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Mature butternut. Photo by T. Davis Sydnor, The Ohio State University, Bugwood.org

Research ReviewUS FOREST SERVICE NORTHERN RESEARCH STATION

Two Fungal Diseases Spreading and Endangering Walnut Species:Butternut Canker and Thousand Cankers Disease

Invasive fungal diseases and insects are assaulting hardwood tree species in our eastern forests. Although the insects receive more attention (they are more visible after all), scientists, foresters, and environmentalists are concerned that invasive fungi are endangering many hardwoods, with fungus-caused canker diseases the most destructive. Trees that produce mast—beechnuts, butternuts, walnuts, and acorns, which are food sources for many animals—are especially at risk. The most tragic example is the American chestnut, once the “queen of the eastern forests,” which has essentially been extirpated in the wild in its native range because of the canker disease chestnut blight.

Cankers are localized areas of fungal infection of tree bark and cambium on branches and stems. Numerous infections and subsequent coalescing of cankers will kill trees. Butternut canker and thousand cankers disease of black walnut are cankers affecting eastern hardwoods. Since its discovery in 1967, butternut canker has established itself throughout the native range of butternut in the East, killing up to 90% of the trees in some states. Since 1990, Northern Research Station (NRS) scientists have focused their attention on conservation of butternut and the search for disease resistance in this species. The discovery of thousand cankers disease (TCD) in Colorado in 2007 and its subsequent discovery in the eastern United States have led to predictions of disastrous losses of eastern black walnut trees. NRS scientists and university colleagues are investigating how the insect-pathogen complex causing this disease may build and spread within the East and assessing what resistance exists in black walnut to both the pathogen and the insect pest.

United States Department of Agriculture

US Forest ServiceNorthern Research Station

NO. 28 | JANUARY 2016

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BUTTERNUT CANKER

Butternut canker is a disease that threatens the butternut tree (Juglans cinerea), also known as white walnut. It is mostly found in mixed hardwood forests, sometimes in groups but often as isolated individuals, from New Brunswick to North Carolina, and westward to Minnesota and Missouri. The nuts are important as a mast crop and are also used in baking. The wood looks similar to black walnut but is more easily worked for woodcarving, paneling, and furniture. The first chief of the Forest Service, Gifford Pinchot, paneled his library at Grey Towers with butternut wood. The trees are difficult to propagate and grow in plantations necessary for commercial production. However, butternut is highly valued as a yard tree or in small woodlots.

The fungus causing butternut canker (Ophiognomonia clavigignenti-juglandacearum, abbreviated as Oc-j ) occurs all over North America and was first discovered in 1967 in Wisconsin. The canker has spread rapidly throughout the range of butternut and is extremely virulent. Because of the host trees’ limited resistance, the fungus is believed to be of nonnative origin. Survival of the butternut species is threatened and the severity of the disease has prompted the U.S. Forest Service to consider butternut a “species at risk.” The fungus attacks other members of the walnut family, but butternut is the most susceptible native species.

Infection by this canker results in sunken, elliptical cankers on branches, stems, and buttress roots and dark brown, elliptical stains beneath the bark; these become large and girdle branches and twigs, then trunk or roots, which kills the tree.

Northern Research Station Science

Investigating the biology of the fungus causing butternut canker has provided useful insights. The canker predominantly affects butternut, but naturally produced cankers have been found on other Juglans species, namely black walnut (J. nigra) and Japanese walnut (J. ailantifolia). Artificial inoculations have found that Oc-j can colonize and survive in the wood of several genera, including Carya (hickory), Corylus (hazelnut), Prunus (stone fruits), and Castanea (chestnut). This is evidence that the fungus may be surviving on other trees in a nonpathogenic state as a reservoir when butternuts are not present.

Finding insect vectors of butternut canker is another important activity of NRS scientists. Rain splash and wind have been shown to spread Oc-j conidiospores up to about 45 meters. For long-distance transport, the sticky spores can adhere to insects that then carry the spores to isolated butternut trees. A number of insects have been found capable of disseminating Oc-j conidia, and three beetle species were found able to carry viable conidia for up to 16 days. These potential vectors may explain how apparently isolated butternuts become infected, but more studies are needed on conditions and timing.

