activities of cinnamaldehyde and eugenol derivatives revealed that α-methylcinnamaldehyde, α-methylcinnamic acid, methyleugenol, acetyleugenol, isoeugenol, methylisoeugenol, and acetylisoeugenol showed good antifungal activities against R. solani and F. oxysporum. In structure-activity relationships, the fungicidal activity of cinnamaldehyde derivatives could be related to conjugated double bond and the length of CH chain outside the ring. In addition, the presence of the lipophilicity may have a considerable influence on the toxicity of phenylpropenes. The present approach may help future work in the search for fungicidal compounds.