Mechanism of the antifungal activity of the diterpenoid aldehyde traversianal against the cucumber anthracnose fungus Colletotrichum orbiculare

The development of fungicides with novel modes of action is essential for addressing the emergence of drug resistance in plant pathogens caused by the long-term use of chemical pesticides. In this study, we investigated the mechanisms underlying the antifungal activity of traversianal, a diterpenoid aldehyde isolated from a culture filtrate of Cercospora sp. ME202, against Colletotrichum orbiculare , the causal agent of cucumber anthracnose. In antibacterial assays using two Pseudomonas species and one Bacillus species, traversianal showed growth-inhibitory activity against the Bacillus species. The fungicidal activity of traversianal against C. orbiculare was attenuated by the addition of lecithin, which consists primarily of phospholipids. Compared with that in the control, we observed a significant increase in the number of colonies of C. orbiculare on potato sucrose agar following treatment with traversianal in the presence of lecithin, thus indicating the competitive inhibition of fungicidal activity by lecithin. However, this antagonistic effect was not observed with the addition of l-α-phosphatidylcholine dioleoyl, whereas it was detected in the presence of l-α-phosphatidylethanolamine dioleoyl. These findings provide evidence that the cell membrane component phosphatidylethanolamine may be specifically targeted in the fungicidal activity of traversianal, which is consistent with our transmission electron microscopy observations revealing the fragmentation of plasma membranes and disappearance of cell organelles in the conidia of C. orbiculare treated with traversianal. These findings thus indicate that traversianal may have potential application as a novel fungicide that targets phosphatidylethanolamine.