Breeding for resistance to butternut canker is probably the most important aspect of NRS research for fighting butternut canker. Beginning in the 1990s, healthy butternut trees were being found despite growing in close vicinity to diseased trees. Numerous surveys by forest agencies as well as contacts from private individuals recorded a number of possibly resistant butternut trees throughout the range. These trees offer some hope of finding resistance to butternut canker within the natural population. Starting in 1990, NRS scientists Mike Ostry and Darroll Skilling of St. Paul, MN (now both retired), Melanie Moore of St. Paul, and Paula Pijut, Keith Woeste, and Jim McKenna of the Hardwood Tree Improvement and Regeneration Center (HTIRC) at Purdue University (West Lafayette, IN) have been deeply involved in research on butternut canker. They have collected twig samples of possibly resistant butternuts, grafted them onto rootstocks, and planted this material in orchards in many different locales and environmental conditions. These places include an experimental station at Rosemount, MN; the Oconto River Seed Orchard on the Chequamegon-Nicolet National Forest in Wisconsin; the HTIRC at Purdue University, and several Ministry of Natural Resources locations in Ontario, Canada. Butternut trees at these sites are used in inoculation studies or allowed to become infected naturally. Survivors can then be used for further breeding. Asian butternuts do show resistance to Oc-j and are being used in hybridization efforts.

Butternut canker 17 months after innoculation. Photo by Michael Ostry, U.S. Forest Service.

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During the search for resistant trees, an unusual appearance was discovered among many of the healthier trees. A large proportion of the healthier, possibly resistant, butternuts had distinctive bark that was darker than the typical light gray with deeper gray furrows. At first glance, it was easy to mistake these for black walnut trees and there was a correlation found between tree health and bark characteristics. Preliminary molecular studies of leaf samples of this population are underway to discover if there is any relationship between bark type and disease resistance. If this is indeed so, the process of selecting resistant trees will be much more efficient. Other studies on bark chemistry have shown a possible correlation between canker resistance and chemical inhibition of conidial germination, which could be developed as a screening tool for finding resistance.

THOUSAND CANKERS DISEASE

Thousand cankers disease (TCD) is the newest “invasive” insect-disease complex coming to eastern forests. Black walnut is an eastern tree that produces wood so highly desirable for furniture and cabinet making that it has been planted all over the United States. In the East, the tree grows in forests and in the countryside, often along fence rows; it is also grown in plantations as an investment. It is allelopathic, meaning that it produces phytochemicals that hinder the growth of other plants near its roots. Unfortunately, in the West, black walnut was planted in the native range of Arizona walnut (J. major) in Arizona, New Mexico, and northern Mexico, which allowed it to become infested with the walnut twig beetle (Pityophthorus juglandis), which has historically fed on Arizona walnut and perhaps on southern California walnut (J. californica). The walnut twig beetle carries with it the fungus Geosmithia morbida, which causes small

Black walnut tree showing early symptoms of thousand cankers disease, in Tennessee. Photo by Jennifer Juzwik, U.S. Forest Service.

cankers beneath the bark of the branches and stems of walnut trees. These numerous small cankers, which give the disease its common name, kill infested trees by restricting nutrient flow; a tree typically dies within 3 to 4 years of infection. The fungus was described as new to science in 2010. Thus, this new disease complex has expanded its geographic range and was able to switch to new host species with no co-evolved resistance, acting as a nonnative pest of black walnut in North America and now also in Europe.

Since the 1990s, black walnut trees planted in the West have shown widespread decline. In 2004, extensive mortality in the Front Range of Colorado was linked to the presence of the TCD beetle and fungus complex. The disease is now present in nine western states and continues to spread into the native range of black walnut. In 2010, it was found in Tennessee; in 2011, in Virginia and Pennsylvania; and in 2013 in Maryland and Ohio. In late 2013, both the beetle and fungus were found in black walnut trees in northern Italy. In 2014, the fungus was reported on a weevil species that emerged from stressed black walnut in Indiana, although TCD symptoms and the beetle were not present. The TCD beetle has since been found on logs at a mill site in Indiana. Long-distance movement of infested logs probably transported the beetle.

Northern Research Station Science

In 2012, NRS researcher Robert Venette began work with scientists at the Pacific Southwest Research Station of the U.S. Forest Service; and at University of Minnesota, University of Missouri, and Purdue University, to better understand the threat that walnut twig beetle poses to black walnut and closely related Juglans species. They studied cold tolerance of walnut twig beetle, the flight capacity of beetles, and the quality of different Juglans species for beetle reproduction and found that cold winter temperatures may cause substantial mortality of walnut twig beetle in portions of Minnesota, Wisconsin, South Dakota, and Iowa. In most places where black walnut grows, though, walnut twig beetle could survive the winter. Walnut twig beetles do not appear to be strong fliers; most fly less than a quarter mile in their lifetime. These results confirm the significant role that movement of infested wood products plays in the long distance movement of the insect. Beetle reproduction is affected by different cultivars of black walnut and other Juglans species, offering hope for sources of resistance.

In 2016, NRS scientist Jennifer Juzwik will work with the Animal and Plant Health Inspection Service (USDA APHIS) and Virginia Tech collaborators

Walnut twig beetle, greatly magnified; true size is 1.5 to 1.9 mm, smaller than half a grain of rice. Photos by Steven Valley, Oregon Department of Agriculture, Bugwood.org.

