United States Department of Agriculture

US FOREST SERVICE NORTHERN RESEARCH STATION Research Review

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.

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

US Forest Service NO. 28 | JANUARY 2016 1 Northern Research Station continued from page one

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, Butternut canker 17 months after innoculation. stems, and buttress roots and dark brown, elliptical stains beneath the bark; Photo by Michael Ostry, U.S. Forest Service. these become large and girdle branches and twigs, then trunk or roots, which kills the tree. Breeding for resistance to butternut canker is probably the most important aspect of NRS research for fighting butternut canker. Beginning in Northern Research Station Science the 1990s, healthy butternut trees were being found despite growing in close Investigating the biology of the fungus causing butternut canker has vicinity to diseased trees. Numerous surveys by forest agencies as well as provided useful insights. The canker predominantly affects butternut, but contacts from private individuals recorded a number of possibly resistant naturally produced cankers have been found on other Juglans species, butternut trees throughout the range. These trees offer some hope of finding namely black walnut (J. nigra) and Japanese walnut (J. ailantifolia). Artificial resistance to butternut canker within the natural population. Starting in inoculations have found that Oc-j can colonize and survive in the wood 1990, NRS scientists Mike Ostry and Darroll Skilling of St. Paul, MN (now of several genera, including Carya (hickory), Corylus (hazelnut), Prunus both retired), Melanie Moore of St. Paul, and Paula Pijut, Keith Woeste, (stone fruits), and Castanea (chestnut). This is evidence that the fungus may and Jim McKenna of the Hardwood Tree Improvement and Regeneration be surviving on other trees in a nonpathogenic state as a reservoir when Center (HTIRC) at Purdue University (West Lafayette, IN) have been deeply butternuts are not present. involved in research on butternut canker. They have collected twig samples of possibly resistant butternuts, grafted them onto rootstocks, and planted Finding insect vectors of butternut canker is another important activity this material in orchards in many different locales and environmental of NRS scientists. Rain splash and wind have been shown to spread Oc-j conditions. These places include an experimental station at Rosemount, conidiospores up to about 45 meters. For long-distance transport, the sticky MN; the Oconto River Seed Orchard on the Chequamegon-Nicolet National spores can adhere to insects that then carry the spores to isolated butternut Forest in Wisconsin; the HTIRC at Purdue University, and several Ministry trees. A number of insects have been found capable of disseminating Oc-j of Natural Resources locations in Ontario, Canada. Butternut trees at these conidia, and three beetle species were found able to carry viable conidia for sites are used in inoculation studies or allowed to become infected naturally. up to 16 days. These potential vectors may explain how apparently isolated Survivors can then be used for further breeding. Asian butternuts do show butternuts become infected, but more studies are needed on conditions and resistance to Oc-j and are being used in hybridization efforts. timing.

2 During the search for resistant trees, an unusual appearance was discovered Walnut twig beetle, greatly among many of the healthier trees. A large proportion of the healthier, magnified; true size is 1.5 to possibly resistant, butternuts had distinctive bark that was darker than 1.9 mm, smaller than half a the typical light gray with deeper gray furrows. At first glance, it was easy grain of rice. to mistake these for black walnut trees and there was a correlation found Photos by Steven Valley, Oregon Department of Agriculture, between tree health and bark characteristics. Preliminary molecular studies Bugwood.org. 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, cankers beneath the bark of the branches and stems of walnut trees. These the process of selecting resistant trees will be much more efficient. Other numerous small cankers, which give the disease its common name, kill studies on bark chemistry have shown a possible correlation between canker infested trees by restricting nutrient flow; a tree typically dies within 3 to resistance and chemical inhibition of conidial germination, which could be 4 years of infection. The fungus was described as new to science in 2010. developed as a screening tool for finding resistance. 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 THOUSAND CANKERS DISEASE a nonnative pest of black walnut in North America and now also in Europe.

