Butternut canker is a fungal disease that attacks Juglans cinerea, also known as butternut or white walnut. The disease is caused by the pathogen, Sirococcus clavigignenti-juglandacearum. S. clavigignenti-juglandacearum is believed to have originated in Asia. It was first documented in North America in 1967. The pathogen was discovered plaguing butternut trees in Wisconsin. It has since become widespread across the eastern United States and southeastern Canada.
Butternut canker is the most significant disease of butternut. No cure is presently known. Butternut canker induces the formation of cankers on infected trees. The cankers rapidly enlarge, eventually girdling the affected portion of the tree. Due to the virulent nature of the disease, infected butternuts are typically killed quickly. Butternut canker has ravaged butternut populations in their native range. The United States and Canada are currently engaged in preservation efforts to protect butternut from further devastation.
Distribution & Habitat
Butternut canker is prevalent across the midwestern and northeastern United States. In Canada, it has been reported across the entire native range of butternut, including New Brunswick, southern Ontario, and southwest Quebec.
Butternut canker primarily infects the butternut tree. Infections may also occur on hickories, and other members of the genus Juglans, albeit with less frequency.
The butternut canker pathogen overwinters on previously infected plant material. In spring, when conditions are sufficiently moist, fruiting bodies called pycnidia burst and expel asexual conidiospores into the air. The spores are released in wet, creamy masses, and spread to vulnerable trees via splashes of rain, air currents, insects vectors, birds, or rodents. The spores invade trees through leaf and branch openings. They germinate in the tree, quickly becoming established. Trees may be infected repeatedly as spores drip or wash down onto the main trunk. The nuts produced by butternut may also be infected.
Sunken, elliptical cankers develop beneath the bark of infected trunks, stems, and branches, as well as on exposed buttress roots. As the disease progresses, the cankers grow larger, eventually girdling the affected portion of the tree. Cankers that form on the trunk disrupt the ability of the tree to distribute water and nutrients throughout the crown. This causes infected butternuts to quickly decline. Butternut infections may continue to occur throughout the growing season.
Symptoms of Infection
Cankers form on infected stems, branches, twigs, and buttress roots. Long fissures will appear in the bark of infected trees. As cankers increase in size, they may coalesce, causing the bark to assume a malformed appearance. Small sunken, black cankers with white margins may be observed around leaf or branch scars. A black substance will often ooze from bark cracks on infected trees. When the substance dries, it leaves a dark stain on the bark. Epicormic sprouts will typically grow just below dead or infected sections of the tree. The sprouts usually become infected, and die back. Infected trees may have loose bark. Bark may slough off of infected trees. Armillaria root rot often infects trees suffering from butternut canker. A secondary fungus, Melanconium oblongum, is regularly found on dead butternut branches. While it is not as aggressive as Sirococcus clavigignenti-juglandacearum, M. oblongum can hasten the dieback of infected branches.
- Fungicidal treatments are ineffective against butternut canker.
- If butternut canker is detected early, infected branches can be removed to prevent the spread of the disease. Prune out branch cankers by cutting six to eight inches below the canker. Disinfect pruning tools after each cut.
- Butternut canker infections are most common on trees that have been weakened by environmental stressors, or mechanical injuries. As such, maintain tree vigor through sound cultural practices.
- Ensure that butternut trees are watered, especially during extended periods of drought.
- Apply a layer or organic mulch around the base of vulnerable trees to improve the soil conditions, and retain soil moisture.
- Avoid composting diseased plant material.
- When planting, select disease resistant trees.
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