|Rapid crown decline and mortality of hickory associated with numerous Ceratocystis smalleyi infections and mass attacks by the hickory bark beetle|
J. JUZWIK (1), J. H. Park (2). (1) U.S. Forest Service, St. Paul, MN, U.S.A.; (2) Korea Forest Research Institute, Seoul, South Korea
Episodes of widespread and locally severe mortality of hickory (<i>Carya</i> spp.) have occurred periodically in the eastern USA since the early part of the 20th century. Historically, this mortality was attributed to outbreaks of the hickory bark beetle (Hbb) (<i>Scolytus quadrispinosus</i>) during extended periods of drought. Observation of discolored wood and bark cankers on Hbb-attacked hickories in the early 1990’s led to discovery of a new <i>Ceratocystis</i> species subsequently described as <i>C. smalleyi</i>. In an observational field study, tens to hundreds of bark cankers with xylem lesions and hundreds to thousands of bark beetle attacks were documented on main stems of <i>C. cordiformis</i> (bitternut hickory) with actively declining crowns. Hbb galleries or entry/exit holes were found within 92 to 94% of the xylem lesions. The pathogen was also isolated from a high percentage of Hbb initiating attack on stems of hickory trees. Xylem sap flow rates were significantly lower for <i>C. smalleyi</i> inoculated bitternut hickory trees than water controls in field studies. The numerous cankers and xylem dysfunction resulting from multiple infections suggests the fungus plays an important role in rapid crown decline. It is hypothesized that the Hbb-<i>C. smalleyi</i> interaction leads to rapid tree wilt and death, especially following predisposing abiotic events. This bark beetle-fungal pathogen complex may have been a black swan event 115-plus years ago, but it does not resemble one today.