Dogwood (Cornus spp.) - Collar Rot
Cause Several fungi have been associated with necrotic roots of declining trees, but their importance is undetermined. Dogwoods weakened by adverse environmental conditions such as drought stress and poor cultural care are most susceptible to root rot. Trees planted too deeply, in full sun, in poorly drained areas, and those damaged by mechanical injury are at risk. Phytophthora sp. has been found by the OSU Plant Clinic. Flowering dogwood and Cornus nuttallii have been experimentally infected with P. cinnamomi. Flooding encourages disease development and longer periods of flooding increase disease pressure. Drought stressed seedlings were also found to be more susceptible to P. cinnamomi.
Symptoms Root rots cause crown thinning, twig dieback, foliar discoloration, marginal leaf scorch, wilt, and tree death. Phytophthora infected roots will be discolored in the vascular cambium. Secondary roots may be pinched off, leaving only discolored stubs, or may be lacking altogether. Using a pocket knife to expose the cambium, look for a transition zone of discolored tissue below and healthy tissue above.
Cultural control Preventive measures are paramount.
- Plant no deeper than the tree grew in the nursery. Dogwoods have shallow root systems, and planting too deeply can mean the tree will fail to thrive.
- Dogwoods are understory trees, so plant where they will have at least partial shade.
- Provide supplemental water during the summer; water with a soaker hose 6 inches deep once a week. Do not overwater.
- Provide good drainage, plant on a mound, or consider planting a different tree when planting into a wet site.
- Mulch under the tree to conserve moisture during the summer and insulate the shallow roots from heat and cold. Mulch 4 to 6 inches deep out as far as the branches reach (drip line). This also helps prevent injury to the trunk from string weeders or lawn mowers.
- Once a tree is in decline from root rot, it is best to remove it. Do not replant with another dogwood before determining why the first one died and correcting the conditions that led to its demise.
Chemical control Use fungicides as preventative treatments. Use after short periods of flooding has limited utility and is not recom-mended after long periods. The Group 4 and Group 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. Alternate or tank-mix products from different groups that have different modes of action. Limit the use of any one group during crop production.
- 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. 24-hr reentry.
- 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.
- 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 as a foliar spray or at 12 to 24 fl oz/100 gal water as a soil drench. Do not use copper products within 20 days of treatment. 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 soil drench. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- Mural at 3 oz/100 gal water applied as a drench. Group 7 + 11 fungicide. 12-hr reentry.
- 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
- 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. No restrictions on reentry when used as a soil drench.
Biological control Use in combination with other tactics. Will not be effective under high-disease pressure or after flooding.
- RootShield Plus Granules (Trichoderma harzianum Rifai strain T-22 and T. virens strain G-41) at 1 to 3 lb/cubic yard of soil mix or 0.5 to 7.5 lb/1,000 sq ft. No restrictions on reentry required when soil incorporated. O
- Bio-Tam 2.0,Tenet WP, or Obtego (Trichoderma asperellum and T. gamsii) is registered and may be effective. Use 0.5 to 1.5 lb/cubic yard of substrate. See label for details. No restrictions on reentry when soil incorporated. O
- Stargus (Bacillus amyloliquefaciens strain F727) at 2 to 4 quarts/A plus a nonionic surfactant. Group BM02 fungicide. 4-hr reentry. O
Reference Neupane, K., Alexander, L. and Baysal-Gurel, F. 2022. Management of Phytophthora cinnamomi Using Fungicides and Host Plant Defense Inducers Under Drought Conditions: A Case Study of Flowering Dogwood. Plant Disease. 106:475-485.