Sirococcus clavigignenti-juglandacearum (butternut canker)

Cankers can occur on all woody parts of J. cinerea, including twigs, branches, stems and buttress roots. Stem cankers, usually many per tree, separate or joined together, can be present on all sides of the trunk, especially the lower portion. Trees of all ages and sizes, regardless of site conditions, can be infected. Tisserat and Kuntz (1984) found that the disease develops in a characteristic pattern. Young cankers are elongated, sunken areas, which usually originate at leaf scars, lenticels, lateral buds, stomata, bark wounds, natural bark cracks, and even sporadically at points apparently free of any injury. These initial cankers are generally found in the lower crowns. In spring, an inky black, watery exudate issues from canker fissures. In summer, dry sooty black patches remain, often with a whitish margin. If the bark is removed, a brown to black elliptical area of killed cambium can be seen. Small branches and young saplings die rapidly, but most stem cankers of older trees become perennial. These occur mostly on the lower stem and on exposed parts of the roots. Older cankers are surrounded by layers of callus and are vertically oriented, large, open or partly covered by shredded bark. Trees are killed progressively as cankers girdle the stem or kill the crown of the tree. Trunk sprouts and branch suckers often emerge below the girdled area but are infected subsequently and rapidly killed (Kuntz et al., 1979; Ostry, 1997b). Fruits from infected trees are sound and edible according to Kuntz et al. (1979). For a detailed description of symptoms on J. regia and J. nigra, see Orchard (1984). Another fungus, Melanconis juglandis (anamorph: Melanconium oblongum Berk.), often causes secondary infection (Nicholls et al., 1978), and has been confused with S. clavigignenti-juglandacearum. M. juglandis does not cause cankers. Its anamorphic fruiting bodies appear as very small, dark, smooth bumps on the bark, releasing inky black masses of spores. Spores are usually asymmetrically ovoid, dark and non-septate (Nicholls et al., 1978).

Up to now, there is no effectively applied control against S. clavigignenti-juglandacearum. The fungicides benomyl and chlorothalonil have been found to be effective preventive products in laboratory experiments (Nicholls, 1979), but were not approved in the field. Even if an effective mechanical method, fungicide or biocontrol agent existed, it would be difficult to apply it due to the scattered distribution of susceptible trees. Removal of recently killed and visibly infected trees may reduce the amount of inoculum only locally (Nair, 1999). The only possibility for successful control of the fungus seems to be the selection of resistant forms. Resistance varies greatly between individual trees and is possibly heritable (Ostry, 1997b). Studies are in progress to find, graft, propagate and plant resistant trees of J. cinerea in the USA (Ostry, 1997b).

S. clavigignenti-juglandacearum is a very aggressive plant pest. From 1981 to 1996, in the USA, it killed 58% of the trees of J. cinerea in Wisconsin and 84% in Michigan (Ostry, 1997b). Cummings-Carlson (1993) reported that 91% of J. cinerea trees in Wisconsin were already infested at the time of her survey. In North and South Carolina, the fungus had eliminated J. cinerea to a large extent by 1977 (Anderson and Lamadeleine, 1977) and 77% of J. cinerea trees had been killed in North Carolina and Virginia by 1994 (USDA, 1995; Tainter and Baker, 1996). In USA, good-quality wood of J. cinerea is second in economic value only to J. nigra (Peterson, 1977), for use in interior woodwork, household woodenware, furniture, cabinetry and dye production. Nuts present an important source of wildlife food. S. clavigignenti-juglandacearum presents a threat to biodiversity in North America. Due to the extensive mortality caused by the fungus and the large amount of already infected trees, J. cinerea was listed as a threatened and endangered species in the USA (Category 2 - more information required prior to formal listing; Ostry et al., 1994), but this category has been eliminated and currently J. cinerea has no official listing status. In Canada, butternut was listed as an endangered species by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in November 2003.