Limited asymptomatic colonization of apple tree shoots by <i>Neonectria ditissima</i> following infection of leaf scars and pruning wounds

Abstract

Abstract European apple canker, caused by Neonectria ditissima , is an important disease of apple ( Malus domestica ). The fungus may reside in the tree without causing symptoms for up to a few years, thus making canker control difficult. Asymptomatic infections established in the nursery can result in severe canker outbreaks in newly established apple orchards. It has been suggested that N . ditissima might colonize the tree beyond the infection point during the asymptomatic stage. We investigated whether N . ditissima can colonize the internal tissues of apple shoots, both prior to and after visual symptoms. Apple trees were artificially inoculated via pruning wounds and leaf scars; then the pathogen was tracked at the inoculation point and beyond with isolation or real‐time quantitative PCR (qPCR). Before visual symptoms, N . ditissima could be detected in the infected pruning cut or leaf scar, but not at a distance of 10–15 mm from the entry point, or greater. Conversely, after symptom expression, the pathogen could be detected in the symptomless tissue at 10–15 mm from a canker lesion. This study demonstrated that the asymptomatic infection by N . ditissima can be detected using qPCR and that the pathogen does not grow systemically much beyond the initial entry point inside the plant before visual canker symptoms appear.