See:
Dogwood (Cornus spp.) - Collar Rot
Cause Phytophthora cactorum, a fungus-like microorganism that lives in soil and also attacks dogwood, apple, and other trees and plants. The disease also is distributed through most of the madrone range. It is found in nurseries with poor drainage. The pathogen survives as spores in the soil, organic debris or infected tissue. Root or root crown infection occurs in spring when soils are at or near saturation. Infection can also occur directly through pruning wounds. Trees are sensitive to overwatering in garden plantings.
Although madrone is susceptible to a leaf spot and shoot dieback caused by Phytophthora ramorum, it has not been found on madrone in the PNW.
Symptoms Leaves of infected trees may be stunted or may die. Cankers are at or near the base of the trunk. Cankers are sunken and may appear water soaked, brown, or blackish; each has a definite margin and sometimes exudes a black liquid.
Cultural control
- Avoid wounding trees.
- Small cankers may be removed surgically with some success if all discolored bark and wood, including a 1-inch border area of apparently healthy bark and wood, also are removed and destroyed. Leave area open to the air to facilitate drying. Wash and sterilize tools afterward.
- Plant only in well-drained areas. Correct drainage if necessary.
- Do not let overhead irrigation water hit and run down trunks and collect at the base of trees in landscapes. Infrequent deep waterings weeks apart are better than frequent light waterings everyday.
- Remove and destroy all infected plants and plant debris.
- Avoid piling more soil, trenching, or compacting soil in the area within the tree's drip zone. Avoid mulching around the base of the tree.
Reference Stuntz, D.E. and Seliskar, C.E. 1943. A stem canker of dogwood and madrona. Mycologia 35:307-221.