Butternut (= white walnut or oilnut), Juglans cinerea, is a widespread but uncommon tree in the eastern hardwood forest. Its range extends from New Brunswick to Minnesota, North Carolina to Missouri. Densest populations were in the upper Midwest, especially the North-Central Driftless and Escarpment and Western Superior Highlands ecoregions (Morin et al. 2018). Genetic diversity is highest in the southern part of its range (Bechman et al. 2021).
Butternut is an important nut species and can produce copious amounts of nuts fed on by a variety of wildlife species and people (Millikan and Stefan, 1989). In the past, the wood was prized for veneer, cabinetry, and especially carving.
Butternut populations have been significantly reduced by an apparently introduced fungal disease that causes multiple branch and trunk cankers. The host tree is killed when multiple trunk cankers join and girdle the tree. The causal agent is Ophiognomonia clavigignenti-juglandacearum.
Although the disease was first discovered in 1967 in southwestern Wisconsin (Renlund, 1971), coring of infected trees in the southern portion of the butternut’s range suggest that the disease was introduced into the southeast around the beginning of the 20th Century (Anderson and LaMadeleine, 1978). Some scientists conclude that two different strains of O. clavigignenti-juglandacearum might have been introduced: a more virulent strain in Minnesota and Wisconsin in the 1960’s and a less virulent strain that might have been present in the Northeastern states for far longer. It is also possible that a native fungal endophyte or minor pathogen “jumped” to a new host, e.g., butternut, where it became pathogenic (Morin et al. 2018).
Butternut canker has spread throughout much of the tree’s range, reaching Canada in 1990. Analysis of USDA Forest Service Forest Inventory and Analysis (FIA) data indicates a 58% decrease in butternut trees across the United States’ range (Morin et al. 2018). Other sources put the loss higher, at nearly 80% (Beckman et al. 2021). Particularly severe losses have occurred in areas supporting the densest populations: Minnesota, Wisconsin, and Iowa. The annual mortality rate for butternut averaged 5.6%, or more than 430,000 trees per year (Morin et al. 2018). It is unclear whether the variation in tree mortality in parts of the species’ range indicates differences in the virulence of the fungus, the presence of disease-resistant butternut trees, or ecological site differences (Morin et al. 2018).
Unlike American chestnuts and chinkapins (Castanea spp.), butternut will not sprout from the root crown when the top is killed by cankers. Seedlings, young sprouts, and mature trees are all killed by the disease (Prey and Kuntz, 1982). Therefore, when butternut canker disease destroys a population, that particular gene pool is lost forever, as there is no possibility for natural reproduction. The rapid demise of the species has led the U. S. Fish and Wildlife Service to declare the butternut “a species of concern.” The International Union for the Conservation of Nature (IUCN) designated the species as Endangered on its Red List (Beckman et al. 2021).
USFS scientists and managers developed a conservation priority-setting framework for forest tree species at risk from pest & pathogens and other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The scientists prioritized 419 tree species native to the North American continent. The analysis identified 15 taxonomic groups requiring the most immediate conservation intervention because of the tree species’ exposure to an extrinsic threat, their sensitivity to the threat, and their ability to adapt to it. Each of these 15 most vulnerable species, and several additional species, should be the focus of both a comprehensive gene conservation program and a genetic resistance screening and development effort. Butternut is one of six species that face severe pest threats, and although they are highly sensitive to that pest, they have high capacity to adapt (according to CAPTURE project). The project urges immediate conservation and the facilitation of resistance through breeding.
Juglans spp. (including butternut) seeds cannot be stored long-term in conventional seed bank conditions of low temperature and moisture, so living collections are an important conservation tool. The Morton Arboretum found that botanical gardens and similar institutions hold 487 individual trees, but nearly 80% are of unknown or horticultural origin so their contribution to conserving genetic diversity of the species is probably minimal (Beckman et al. 2021).
During the 1990s, there was apparently some optimism about breeding trees resistant to the disease.
Scientists at the USDA Forest Service North Central Experiment Station in Minnesota and at the University of Tennessee began exploring the possibility (Ostry et al. 1994, 1996; Schlarbaum et al. 1997; van Manen et al. 2002). However, little progress has been made in the following decades. It has been discovered that most surviving or vigorous trees in habitat are hybrids with the introduced Japanese butternut Juglans ailantifolia (C. Pike, personal communication, 2021) Hybridization rates are particularly high (more than 60%) near agricultural lands (Beckman et al. 2021). Hybrids are virtually indistinguishable from true butternut (Morin et al. 2018).
