Physical Description

Infection by the S. clavigignenti-juglandacearum fungus causes cankers to form on the bark of the trunk, branches, and exposed roots of vulnerable trees. Cankers first appear as sooty dark patches on the bark, and develop into broad, sunken dead areas, dark brown to black in colour. Beneath the bark, S. clavigignenti-juglandacearum produces a thick, black mat of hypha. Eventually, the bark infected by cankers will begin to crack and fall off (University of Vermont, 2002).

A fungal infection by S. clavigignenti-juglandacearum produces thick, black hyphal pegs or stromatal columns underneath the bark of vulnerable host trees. Eventually, these growths will cause the bark to split open and sometimes shed from the tree. Pycnidia (asexual fruiting bodies) develop from the mat of hypha, and release spores in creamy masses. Spores are released throughout the summer by a number of possible vectors; evidence shows that rain, wind, insects, birds, and rodents could all play a role in the spread of spores to new host trees. Fruiting structures have been observed to develop on dead branches in the canopy first, and spores spread to lower areas of the host tree through rain splash and runoff (University of Vermont, 2002).

The main host tree is butternut (Juglans cinerea), although it has the potential to infect other species in the walnut (Juglandaceae) family (NRCan, 2014).

Photo:  Rob Routledge, Sault College, Bugwood.org

Signs and symptoms of butternut canker disease include:

• dark, sooty patches on bark

• canopy decline

• cracks in bark

Photo: David Nisbet, Invasive Species Centre

Some common signs and symptoms of butternut canker disease.

The cankers on the bark are the most obvious symptom of the fungal infection; however, cankers are not unique to S. clavigignenti-juglandacearum. Many species of fungi cause canker disease on a variety of trees, with different levels of severity. In addition, some cankers can even be caused by abiotic influences such as winter frost damage. 

Photo: Tom Creswell, Purdue University, Bugwood.org

Signs of butternut canker disease on the surface, and below the bark. Beneath the bark, S. clavigignenti-juglandacearum produces a thick, black mat of hypha, which creates broad dead areas of bark.

Butternut canker is distributed throughout the native range of butternut in North America, raising questions about how the fungus spreads. Scientists believe that transportation vectors, likely flying insects, could specifically target butternut trees (University of Vermont, 2002).

Map:  U.S. Geological Survey 

Native range of the butternut tree

Ecological Impacts

Canker development girdles the tree, leading to canopy dieback and eventually tree mortality. Once infected, the tree can take up to 40 years to die. The rate of mortality depends on the age and size of the tree, as younger trees are killed faster. Roughly 1/3 of Ontario’s butternut trees have already been killed due to the canker disease, and estimates suggest over 90% of the remaining trees are infected (OMNRF, 2013) and will eventually be lost.  

Butternut trees are found throughout the northeast of the United States and southern areas of Ontario and Quebec, although it rarely occurs as a dominant tree species on the landscape. Therefore, scientists anticipate that the ecological impacts resulting from the loss of butternut will be less significant than the loss of other more dominant tree species (e.g. ash or beech). Although butternuts are not a dominant tree, they are a high quality tree resource and provide a nutritious food source (mast) for wildlife and humans (Loo, 2009; Waldron, 2003). In certain environments where other high-quality mast producing trees (e.g. oak) are not present, the impacts that result from the loss of butternut could be greater (Loo 2009; Ostry et al. 1994).

Economic and Social Impacts

 
Butternut wood is prized for woodworking and specialty cabinet making (Poisson and Ursic, 2013). Landowners have begun harvesting butternut in anticipation of mortality (Poisson and Ursic, 2013; Ostry and Pijut 2000), and an increased amount of butternut is already evident in Ontario markets (Poisson and Ursic, 2013). The tree produces rich oily nuts, which have been consumed throughout human history in North America, once being an important food source for Native Americans and early settlers. In addition, the inner bark of the butternut can be boiled to produce a natural yellow dye used for clothing and textiles (Sheu, n.d.). If butternut is lost from the landscape, these traditional uses will no longer be possible.

Photo: USDA Forest Service - Forest Health Protection - St. Paul Archive, USDA Forest Service, Bugwood.org