Thousand cankers disease (TCD) is the newest “invasive” insect-disease Since the 1990s, black walnut trees planted in the West have shown complex coming to eastern forests. Black walnut is an eastern tree that widespread decline. In 2004, extensive mortality in the Front Range of produces wood so highly desirable for furniture and cabinet making Colorado was linked to the presence of the TCD beetle and fungus complex. that it has been planted all over the United States. In the East, the tree The disease is now present in nine western states and continues to spread grows in forests and in the countryside, often along fence rows; it is also into the native range of black walnut. In 2010, it was found in Tennessee; grown in plantations as an investment. It is allelopathic, meaning that it in 2011, in Virginia and Pennsylvania; and in 2013 in Maryland and Ohio. produces phytochemicals that hinder the growth of other plants near its In late 2013, both the beetle and fungus were found in black walnut trees roots. Unfortunately, in the West, black walnut was planted in the native in northern Italy. In 2014, the fungus was reported on a weevil species that range of Arizona walnut (J. major) in Arizona, New Mexico, and northern emerged from stressed black walnut in Indiana, although TCD symptoms Mexico, which allowed it to become infested with the walnut twig beetle and the beetle were not present. The TCD beetle has since been found on (Pityophthorus juglandis), which has historically fed on Arizona walnut logs at a mill site in Indiana. Long-distance movement of infested logs and perhaps on southern California walnut (J. californica). The walnut twig probably transported the beetle. beetle carries with it the fungus Geosmithia morbida, which causes small 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

Black walnut tree showing early symptoms of thousand cankers 3 disease, in Tennessee. Photo by Jennifer Juzwik, U.S. Forest Service. to evaluate the effectiveness of vacuum-steam treatment in eradicating BIOGRAPHIES TCD fungus and walnut twig beetles from veneer logs destined for export Jennifer Juzwik is a research plant to overseas markets. Current quarantines in numerous countries require pathologist in the “Restoration and methyl bromide fumigation of such logs because of TCD presence in the Conservation of Rural and Urban United States. She is also collaborating with Purdue and West Virginia Forests” unit in St. Paul, MN. She University scientists to study the role of naturally occurring canker diseases, received her PhD from the University and other insect pests on the development of branch dieback and tree crown of Minnesota, MS from Colorado State death in TCD-affected black walnut in Ohio and Tennessee. The ability of University, and BS from Fairmont the fungus causing Fusarium canker and the fungus causing Botryosphaeria branch dieback are being compared to that of the TCD fungus in State College, West Virginia. The etiology, epidemiology, and experiments underway in urban parks in Butler County, Ohio, and in a management of hardwood tree diseases, most recently thousand quarantined black walnut plantation in central Indiana. These other fungi had cankers disease, have dominated her research career. been commonly isolated from cankers on TCD trees in Tennessee and Ohio. James McKenna is a biologist at the Hardwood Tree In addition, the colonization of TCD-affected trees by invasive ambrosia Improvement and Regeneration Center (HTIRC) in West beetle species as well as the walnut twig beetle suggests that several insects Lafayette, IN. He has a BS in plant biology from the University of may contribute to the development of TCD. Recently, a weevil species and California. two of the exotic ambrosia beetle species that commonly colonize stressed walnut, including TCD trees, were found to carry the TCD fungus. The role Melanie Moore is a biological science of these other insects in spread of the disease is under investigation. technician at St. Paul, MN, who frequently published with Mike Ostry (retired). Although TCD killed over 60% of black walnuts planted in Boulder, CO, She recently received her MS from the where it was discovered, the disease in the East has not progressed in University of Minnesota and is working many affected trees in Tennessee and Virginia during years of normal with Jennifer Juzwik. 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 Paula M. Pijut blight in the eastern United States. It may behave more like oak decline, is a research which requires stress events to predispose black walnut to the complex plant physiologist with the HTIRC in of insect attack and canker disease development. Disease monitoring and West Lafayette, IN. She received her continued research will test these hypotheses. 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. Black walnut branch with thousand cankers disease. Photo by Jennifer Juzwik, U.S. Forest Service. 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 2 3 impacts of invasive species. 4 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.

