Characterization of phenolic compounds in the bark and phloem of black walnut and their role in resistance against thousand cankers disease

Thousand cankers disease (TCD) has caused widespread mortality of eastern black walnut (Juglans nigra L.) 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 complex 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 little is known about their role in resistance against TCD. In this study, we investigated the role of constitutive and induced chemical defenses within the bark and phloem of black walnut in providing resistance to G. morbida. Specifically, we artificially inoculated seedlings of five genotypes of black walnut with G. morbida and monitored changes in local and systemic 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 improves our understanding of chemical defenses of black walnut against this disease complex, but may also find application in breeding resistant phenotypes.