Another problem arises such that, since trees are killed slowly by many cankers, each tree’s response to any single infection is weak. This makes it extremely difficult to test seedlings or saplings for potential resistance to the disease. Finally, vigorously growing seedlings in greenhouses or other favorable conditions are likely to exhibit low infection rates (C. Pike, pers. comm., 2021). Still, disease killed most of the trees planted at Purdue University in an earlier test. A new pathologist on staff hopes to revive the project (C. Pike, pers. comm., 2021).
The Ontario Forest Research Institute attempted to screen seedlings in a greenhouse with a misting procedure, but they were not able to elicit a response and sort out resistant genotypes (C. Pike, pers. comm. 2021). The USFS and Wisconsin Department of Natural Resources recently installed a progeny test with 50 butternut families and confirmed that all the genotypes were 100% pure butternut. They are hoping that the disease will spread by natural means from artificially inoculated trees to non-inoculated, thus mimicking natural spread. This would enhance the value of the test of individual trees possible resistance to butternut canker (C. Pike, pers. comm. 2021).
Future conservation action, including seed collection and resistance breeding, for J. cinerea may be assisted by its recent addition to TreeSnap, an app that allows citizens to record the species’ occurrence, disease condition, seed production, and other information (see treesnap.org) (Beckman et al. 2021).
The Geosmithia fungus that causes thousand cankers disease on walnuts (which are in the same genus as butternut) has caused canker formation on butternut following artificial inoculations, but it is not yet certain that butternut is host of the walnut twig beetle (Tisserat pers. comm., February 2010). The Morton Arboretum assessment assigns a score of five out of 10 for the threat thousand cankers disease poses to butternut.
Anderson, R.L., and L.A. LaMadeleine. 1978. The distribution of butternut decline in the eastern United States. United States Department of Agriculture, Forest Service, Northeast Area, State and Private Forestry Report S-3-78. 4 pp.
Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. 2021. Conservation Gap Analysis of Native U.S. Walnuts. Lisle, IL: The Morton Arboretum. August 2021.
Burkman, W. G., J. S. Vissage, W. H. Hoffard, D. A. Starkey, and W. A. Bechtold. 1998. Summary Report: Forest Health Monitoring in the South, 1993 and 1994. United States Department of Agriculture Forest Service Southern Research Station, Resource Bulletin SRS-32.
Millikan, D. F. and S. J. Stefan. 1989. Current status of the butternut, Juglans cinera L. Ann. Rep. North. Nut Growers Assoc. 80: 52-54.
Morin, R.S., K.W. Gottschalk, M.E. Ostry, and A.M. Liebhold. 2018. Regional patterns of declining butternut (Juglans cinerea L.) suggest site characteristics for restoration. Ecology and Evolution, 2018;8-546-5559.
Ostry, M. E., M. E., Mielke, and D. D. Skilling. 1994. Butternut – strategy for managing a threatened tree. USDA For. Ser. Gen. Tech. Rep. NC 165. 7 pp.
Ostry, M. E., M. E. Mielke, and R. L. Anderson. 1996. How to identify butternut canker and manage butternut trees. USDA For. Ser., North Central Exp. Stn., Northeast. Area State and Private Forestry, and Region 8, State and Private Forestry, HT 70.
Pike, Carolyn. 2021. Purdue University Hardwood Tree Improvement and Regeneration Center. Personal communication to F.T. Campbell September 2021
Potter, K.M., Escanferla, M.E., Jetton, R.M., Man, G., Crane, B.S. 2019. Prioritizing the conservation needs of US tree spp: Evaluating vulnerability to forest insect and disease threats, Global Ecology and Conservation, doi: https://doi.org/10.1016/
Prey, F. J. and J. E. Kuntz. 1982. The distribution and impact of butternut canker. In Black walnut for the future. USDA For. Serv. Gen. Tech. Rep. NC-74. pp. 23-26.
Renlund, D. W. (ed.). 1971. Forest pest conditions in Wisconsin. Wis. Dept. Nat. Res. Ann. Rep. 53 pages.
Schlarbaum, S. E., F. Hebard, P. C. Spaine and J. C. Kamalay. 1997. Three American tragedies: chestnut blight, butternut canker, and Dutch elm disease. Proc. Exotic Pest of Eastern Forests, April 8-10, 1997, Nashville, TN, pp. 45-54.
van Manen, F. T., J. D. Clark, S. E. Schlarbaum, K. Johnson, and G. Taylor. 2002. A model to predict the occurrence of surviving butternut trees in the southern Appalachian region. In Predicting Species Occurrences: Issues of Scale and Accurancy, J. M. Scott, P. J. Heglund, M. L. Morrison, J. B. Haufler, M. G. Raphael, W. A. Wall, and F. B. Samson, eds. Island Press. Chapter 43, pp. 491-497.