Survival of butternut (Juglans cinerea) throughout its range in the eastern United States and southeastern Canada is threatened by butternut canker caused by the fungus Sirococcus clavigignenti-juglandacearum. The severity of the disease has prompted the United States to consider butternut a "species at risk." Because of the rapid movement of the fungus, its extreme virulence, its lack of genetic diveristy, and limited resistance displayed by the host, the fungus is believed to be an exotic species of unknown origin introduced to North America.
Infection by S. clavigignenti-juglandacearum results in sunken, elliptical cankers on branches, stems, and buttress roots and dark brown, elliptical stains beneath the bark. If large cankers (or many coalescing smaller cankers) form, branches and twigs become girdled, resulting in dieback. The stem or buttress roots can also become girdled, ultimately killing the tree. Trunk sprouts and epicormic shoots usually become infected and die rapidly. Nuts have also been reported to be infected and upon germination, seedlings may become infected and die. Armillaria root rot is often associated with dying trees and may hasten mortality.
In Vermont, infection levels observed on 18 sites during a survey conducted from 1993 to 1995 ranged from 69 to 100%. A survey of the same trees conducted during 2001 and 2002 indicated infection rates of 71 to 96% at individual sites and an overall infection rate of 82%. All tree locations were entered into a database using GPS and GIS. Physical site attributes (soil type, elevation, hydrology, aspect), which may effect disease incidence and severity, were also obtained. To better understand the role of each of these site factors, site attribute data will be combined with butternut health data to determine whether there are correlations.
Beneath the bark, S. clavigignenti-juglandacearum produces thick, black hyphal pegs or stromatal columns that cause the bark to blister and split open. Pycnidia develop amid the hyphal pegs, and conidiospores are released in gelatinous tendrils or creamy masses. Spores are disseminated throughout the growing season by rain splash, wind, and probably by insects, birds, and rodents. Fruiting structures are more likely to develop on dead branches than on cankered areas of the stem, and rain splash and stem runoff spread spores to lower portions of the tree.
Rain splash and wind have been shown to spread conidiospores up to about 45 meters, although longer distances are possible. Since its discovery in 1967, the fungus has spread rapidly and efficiently throughout the range of butternut, raising questions about the mode of dissemination. Vectors specifically targeting butternut trees may be involved. Sticky conidiospores could adhere easily to the exoskeleton of winged insects and be transported long distances. And so began our search for potential insect vectors of the butternut canker fungus in Vermont.
Sirococcus clavigignenti-juglandacearum is easily isolated from the margins of cankers onto malt extract agar. At 20 degrees Celsius in the dark, the fungus produces brown to black pycnidia in about 14 days.
Conidiospores are hyaline (clear), fusiform (spindle-shaped), 2-celled, and measure 9-17 micrometers x 1-1.5 micrometers. A perfect stage of the fungus remains unknown.
A secondary fungus, Melanconium oblongum (perfect stage: Melanconis juglandis), is often found fruiting on dead butternut branches and is often confused with S. clavigignenti-juglandacearum. Melanconium oblongum produces black acervuli and ovoid to ellipsoid, 1-celled, dark conidiospores that average 19 micrometers x 9 micrometers.
Considered a weak parasite, this fungus invades weakened or dead tissue and causes what is referred to as Melanconis dieback. Both S. clavigignenti-juglandacearum and M. oblongum can be found fruiting on the same branch.
Total numbers of conidiospores estimated per beetle ranged from 200 to 1.6 million. Beetles found in greatest abundance from freshly cut logs and branches of butternut upon which the fungus was fruiting included: Acoptus suturalis (Curculionidae), Astylopsis macula (Cerambycidae), Eubulus parochus (Curculionidae), and Hyperplatys maculata (Cerambycidae).
In 1999, 37 to 74% of each of these four species was carrying conidiospores of the butternut canker fungus. We observed these beetles feeding on hypal pegs and pycnidia of the fungus.
They were also found in crowns of living trees where they are probably attracted to dead tissue. They may be carrying spores of S. clavigignenti-juglandacearum to both living and recently dead branches and increasing infections in the crowns.
If you would like to find out more, see our publication:
Halik, S. and Bergdahl, D.R. 2002. Potential beetle vectors of Sirococcus clavigignenti-juglandacearum on butternut. Plant Disease 86:521-527.