Dogwood (Cornus spp.)-Collar Rot

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Cause Phytophthora cactorum, a soilborne fungus-like microorganism. This organism can survive as a resting structure (chlamydospores and oospores), as hyphae in the roots of diseased plants and in crop debris. It is easily spread in contaminated potting media, disease cuttings and splashing or flowing water. Recycled irrigation water can also be a source. Generally it enters through trunk wounds but also can enter from infected roots. Saturated soils due to rain or overwatering favor the disease.

Symptoms As a result of the developing canker on the trunk near the soil or on the main roots, usually the first symptom is the lack of vigor and unthriftiness of the tree top. Leaves are small and light green. As the disease progresses, twigs and branches die. At first only one part or side of the tree may show symptoms; however, within 2 to 3 years, symptoms may be apparent over the entire tree.

In the canker at the base of the trunk, the inner bark, cambium, and sapwood are discolored. As the canker progresses, the affected area becomes sunken; the bark falls away, exposing the wood. The tree dies when the canker girdles the trunk or root crown.

Cultural control

  • 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. Leave wound exposed to air during the summer and do not cover with soil or mulch.

Chemical control Use fungicides as preventative treatments. The Group 4 and P7 fungicides used to manage Phytophthora do not kill this organism. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant thereby delaying symptoms that might have developed. Once chemical activity has subsided with time, the organism can resume growth within infected plants. Rotate fungicides from different groups that have a different mode of action for resistance management.

  • Alude at 51 fl oz/74 fl oz water plus an organosilicone-based surfactant as a basal bark spray. Group P7 fungicide. 4-hr reentry.
  • Although not specifically registered for dogwood, according to IR-4 data, Aliette is safe on this crop. This product provides good control on many other crops. Do not use with adjuvants. Group P7 fungicide. 48-hr reentry.
  • ArborFos is registered for tree injections. The number of capsules used is based on tree size. Group P7 fungicide.
  • Empress at 2 to 6 fl oz/100 gal water. Group 11 fungicide. 12-hr reentry.
  • Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
  • Mefenoxam 2 AQ at 0.98 to 1.96 fl oz/100 gal water as a soil drench or at 1.23 to 2.45 fl oz/1,000 sq ft followed by at least 0.5 inch rain or irrigation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
  • MetaStar 2E at 1 to 4 fl oz/100 gal water as a drench. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
  • Organocide Plant Doctor at 16 fl oz in 16 fl oz water plus 1 oz Pentra-Bark as a basal trunk spray. Also labeled for injection, see label for details. Can be used in landscape sites. Group P7 fungicide. H
  • Phospho-Jet at 1 to 2 quarts/100 gal water as a foliar spray. Group P7 fungicide. 4-hr reentry.
  • Segovis at 4.8 to 38.6 fl oz/100 gal water as a soil application. Group 49 fungicide. 4-hr reentry.
  • Stature SC at 6.12 to 12.25 fl oz/50 to 100 gal water. Use as a drench. Group 40 fungicide. 12-hr reentry.
  • Subdue MAXX at 1.25 to 2.5 fl oz/1,000 sq ft, irrigated in with 0.5 inch water within 24 hours. Group 4 fungicide. 48-hr reentry.

Reference Stuntz, D.E. and Seliskar, C.E. 1943. A stem canker of dogwood and madrona. Mycologia 35:307-221.