BIOGRAPHIES

Jennifer Juzwik is a research plant pathologist in the “Restoration and Conservation of Rural and Urban Forests” unit in St. Paul, MN. She received her PhD from the University of Minnesota, MS from Colorado State University, and BS from Fairmont State College, West Virginia. The etiology, epidemiology, and management of hardwood tree diseases, most recently thousand cankers disease, have dominated her research career.

James McKenna is a biologist at the Hardwood Tree Improvement and Regeneration Center (HTIRC) in West Lafayette, IN. He has a BS in plant biology from the University of California.

Melanie Moore is a biological science technician at St. Paul, MN, who frequently published with Mike Ostry (retired). She recently received her MS from the University of Minnesota and is working with Jennifer Juzwik.

Paula M. Pijut is a research

plant physiologist with the HTIRC in West Lafayette, IN. She received her PhD in horticulture from The Ohio State University, a MS in horticulture from Murry State University, and a BS in medical technology from Maryville College of the Sacred Heart. Paula has worked for the USDA Forest Service since 1989 and joined the HTIRC in 2001. Paula’s research program focuses on forest tree biotechnology and vegetative propagation for tree improvement and conservation.

Keith E. Woeste is a research geneticist for the HTIRC since 1999. His interests include the genetics and breeding of hardwood trees, especially fine hardwoods such as walnut, oak, and black cherry. He earned both MS and PhD degrees from the University of California, Davis; his BS in botany was from the University of Florida.

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Robert C. Venette is a research biologist in the “Restoration and Conservation of Rural and Urban Forests” unit in St. Paul, MN. He received his PhD from the University of

California, Davis, and BS from the University of Minnesota. He specializes in ecological research to project the distribution and impacts of invasive species. 4

to evaluate the effectiveness of vacuum-steam treatment in eradicating TCD fungus and walnut twig beetles from veneer logs destined for export to overseas markets. Current quarantines in numerous countries require methyl bromide fumigation of such logs because of TCD presence in the United States. She is also collaborating with Purdue and West Virginia University scientists to study the role of naturally occurring canker diseases, and other insect pests on the development of branch dieback and tree crown death in TCD-affected black walnut in Ohio and Tennessee. The ability of the fungus causing Fusarium canker and the fungus causing Botryosphaeria branch dieback are being compared to that of the TCD fungus in experiments underway in urban parks in Butler County, Ohio, and in a quarantined black walnut plantation in central Indiana. These other fungi had been commonly isolated from cankers on TCD trees in Tennessee and Ohio. In addition, the colonization of TCD-affected trees by invasive ambrosia beetle species as well as the walnut twig beetle suggests that several insects may contribute to the development of TCD. Recently, a weevil species and two of the exotic ambrosia beetle species that commonly colonize stressed walnut, including TCD trees, were found to carry the TCD fungus. The role of these other insects in spread of the disease is under investigation.

Although TCD killed over 60% of black walnuts planted in Boulder, CO, where it was discovered, the disease in the East has not progressed in many affected trees in Tennessee and Virginia during years of normal precipitation. Some trees even seem to have “recovered.” Results of studies to date have led to guarded optimism that TCD will not be another chestnut blight in the eastern United States. It may behave more like oak decline, which requires stress events to predispose black walnut to the complex of insect attack and canker disease development. Disease monitoring and continued research will test these hypotheses.

Black walnut branch with thousand cankers disease. Photo by Jennifer Juzwik, U.S. Forest Service.

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RESOURCES AND REFERENCESWeb Resources:

USDA Forest Service: www.fs.fed.us/research/invasive-species/plant-pathogens/

USFS S&PF Northeastern Area: www.na.fs.fed.us/spfo/pubs/howtos/ht_but/ht_but.htm

USFS R&D Northern Research Station: www.thousandcankers.comwww.thousandcankers.com/state-info.php?state=Colorado

USFS Southern Research Station:www.srs.fs.usda.gov/pubs/ja/ja_schlarbaum002.htm

USDA National Agriculture Library: www.invasivespeciesinfo.gov/microbes/thousandcankers.shtml

Northern Woodlands magazine:northernwoodlands.org/outside_story/article/their_goal_saving_the_butternut_tree

References:

Ginzel M, Juzwik J. 2014. Geosmithia morbida, the causal agent of thousand cankers disease, found in Indiana. HN-89-W. West Lafayette, In: Purdue Extension, Department ofEntomology. 2 p.

Juzwik J, Banik MT, Reed SE, English JT, Ginzel MD. 2015. Geosmithia morbida found on weevil species Stenominus pallidus in Indiana. Plant Health Progress doi, 10.1094/PHP-RS-14-0014.

