Tree Diseases: Hypoxylon Canker of Oaks (Hypoxylon atropunctatum)
Hypoxylon canker of oaks is a fungal disease caused by the pathogen, Hypoxylon atropunctatum. The fungus infects weakened, injured, and dying wood on oak trees. The fungus generally infects trees that have sustained mechanical injuries, or are suffering from environmental stress. Oak trees may be readily infected in landscape and forested settings. Infected trees will generally succumb to infection within 1 to 2 years.
Distribution & Habitat
Hypoxylon canker of oaks is common across southern Canada. It is also widespread across the United States, pariticularly the Northern, Southern, and Midwestern states.
Black, blackjack, pin, post, red, water, and willow oak are the most prone to infection. Burr, chestnut, swamp white, and white oak are infected as well, albeit with less frequency. While the fungus generally colonizes live plant material, it also exhibits sacrophytic behavior, infecting the dead wood of basswood, beech, hickory, hornbeam, maple, and sycamore.
From late spring to early summer, the fungus infiltrates susceptible trees through wounds on the branches or trunk. The fungus feeds on the conductive tissue within the sapwood, effectively killing it. As the fungus progresses downward, the infected branches gradually die back. Dead, sunken patches will become visible on infected branches and trunks. A soft, silver-colored sheet of fungal material called stromata develops beneath the bark, on the infected wood. Minute, black dots arise on the surface of the stromata. These dots release spores, which are dispersed by air currents, splashes of rain, or various rodents, birds, and insects to nearby trees, where they initiate new infections. Meanwhile, the fungus continues to advance within the tree, eventually killing it.
Symptoms of Infection
At the onset of infection, leaves may become discolored, and wilt. The most characteristic symptom is the sloughing of bark on infected branches and trunks. As the bark sloughs off, it exposes the stromata on the diseased wood. In spring or early summer, powdery, green to brown masses of spores called conidia are produced on the surface of the stromata. After the conidia have been released, the stromata thicken, and become firm. During this period, the stromata turn brown to black in color. When mature, the stromata may extend from several inches to a few feet in length. Sunken or depressed areas will appear on infected sections of the tree prior to the formation of the stromata. As the infection advances, the tree will undergo extensive foliar dieback. This will eventually result in plant mortality.
- There is currently no effective control method for hypoxylon canker.
- If over 15% of the crown has been infected, it is recommended that the tree be felled, and the debris safely disposed of. Avoid using diseased plant material as compost.
- Following the removal of infected trees, avoid planting in previously diseased areas for at least six months.
- If 15% or less of the crown is infected, prune out all infected branches and cankers. Dispose of the diseased material.
- Maintain plant vigor through sound cultural practices.
- Ensure that plants are sufficiently watered, especially during extended periods of drought.
- Apply a layer of organic mulch around the base of trees to improve soil quality, moderate soil temperature, and retain soil moisture.
- Avoid mechanical injuries to trees, and unneccesary disturbances to the soil.
- Monitor for the presence of insects. Treat infested plants accordingly to eliminate insects, and prevent damage to vulnerable plants.
Photo courtesy of Texas Forest Service