Butternut Canker

Bye Bye Butternut?

Host

Butternut (Juglans cinerea) is naturally an uncommon tree through most of its range (US Northeast, Midwest and northern Southeast, and adjacent provinces of Canada). Beginning in 1967, increasing mortality was observed and soon linked to the disease. No other hosts are significantly affected.

Pathogen

The pathogen was previously unknown.  Sadistic mycologists gave it a tortured name that noone can pronounce: Sirococcus clavigignenti-juglandacearum.  Then, although it only reproduces asexually (pycnidia), they later identified the genus it belongs to phylogenetically, and started using it: Ophiognomonia ​[1]​.  Just wanted to keep your head spinning ;-).

Environment

As with many introduced diseases, environmental factors don’t seem to be very effective in regulating the amount of disease.

Disease Cycle

Conidia are available throughout the growing season ​[2]​.  Infection courts include leaf scars and buds, but the fungus also seems able to enter through secondary bark.  Cankers may appear anywhere on the tree. They are common on the stem base and on exposed roots, presumably because spores are washed there in rain and accumulate.  Cankers exude black liquid in spring, leaving a sooty residue.  Older cankers are perennial and have successive callus layers.

Where bark is somewhat thin, the fungus produces pegs or pillars beneath the outer bark layer.  The pegs (stromata) extend and rupture the outer bark to allow spore release, as shown in the featured image above.  Pycnidia are produced among the stromata.

Origin and Distribution

The disease was first noted in Wisconsin in 1967, but was likely present earlier.  A new species such as this causing novel, severe damage to a host in its native range leads us to suspect that the pathogen is introduced, but there is no direct evidence.

The disease has spread virtually throughout the host range, and live butternut has decreased as much as 90 percent.  Most remaining trees are diseased. In many areas, it is anticipated that butternut will be nearly extirpated in a few decades.

Some states have declared a moratorium on harvest of live butternut. It is now listed in category 2, under consideration, on the list of Endangered and Threatened Plants, it is a sensitive species on National Forest System lands, and in Canada it was listed as Endangered in 2005.