<b>Evaluating Resistance to Thousand Cankers Disease in Black Walnut: Interactions Between Genetic Variation and Phenolic Defenses</b>

<p dir="ltr">With ever increasing threats to forests around the world, there is a critical need for effective tools to combat foreign pests and pathogens and protect trees. A promising approach is breeding resistance into trees to protect them from these threats. However, progress is often hindered by a poor understanding of specific resistance mechanisms and the challenges involved in selecting resistant individuals, especially in long-lived species, which require substantial time, space, and financial investment to cultivate. To maximize breeding success, it is essential to identify and propagate only the most promising genetic material. Eastern black walnut (<i>Juglans nigra</i>) has emerged as a high-priority candidate for resistance breeding due to its ecological and economic value and its susceptibility to thousand cankers disease (TCD), a disease complex involving the walnut twig beetle which vectors the fungal pathogen <i>Geosmithia morbida</i>. TCD has devastated black walnut populations in the western United States, yet little is known about potential resistance within the species. Recent findings suggest high genetic variability in resistance traits, indicating promising opportunities for targeted breeding efforts. To further this goal, we conducted three interrelated studies. First, we developed a rapid, high-throughput assay to estimate black walnut resistance to the walnut twig beetle. Second, we explored potential defense mechanisms in walnut, focusing on phenolic compounds, secondary metabolites commonly associated with disease resistance in many systems. We analyzed changes in phenolic profiles in the bark and phloem of black walnut following inoculation with <i>G. morbida</i>, compared to mechanical wounding. Results suggest that certain phenolics may serve as biomarkers of resistance, though further validation is needed. Additionally, we investigated short-term phenolic responses during the early stages of infection, a critical temporal window that may disproportionately influence disease outcomes. Finally, we examined how seasonal timing affects both phenolic defenses and disease severity. Inoculations were performed at three phenologically distinct points in the growing season: early summer (post leaf-out), mid-summer, and late fall (onset of senescence), to assess how shifts in tree physiology influence defense allocation. Together, these studies lay foundational groundwork for understanding phenolic-based defenses in black walnut and offer promising directions for future research which may lead to the development of effective resistance breeding strategies against TCD.</p>