Ilicicolin Biosynthesis by Neonectria ditissima: A Putative Determinant of Virulence in Apple Diseases

Diseases caused by Neonectria ditissima result in major crop losses in apple production worldwide. Areas with mild and humid climates are particularly affected. In addition to canker on apple trees, infections by N. ditissima may also cause pre- and post-harvest fruit rots. The molecular basis of disease development is as yet unclear. Secondary metabolite profiles were analysed in naturally infected apple tissues exhibiting disease symptoms collected from orchards, in N. ditissima pure cultures at different developmental stages, and in co-cultivations with other microorganisms from the apple ecosystem. Universally, two classes of secondary metabolites, the meroterpenoid-type ilicicolins C, D, E, F and 4',5'-dihydro-4'-hydroxyascochlorin (DHAC) and the chemically unrelated pyridone ilicicolin H, were identified. The production of ilicicolin H, ilicicolin D and DHAC was reported for the first time in N. ditissima. Phytotoxic and antifungal activities were demonstrated for ilicicolin D and DHAC. Furthermore, gene clusters for ilicicolin biosynthesis in N. ditissima were identified by bioinformatic analyses. RNA sequencing revealed expression of the predicted biosynthetic genes under ilicicolin-producing conditions, which was confirmed by quantitative PCR for the core biosynthetic genes. Together, these findings support a potential role of ilicicolins in host infection and ecological interactions of N. ditissima and provide a basis for further functional studies of their contribution to apple disease development.