Moore MJ, Ostry ME, Hegeman AD, Martin AC. 2014. Inhibition of Ophiognomonia clavigignenti-juglandacearum by Juglans species bark extracts. Plant Disease 99:401-408.

van Sambeek J, Juzwik J. 2010. What’s killing my walnuts—how to find help. Walnut Council Bulletin 37(1):10-12.

Woeste K, Farlee L, Ostry M, McKenna J, Weeks S. 2009. A forest manager’s guide to butternut. Northern Journal of Applied Forestry 26(1):9-14.

Vegetative propagation (rooted cuttings) and in vitro culture (micropropagation) methods are important in conserving and producing clones of possibly resistant butternut or black walnut trees selected or genetically improved for canker resistance. Paula Pijut at the HTIRC has developed methods for vegetative propagation by rooted cuttings by using carefully controlled conditions. Propagation of 5- and 6-year-old butternut trees by hardwood and softwood cuttings has been successful and rooted cuttings overwintered in cold-storage had high survival and good growth when field-planted the following year. In vitro micropropagation of black walnut produces a large volume of genetically identical material in a relatively short period of time. This material can then be used in research on adventitious root formation in black walnut, which is a limiting factor in clonal walnut propagation. By using a fog system, black walnut softwood cuttings from juvenile seedlings were successfully rooted at rates as high as 71%. These clonal propagation methods developed at HTIRC are a necessary tool for tree breeders and for conservation efforts, especially because of butternut canker and thousand cankers disease. Vegetative propagation of butternut-rooted cuttings.

Photo by Paula Pijut, U.S. Forest Service.

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For additional printed copies or to receive this publication in electronic format, email nrspubs@fs.fed.us or call 740-368-0123.

NRS Research Review is published quarterly by the Communications and Science Delivery Group of the Northern Research Station (NRS), U.S. Forest Service. As part of the Nation’s largest forestry research organization, NRS serves the Northeast and Midwest and beyond, providing the latest research on current problems and issues affecting forests and the people who depend on them. Our research themes are (1) Forest Disturbance Processes, (2) Urban Natural Resources Stewardship, (3) Sustaining Forests, (4) Providing Clean Air and Water, and (5) Natural Resources Inventory and Monitoring.

There are 111 NRS scientists working at 20 field offices, 24 experimental forests, and universities located across 20 states, from Maine to Maryland, Missouri to Minnesota.

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Additional References

Juzwik J, Haugen L, Park JH, Moore M. 2008. Fungi associated with stem cankers and coincidental scolytid beetles on declining hickory in the upper Midwest. In: Jacobs DF, Michler CH, eds. 2008. Proceedings, 16th Central Hardwood Forest Conference. GTR-NRS-P-24. Newtown Square, PA: USDA Forest Service, Northern Research Station: 476-482.

Krochmal A, Krochmal C. 1982. Uncultivated nuts of the United States. Ag. Info. Bull. 450. Washington, DC. USDA Forest Service.

Moore MJ, Ostry ME. 2015. Influence of temperature and humidity on the viability of Ophiognomonia clavigignenti-juglandacearum conidia. Plant Disease, PDIS-09-14-0976-RE.

McKenna JR, Ostry ME, Woeste K. 2011. Screening butternut and butternut hybrids for resistance to butternut canker. In: Fei S, Lhotka JM, Stringer JW, e al., eds. Proceedings, 17th Central Hardwood Forest Conference. NRS-P-78. Newtown Square, PA: USDA Forest Service, Northern Research Station: 460-474.

Ostry ME, Moore M. 2007. Natural and experimental host range of Sirococcus clavigignenti-juglandacearum. Plant Disease 91(5):581-584.

Ostry ME, Moore MJ, Worrall SAN. 2003. Butternut (Juglans cinerea) annotated bibliography. GTR-NC-235. St. Paul, MN: USDA Forest Service, North Central Research Station. 28 p. (Available online at www.ncrs.fs.fed.us/pubs/).

Pijut PM, Lawson, SS, Michler CH. 2011. Biotechnological efforts for preserving and enhancing temperate hardwood tree biodiversity, health, and productivity. In Vitro Cellular and Developmental Biology-Plant 47:123-147.

Pijut PM, Moore M, 2002. Early season softwood cuttings effective for vegetative propagation of Juglans cinerea. HortScience 37(4):697-700.

Pijut PM, Woeste KE, Michler CH. 2011. Promotion of adventitious root formation of difficult-to-root hardwood tree species. In: Janick J, ed. Horticultural Reviews 38:213-251.

Reed SE, Juzwik J, English JT, Ginzel MD. 2015. Colonization of artificially stressed black walnut trees by ambrosia beetle, bark beetle, and other weevil species (Coleoptera: Curculionidae) in Indiana and Missouri. Environmental Entomology doi: 10, 1093/33/nvv126.

Woeste K, Pijut PM. 2009. The peril and potential of butternut. Arnoldia 66(4):2-12.