RESOURCES AND REFERENCES Web Resources: References: Ginzel M, Juzwik J. 2014. Geosmithia morbida, the causal USDA Forest Service: www.fs.fed.us/research/invasive-species/ agent of thousand cankers disease, found in Indiana. HN- plant-pathogens/ 89-W. West Lafayette, In: Purdue Extension, Department of USFS S&PF Northeastern Area: www.na.fs.fed.us/spfo/pubs/ Entomology. 2 p. howtos/ht_but/ht_but.htm Juzwik J, Banik MT, Reed SE, English JT, Ginzel MD. 2015. USFS R&D Northern Research Station: www.thousandcankers. Geosmithia morbida found on weevil species Stenominus com pallidus in Indiana. Plant Health Progress doi, 10.1094/ PHP-RS-14-0014. www.thousandcankers.com/state-info.php?state=Colorado Moore MJ, Ostry ME, Hegeman AD, Martin AC. 2014. Inhibition USFS Southern Research Station: of Ophiognomonia clavigignenti-juglandacearum by www.srs.fs.usda.gov/pubs/ja/ja_schlarbaum002.htm Juglans species bark extracts. Plant Disease 99:401-408.

USDA National Agriculture Library: www.invasivespeciesinfo. van Sambeek J, Juzwik J. 2010. What’s killing my walnuts— gov/microbes/thousandcankers.shtml how to find help. Walnut Council Bulletin 37(1):10-12.

Northern Woodlands magazine: Woeste K, Farlee L, Ostry M, McKenna J, Weeks S. 2009. A northernwoodlands.org/outside_story/article/their_goal_ forest manager’s guide to butternut. Northern Journal of saving_the_butternut_tree Applied Forestry 26(1):9-14.

5 Additional References Ostry ME, Moore MJ, Worrall SAN. 2003. Butternut (Juglans cinerea) annotated bibliography. GTR-NC-235. St. Paul, MN: Juzwik J, Haugen L, Park JH, Moore M. 2008. Fungi associated USDA Forest Service, North Central Research Station. 28 p. with stem cankers and coincidental scolytid beetles on (Available online at www.ncrs.fs.fed.us/pubs/). declining hickory in the upper Midwest. In: Jacobs DF, Michler CH, eds. 2008. Proceedings, 16th Central Hardwood Forest Pijut PM, Lawson, SS, Michler CH. 2011. Biotechnological efforts Conference. GTR-NRS-P-24. Newtown Square, PA: USDA Forest for preserving and enhancing temperate hardwood tree Service, Northern Research Station: 476-482. biodiversity, health, and productivity. In Vitro Cellular and Developmental Biology-Plant 47:123-147. Krochmal A, Krochmal C. 1982. Uncultivated nuts of the United States. Ag. Info. Bull. 450. Washington, DC. USDA Forest Pijut PM, Moore M, 2002. Early season softwood cuttings Service. effective for vegetative propagation of Juglans cinerea. HortScience 37(4):697-700. Moore MJ, Ostry ME. 2015. Influence of temperature and humidity on the viability of Ophiognomonia clavigignenti- Pijut PM, Woeste KE, Michler CH. 2011. Promotion of adventitious juglandacearum conidia. Plant Disease, PDIS-09-14-0976-RE. root formation of difficult-to-root hardwood tree species. In: Janick J, ed. Horticultural Reviews 38:213-251. McKenna JR, Ostry ME, Woeste K. 2011. Screening butternut and butternut hybrids for resistance to butternut canker. In: Fei S, Reed SE, Juzwik J, English JT, Ginzel MD. 2015. Colonization Lhotka JM, Stringer JW, e al., eds. Proceedings, 17th Central of artificially stressed black walnut trees by ambrosia Hardwood Forest Conference. NRS-P-78. Newtown Square, PA: beetle, bark beetle, and other weevil species (Coleoptera: USDA Forest Service, Northern Research Station: 460-474. Curculionidae) in Indiana and Missouri. Environmental Entomology doi: 10, 1093/33/nvv126. Ostry ME, Moore M. 2007. Natural and experimental host range of Sirococcus clavigignenti-juglandacearum. Plant Disease Woeste K, Pijut PM. 2009. The peril and potential of butternut. 91(5):581-584. Arnoldia 66(4):2-12.

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NRS Research Review is published quarterly by the There are 111 NRS scientists working at 20 field offices, Communications and Science Delivery Group of the Northern Research 24 experimental forests, and universities located across 20 states, Station (NRS), U.S. Forest Service. As part of the Nation’s largest from Maine to Maryland, Missouri to Minnesota. 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.

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