Butternut Canker

Butternut canker, a disease caused by the nonnative fungus, Sirococcus clavigignenti-juglandacerum (USDA Forest Service 1994),produces multiple cankers on the main stem and branches of butternut (Juglans cinera). First identified in in 1967 (Anderson 1988), it appears that butternut canker has spread throughout the range of butternut in the South, except for some isolated pockets. In the South, butternut mortality in the last 30 years is estimated at 77%. In the Southern Appalachians, the disease has killed 90% of butternut trees. Unfortunately, the fungus went largely unnoticed because butternut trees are generally scattered and death from the disease is slow. Since nuts from infected trees generally are not viable, declining trees do not reproduce (Hoffard and others 1995).

Spores of the causal fungus are disseminated from fruiting bodies by rain splash and possibly by insects. Produced throughout the growing season, the spores can survive and be dispersed long distances during cool weather. Branch cankers caused by the fungus usually occur first in the lower crown, and stem cankers develop later from spores washing down from cankers above. Cankers are elongated sunken areas, often with an inky black center and whitish margin. Brown-to-black elliptical areas of killed cambium can be seen under peeled bark. Older branch and stem cankers are perennial, often covered by shredded bark and bordered by successive callus layers. (Hoffard and others 1995) Genetic resistance to this fungus appears to exist among natural populations of butternut. However, surviving trees are often cut by landowners who fear that the disease will eventually infect and kill them too. This harvest of uninfected trees threatens to severely reduce the remaining genetic pool of resistant butternut, making the identification and protection of surviving uninfected butternut trees crucial. Currently, harvest of all butternut is restricted on federal land (Ostry and others 1994, USDAForest Service1994, SAMAB 1996e). Two research and development efforts have been formed to address the decline of butternut. The USDA Forest Service North Central Experiment Station initiated a cooperative effort with Northern States and Northern National Forests to locate surviving butternuts and graft putative resistant trees into clone banks to preserve the germplasm (Nicholls and others 1978, Ostry and others 1994). Research is being conducted to develop laboratory and field protocolsfor screening trees for resistance andhost range (Pijut 1993) and the role of insects in dissemination of the fungus (Schlarbaum and others 1997). A coalition has also been formed by the University of Tennessee, the USDA Forest Service Southern Region and Southern Research Station, the Great Smoky Mountains National Park, the Tennessee Division of Forestry, and the USGS Biological Research Division. This coalition is working to locate surviving trees or populations, characterize sites, identify trees with putative resistance, develop methods for screeningfor disease resistance, study fungal physiology, and preserve germplasm (Schlarbaum and others 1997).