Profiling the nematicidal activity of linear and cyclic compounds on the pinewood nematode

The pinewood nematode (PWN), Bursaphelenchus xylophilus, has become one of the most damaging pests to forest health, in Asia and Europe. This phytoparasite causes Pine Wilt Disease (PWD) by attacking susceptible Pinus trees and inducing shoot yellowing (chlorosis) and wilting (caused by embolism), which can lead to the death of an adult tree in less than 50 days. In the search for sustainable biopesticides, research has focused on plant derived natural products, e.g., essential oils and extracts, known to be comprised of highly active secondary metabolites. Many of these compounds have shown promising results in direct contact bioassays, depending on their chemical structure. In the present work, 25 linear and aromatic pure compounds, commonly found in essential oils and extracts, were preliminarily tested in direct contact bioassays, against the PWN, to understand how distinct chemical structures can be related to a stronger nematicidal activity. Fifteen compounds showed complete mortality (100%) at 1 µL per mL of assay solution. Activity appeared to be strongly related with specific functional groups, isomerism or with the length of the linear carbon chain. In this context, compounds containing an alcohol group showed high activity, e.g., the linear geraniol, linalool or menthol. For the enantiomers of the aldehyde citronellal, complete mortality was only attained by its (-)-citronellal enantiomer. Additionally, the aliphatic decanoic (C10 carbon chain) and undecanoic acids (C11 carbon chain) induced complete mortality but not dodecanoic acid (C12 carbon chain). Uncovering the variation in the structure-activity relationships of anti-PWN compounds contributes to the identification of the nematicidal mechanisms of action. This knowledge is useful for improving sustainable pest management in forest ecosystems.