Collar Rot

If your dogwood, madrone, or buckthorn tree shows unthrifty appearance with small, light green leaves and gradual twig dieback that starts on one side of the canopy, collar rot may be the culprit. The disease develops as a sunken canker at the base of the trunk or on major roots near the soil line; as it expands, bark falls away and wood becomes exposed. The tree eventually weakens completely as the canker girdles the trunk, though this process can take two to three years.

Collar rot thrives in saturated soil conditions common after our wet winters, so drainage is critical. Avoid trunk wounds at all costs, keep overhead irrigation water off the trunk, and don't overwater on cool or cloudy days when soil stays wet. If you catch a small canker early, you can try removing diseased tissue with a sharp knife in dry weather and leaving the wound open to air during summer. For established cankers, the disease is usually fatal; remove and destroy the tree to prevent spread to nearby plants.