Sensing Disease in Apple Trees: Potential Signature Compounds for European Canker (Neonectria ditissima) Infection

Neonectria ditissima is the fungal cause of costly European Canker (EC) in apple trees. A range of secondary metabolites were found at higher concentrations in infected twigs than in disease free twigs. Apple trees were then experimentally inoculated with N. ditissima and analyzed periodically until EC symptoms were visible at 12–13 weeks post-inoculation. Established destructive detection methods used were microscopy, which showed extensive hyphal penetration by 8 weeks post inoculation, and qPCR analyses, which confirmed the presence of N. ditissima. Headspace solid-phase microextraction GC-MS data showed significantly higher concentrations of styrene in apple twigs at six weeks after inoculation, and LC-MS data showed phloretin, triterpene acids, and 1-benzoyl β-d-glucose at raised concentrations after this time. Therefore, these compounds could be useful indicators of N. ditissima infection prior to visible canker formation, suitable for nondestructive disease detection development after further research on apple variety, pathogen specificity, and on field detection technology.