Butternut canker is killing the species over its whole range. The fungal pathogen (Sirococcus clavigignenti-juglandacearum) apparently was introduced from outside of North America. It was first reported from southwestern Wisconsin in 1967 but is believed to have spread from the southeastern US coastal region, where it first appeared about 40-50 years ago. The Forest Service estimated in 1995 that 77% of the butternuts in the Southeast were dead. The fungus infects trees through buds, leaf scars, and possibly insect wounds and other openings in the bark, rapidly killing small branches. Spores produced on branches are spread by rain, resulting in multiple, perennial stem cankers that eventually girdle and kill infected trees – these do not resprout. The cankered portions should be removed and destroyed and the wounds should be covered with fungicidal paint; leaves that might harbor fungus (brown leaf spot) should be destroyed.
A few healthy butternut trees have been found growing among canker-diseased and dying trees and may be resistant. Black walnut apparently is unaffected. A research coalition has been formed to locate surviving trees or populations, characterize sites, identify trees with putative resistance, develop screening methodology for disease resistance, study fungal physiology, and preserve germplasm.
Fire easily top-kills butternut and older trees rarely sprout from the root crown or stump. A single hot fire or repeated cool fires can effectively eliminate the species in mixed hardwood stands
There is commonly a zone of no-growth or inhibited growth around walnut trees, because they produce a naphthoquinone (juglone) that selectively inhibits growth of associated plants. Juglone is concentrated in root tissue and fruit husks, with lesser amounts in the leaves, catkins, buds, and inner bark.
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