Restoration Potential: The restoration potential of butternut is uncertain, owing to the rapid spread of butternut canker. Since most remaining trees are infected with the fungal disease, or will likely be infected, recovery may be extremely difficult. Many of the infected trees are expected to die within the next 20 years, although Anderson (1993) notes that there are many trees in North Carolina that have survived 40 years with canker. There is presently no known cure for the annual fungus. However, several apparently resistant butternut trees have been located adjacent to infected trees in Virginia and elsewhere. It may be possible to clone these and other apparently disease-resistant butternut trees to produce shoots that could be reintroduced throughout the species' range. The U.S. Forest Service and several state forestry units are working to locate and test trees that may be resistant to infection. (Fogelson and Campbell 1990, Skilling 1993).
Activities that should be undertaken to help restore butternut populations include: (1) conducting inventories for butternut to assess its health and status; (2) increasing the understanding of the butternut canker disease cycle; (3) developing silvicultural methods that will ensure effective regeneration; (4) beginning a seed collection program along with test seeding and transplanting plans; (5) locating disease-resistant trees within the forest; (6) establishing clone banks and seed orchards by using propagation techniques, including tissue culture; (7) testing the possibility of developing resistant trees by using intraspecific tree breeding techniques; and (8) monitoring the effect of current management being employed where butternut is growing and making beneficial refinements as needed. (Ostry et al. 1994).
Preserve Selection and Design Considerations: There are no known preserve designs that would optimize the conservation of this species due to the rapid rate of infection and spread of butternut canker disease.
Management Requirements: There are no effective or practical management techniques known that would completely protect butternut due to the butternut canker. Some sites are being managed by extracting infected individuals to inhibit the spread of the canker, a treatment Skilling (1993) states has no known value. In addition, the practice of extracting healthy trees from infected stands to salvage the economically valuable wood serves to further reduce the potential pool of resistant germplasm. At present, the best management prescription is to locate and monitor butternut stands so that potentially resistant germplasm strains can be identified. Criteria for identifying candidate trees include the following: (1) the tree must be in a stand already infected with canker where the possibility of exposure is high; (2) the tree must be canker-free, or if cankers are present, must have overgrown them; (3) the tree must be at least 10 inches in diameter at breast height (dbh) to ensure that it has been exposed to canker over a number of years; and (4) the tree(s) must be owned by someone who is willing to let researchers collect scion wood (grafting material) as well as nuts from the tree(s) over a number of years (Ostry et al. 1994).
At least three fungicides have been tested on butternut canker (Anderson 1988). One of those fungicides is benomyl, which has provided some effective treatment at low concentrations. However, this treatment is not practical for treating scattered trees within a forest stand.
Because butternut is a shade-intolerant species, its natural propagation can be assisted by managing forests to create disturbance conditions (e.g., canopy gaps, openings, soil disturbance) needed for colonization and establishment of new individuals. As openings grow closed over time, they should be made larger so that established butternut trees can achieve full growth and development (Ostry et al. 1994).
All occurrences of healthy (non-infected and resistant) trees should be tracked; resistant trees, which are of the greatest importance for conservation of the species, should be most actively sought and identified.
Management Programs: Concern over the fate of butternut has led to moratoriums on harvesting. In Minnesota, the Department of Natural Resources issued a moratorium in 1992 on cutting healthy butternut trees on state lands administered by the Division of Forestry (Rose 1992, Skilling et al. 1993). The moratorium did not prohibit the salvage of diseased trees, but encouraged private landowners to abstain from harvesting healthy trees to maintain an optimal pool of potentially resistant germplasm. The U.S. Forest Service has placed a moratorium on cutting healthy butternut trees in National Forests as of March 1993. Within Region 9 of the U.S. Forest Service, butternut has been placed on the sensitive species list; in Region 8 it has been placed on the sensitive species list in Mississippi, with similar designation recommended for several other National Forests.
Management for butternut within the Great Smoky Mountains National Park consists of gathering fruits from wild populations and either storing them for several years or planting them immediately. Germination of seeds is slow and may take several months, but the process can be hastened by treating seed coats with a weak acid to soften them. Seeds are germinated in soil-filled medium pots. Approximately 100 seedlings have be germinated and transplanted into suitable habitat within the same year using this method. Long-term monitoring of tagged butternut trees has also been initiated within the Park, and studies have found that virtually all butternut trees within the Park are infected (Rock 1992).
Monitoring Programs: Annual monitoring for this species in Delaware and the Great Smoky Mountains National Park is conducted in order to update occurrence records and to determine the rate of disease infection (DE NHP 1992, Rock 1992). Contact:
William McAvoy Botanist, Delaware Natural Heritage Program Division of Parks and Recreation Department of Natural Resources and Environmental Control, 89 Kings Highway P.O. Box 1401 Dover, DE 19903 Telephone: (302) 739-5285
Janet Rock Botanist, Great Smoky Mountains National Park Natural Resources Management 107 Park Headquarters Road Gatlinburg, TN 37738 Telephone: (615) 436-1244
Management Research Programs: Research into the identification and propagation of resistant strains of butternut to butternut canker disease is being conducted at the North Central Forest Experiment Station in St. Paul, Minnesota. This research focuses on three approaches: (1) the establishment of grafted clonal lines of butternut from putative resistant trees located in the field; (2) the development of methods to propagate these resistant lines so that resistant plant material will be available; and (3) the development of techniques to produce butternut in vitro by embryo, callus, and nodal explants and to develop an in vitro inoculation system to identify somaclones with resistance to butternut canker; (4) establish field plantings of material from objectives 2 and 3 and determine if field resistance is related to laboratory resistance (Skilling et al. 1990, 1992, 1993). Investigations are also being conducted to obtain samples of the canker fungus from throughout the range to determine if there is genetic or pathogenic variation within the fungus population. This information will assist in evaluating whether different isolates are needed to confirm resistance (Skilling et al. 1993). Detailed results of research conducted in the past three years may be found in progress reports prepared by Skilling et al. (1990, 1992, 1993).
Management Research Needs: As resistant strains of butternut trees to butternut canker disease are successfully propagated, management research will need to focus on reintroduction efforts and the preparation and maintenance of site conditions so that growth and reproduction of butternut will be optimized.
Biological Research Needs: In the years since detection of the butternut canker researchers have clarified several aspects of the disease; the major remaining conservation need is related to using resistant material and other techniques to retain and restore the species (Ostry and Woeste 2004). Resistance has been noted in many trees and may originate from natural phenotypic variants (Ostry and Woeste 2004) or possibly from hybridization with a close Asian relative, Japanese walnut (Juglans ailanthifolia) (Michler et al. 2005). Trees with possible resistance are being propagated and assessed but data are needed on the heritability of resistance and in general further genetic characterization is needed before classical breeding or genetic modification can be used to produce canker-resistant trees (Michler et al. 2005).
Anyone observing what they believe to be disease-resistant trees (individual trees that appear to be canker-free within infected stands and are not residential shade trees) should contact Dr. Darroll Skilling, North Central Forest Experiment Station, 1992 Folwell Avenue, St. Paul, MN, 55108, telephone (612) 649-5116.
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