Collar Rot

Phytophthora cactorum

37 host plants

Last updated

Data Maturity Structured

This profile synthesizes data from multiple published sources. Expert field review is in progress.

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.

Quick Reference

Causal Agent
Phytophthora cactorum
Host Plants
37
Favorable Conditions
Saturated or near-saturated soils; excessive irrigation; trunk wounds; contam...

Management

Vulnerability Window

Most active during wet periods in spring and fall when soil is saturated (April-May and September-November in the Puget Sound region). Susceptible plants in containers or poorly drained sites remain vulnerable year-round if overwatered. # Source: PNW Plant Disease Management Handbook

What Triggers Infection

Saturated or near-saturated soils; excessive irrigation; trunk wounds; contaminated propagation media; waterlogged containers or nursery sites; warm (50-80°F) soil temperatures combined with high moisture. # Source: PNW Plant Disease Management Handbook - Collar Rot

Cultural Controls

  • Avoiding trunk wounds is the most important control.
  • Prevent overhead irrigation water from wetting trunks.
  • Do not overwater on low water use (cloudy and/or cool) days.
  • Remove and destroy all infected plants and plant debris.
  • Surgery of small cankers is recommended in BC to prolong tree life.
  • Remove diseased tissue in dry weather using a sharp knife.

Host Plants (37)

Alnus alnobetula Circumpolar alder group Alnus alnobetula subsp. crispa American Green Alder, Mountain Alder, Green Alder Alnus alnobetula subsp. sinuata Sitka Alder, Wavy Leaf Alder Alnus cordata Italian Alder Alnus glutinosa Common Alder, European Alder, Black Alder Alnus incana subsp. rugosa Speckled Alder Alnus incana subsp. tenuifolia Thinleaf Alder, Mountain Alder Alnus japonica Japanese Alder Alnus rhombifolia White Alder Alnus rubra Red Alder Alnus rubra f. pinnatisecta Cutleaf Red Alder Alnus sinuata Sitka Alder Arbutus unedo Strawberry Tree, Killarney Strawberry Tree, Madroño Cornus alba Tatarian Dogwood, Tartarian Dogwood Cornus alternifolia Pagoda Dogwood Cornus amomum Silky Dogwood Cornus canadensis Bunchberry Cornus capitata Evergreen Dogwood, Bentham's Cornel, Himalayan Strawberry Tree Cornus controversa Giant Dogwood Cornus drummondii Rough-leaved Dogwood Cornus eddies Cornus eddies Cornus elwinortonii Cornus elwinortonii Cornus florida Flowering Dogwood Cornus kousa Kousa Dogwood Cornus mas Corneliancherry Dogwood Cornus nuttallii Pacific Dogwood Cornus officinalis Japanese Cornelian Dogwood Korean Cornelian Dogwood, Chinese Cornelian Dogwood Cornus porlock Porlock Dogwood Cornus racemosa Gray Dogwood, Red Panicle Dogwood Cornus rutgersensis Cornus rutgersensis Cornus sanguinea Bloodtwig Dogwood Cornus sericea Cornus sericea Cornus sericea subsp. occidentalis Western Dogwood, Creek Dogwood Cornus stolonifera Redosier Dogwood Cornus torreyi Western Dogwood, Creek Dogwood Cornus venus Venus Dogwood Rhamnus alnifolia Alder Buckthorn, Glossy Buckthorn, Breaking Buckthorn