Stressors of Hardwood Forests
Butternut is being killed throughout its range by butternut decline, a disease that causes multiple cankers on the main stem and branches. Young cankers are elongated sunken areas, often with an inky black center and whitish margin. Peeling the bark reveals brown-to-black elliptical areas of killed cambium. Older branch and stem cankers are perennial, often covered by shredded bark and bordered by successive callus layers. Figure 3 shows a butternut tree exhibiting typical symptoms of butternut canker.
Branch cankers usually occur first in the lower crown, and stem cankers develop later from spores washing down from cankers above. The fungus can survive and sporulate on dead trees for at least 20 months.
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 weather conditions favorable to the disease (cool temperatures and overcast skies).
In 1978, the first survey for butternut canker throughout the eastern States was completed. Initially, an alert was sent to governments of each State in which butternut naturally occurs. This pest alert advised the States of the problem, gave some information on how to diagnose the condition initially, and requested that reports on infected counties be made to the Forest Service. If trees with signs of incipient infection with cankers were found, the USDA Forest Service requested cooperators to send samples of the diseased material to its Forest Health Unit for laboratory culturing of the suspected pathogen.
Based on results of that initial survey and on more recent field evaluations by State and Federal Forest Health personnel, the disease appears to cover practically the entire range of butternut. Figure 4 shows butternut distribution and confirmed disease by county.
Additionally, in 1990, the Forest Inventory and Analysis (FIA) group of the USDA Forest Service was requested to provide information on the change in the number of butternut 5 inches and larger in diameter in North Carolina and Virginia. The data from FIA show a 77 percent reduction in the number of butternut in these two states over the past 30 years. These changes may be attributable in part to causes other than butternut canker.
Although the complete field picture is unclear because of limited data, it appears that butternut canker has spread throughout the range of the tree in the South, except for some isolated pockets. In southern states with confirmed cases, the mortality in the last 30 years is estimated at 77 percent. These data have a large error potential becauseof the limited sample size.
Butternut, including regeneration, will be devastated by this disease in the years to come. Consequently, the species will be replaced by other hardwoods. Some hope lies in promising studies of genetic resistance. If and when resistant trees are found, they will be deployed in the forest. A butternut coalition is being formed, in part to look for and confirm resistant trees.
While no special survey methods for butternut are in development, technologies applicable to other pests should find applications in the butternut arena as well.