Green Horizons

Rapid white oak mortality in Missouri

By Sharon Reed | Research Scientist, Plant Sciences Division, University of Missouri
Simeon Wright | Missouri Department of Conservation

Photo courtesy of Sharon Reed

RWOM Pocket at Huzzah Conservation area in Crawford County, Missouri.

White oak mortality is not new in Missouri but the large number of trees dying and the unusual pattern has forest and land managers concerned. Since 2011,the MDC has received numerous reports of white oak mortality in southeast, central, and east-central Missouri. Unlike other common oak decline patterns, mortality appears to be rapid and affects white oak on high quality sites. Consequently, the phenomenon has been described as "rapid white oak mortality" (RWOM) to distinguish it from other oak decline patterns.

Research efforts
A team of University of Missouri researchers, including Drs. Sharon Reed, Jim English, Rose-Marie Muzika and John Kabrick (USDA-Forest Service) received funding from USDA-Forest Service's Forest Health Protection Evaluation Monitoring program and the MDC for a oneyear study to describe the regional extent of the mortality in Missouri, Arkansas and Iowa, and to identify associated pathogens and insects.

Major findings
A survey was completed by professional foresters and landowners in Missouri at 87 locations. Survey results indicate white oak mortality consistent with RWOM is occurring in pockets in central, east-central and southeast Missouri. More limited pockets of mortality are also occurring in northeast Missouri and southeast Iowa. Mortality in north-central Arkansas is consistent with traditional oak decline patterns. Mortality is occurring on state, federal and private lands with and without active management. Tree mortality is most frequent on lower slopes of all aspects and next to seasonal drainages. Large overstory white oak are affected most often, but other sizes, crown positions, and species, including post oak and red oak species, are also affected. Some healthy white oak remain in most affected stands. A detailed investigation of pathogens and insects associated with declining white oak at two research sites in the Missouri Ozarks is ongoing.

So far, scientists have detected the following pathogens and insects thought to contribute to mortality: Armillaria, Hypoxylon (Biscogniauxia), Phytophthora cinnamomi, two-lined chestnut borer and a wood boring ambrosia beetle Xyleborinus gracilis. All except P. cinnamomi and X. gracilis are commonly associated with oak decline in Missouri. P. cinnamomi is a root rotting organism that has been associated with oak decline in Europe and similar white oak mortality patterns in Ohio. P. cinnamomi causes plant diseases world-wide. It was introduced into the southeast U.S. in the late 1700s or early 1800s and disproportionately affects susceptible species on lower slopes and along drainages.

Future direction
Scientists want to learn which locations are most at risk of RWOM so that management plans can be made. They also want to know if extreme weather events in the past few decades have played a role in RWOM. Some of these events include several localized and regional droughts since the '80s, a severe freeze in early April 2007, and the wettest back-to-back years in state history between 2008 and 2009. Additional funding has been applied for to expand current research regionally as well as to include weather data from the past few decades in the study. Please contact Dr. Reed (ReedSh@missouri.edu) if large numbers of white oaks are dead or dying on your property.