Fall 2010  Volume 19 No. 3

Butternut and Hybrid Butternut

Butternut or "white walnut" is a native hardwood species related to black walnut. Butternut trees are dying throughout their native range because of a fungal disease known as butternut canker. Breeding and selection of trees resistant to butternut canker began first with the efforts by researchers at USDA-Forest Service and the University of Tennessee. In the past decade, Purdue University (Hardwood Tree Improvement and Regeneration Center - HTIRC) and the University of Vermont began breeding programs. Each program aims to identify and propagate healthy trees from forests where other butternut trees have died. Recently, the HTIRC has learned that the vast majority of healthy butternut trees that have been planted in rural areas in Indiana and across the Midwest are hybrids with Japanese walnut. The Japanese walnut, commonly known as heartnut, was introduced into the U.S. in the late 1800’s as a possible cold-hardy nut crop. As these first heartnut trees grew and produced seed crops, pollen from native butternut cross-pollinated the flowers and produced hybrid offspring. When grown, these first generation (F1) hybrids grew faster than pure butternut or pure heartnut, and they produce more nuts as well.

During the last 40 years or so, as the butternut canker epidemic has moved throughout Indiana and the rest of butternut’s native range, large, healthy butternut trees have come to people’s attention. Most healthy planted trees have turned out to be hybrids. As people planted both hybrids and pure butternut seedlings, it appears that natural selection is favoring the hybrids as very few planted pure butternuts survive and remain healthy today.

Over the last 5 years, researchers using DNA fingerprinting techniques have found genetic markers for heartnut in many of these trees. Current testing of pure and hybrid butternut seed sources at Purdue is showing that hybrids may resist natural infection better than pure butternut, and although hybrids are susceptible to butternut canker when experimentally inoculated, hybrids recover better than butternuts do.

Hybrid trees are often extremely large and often bear large yields of seeds that resemble butternut. Heartnut trees look similar to butternut but the seeds are smooth in contrast to the rough texture of butternut. The twigs and bark differ, however, with heartnut having thicker twigs and lighter colored bark relative to butternut. Distinguishing between pure and hybrid butternut is not easy, in part because many of the hybrids are not simple crosses between the parent species. Instead, many hybrids have complicated pedigrees that probably include intercrosses among hybrids or back-crosses to butternut. As a consequence, they show a confusing pattern of characteristics that can befuddle even an experienced dendrologist.

Since hybrids appear to have more resistance to butternut canker than do pure butternuts, the Indiana DNR Division of Forestry began selling hybrid butternut a few years ago. Their preference is to obtain seed from healthy hybrid butternuts that resemble pure butternut. Scientists at the HTIRC in collaboration with tree improvement at the IN-DNR have been working over the past decade to collect butternuts from Indiana and (possibly) resistant trees from the entire range of the species. These trees are a gene bank, and are being used for basic research, butternut restoration efforts, and as a resource for breeding. Important research objectives include understanding how butternut canker fungus infects trees, and what mechanisms trees use to fight the fungus once it is present. Many healthy butternut trees can still be found in the wild, often growing where other butternuts have died. These trees are an important resource, and at HTIRC we are comparing how these "survivor" trees compare with hybrids that seem less susceptible to infection.

It is possible that we will need genes from Japanese walnut to produce hybrid butternuts that can resist butternut canker disease. Similar to the American Chestnut Foundation’s backcross breeding program, highly resistant hybrid butternuts can be crossed back to pure butternut. The long-term goal at HTIRC is to maintain butternut in the forest and to provide adapted and resistant seed to re-establish it back into its former habitats, for people and wildlife. In Canada, butternut has been formally named an endangered species. Researchers at the University of Gwelph, in conjunction with HTIRC, have been working on understanding the fungus that causes butternut canker disease. What they have learned is that the fungus is almost certainly exotic (introduced from outside the U.S.), and that it is actually a close relative to walnut anthracnose, a disease familiar to many who grow walnuts. Walnut anthracnose is a fungus that forms lesions on leaves and can cause premature defoliation. This finding is important, because it helps us to better understand the ecology of butternut canker. Purdue Extension has two new publications entitled Conservation and Management of Butternut, and Identification of Butternut and Butternut Hybrids. You can view or download both publications free of charge at www.htirc.org.

Keith Woeste and Jim McKenna are researchers with the USDA Forest Service - Northern Research station and Lenny Farlee is an extension forester.  They are part of the Hardwood Tree Improvement and Regeneration Center (HTIRC) in the Department of Forestry and Natural Resources at Purdue. The HTIRC is a collaborative regional research, development and technology transfer effort between industry, university, private, state and federal entities to advance tree improvement of central hardwoods for increased forest productivity in hardwood restoration and reforestation programs.