Rapid induction and seasonal response of phenolic compounds in black walnut to Geosmithia morbida, the causal agent of thousand cankers disease

Thousand cankers disease (TCD) has caused widespread mortality of eastern black walnut (Juglans nigra) throughout the western US and threatens the health and productivity of this ecologically and economically important species within its native range in the East. This disease is caused by a fungal pathogen (Geosmithia morbida) that is vectored by the walnut twig beetle (Pityophthorus juglandis). Outbreaks of TCD occur when P. juglandis mass-attack walnuts, which can result in thousands of inoculations of G. morbida beneath the bark of trees. Phenolics are ubiquitous defensive compounds, yet their role in resistance against TCD is unclear. In this study, we used a time-course experiment to track rapid chemical changes in the bark and phloem of black walnut in response to infection with G. morbida. We also explored the extent to which seasonality affects walnut defenses by characterizing the induction of phenolic compounds at three points during the growing season following G. morbida inoculation. Specifically, we artificially inoculated black walnut seedlings with G. morbida and monitored changes in concentrations of sixteen phenolic compounds in the bark and phloem over time. Sham inoculated and artificially damaged seedlings served as controls. This study not only enhances our understanding of chemical defenses of black walnut against TCD but will also inform efforts to breed resistant walnut varieties. =