Characterizing Pathogen-Induced Changes in Black Walnut Volatile Organic Compounds Following Inoculation with Geosmithia Morbida, The Causal Agent of Thousand Cankers Disease

Thousand cankers disease (TCD) is a pathosystem comprised of Juglandacea spp., a pathogenic fungus Geosmithia morbida, and an insect vector, the walnut twig beetle (WTB) (Pityophthorus juglandis). Of the North American Juglans species, Juglans nigra is the most susceptible to TCD and has resulted in significant decline and mortality of urban and plantation trees in the western United States. Geosmithia morbida causes necrotic cankers in the phloem, and infected trees may release an array of volatile compounds that act as important chemical cues to WTB. Here, we aimed to determine how J. nigra volatile profiles respond to G. morbida infection as these changes can offer valuable insights into plant defense mechanisms and potentially influence WTB behavior, thus impacting disease transmission dynamics. In this study, we collected a series of bark and leaf volatiles from J. nigra seedlings inoculated with one of three isolates of G. morbida and a sham-inoculated control. Our results suggest J. nigra bark responds to G. morbida infection, with the western United States isolate (RN-2) eliciting a distinct volatile response compared to other treatments. We identified six out of fourteen compounds that contribute to 80% of the dissimilarity between RN-2 and sham-inoculated control trees. Inoculation with isolate RN-2 elicited the largest change in volatile profiles and resulted in the smallest cankers in the phloem, suggesting these compounds my play important defensive roles in J. nigra against the fungal pathogen that causes TCD.