Spots that usually develop in May or June on leaves and flower bracts are the first symptoms of dogwood anthracnose. Foliar symptoms commonly develop on lower branches first and progress up the tree.
The lesions vary from small circular spots to irregularly shaped blotches and usually are tan or brown in color with a reddish-purple margin. On the underside of the leaf spots the fruiting structures of the fungus may be visible as small raised tan or brown specks scattered randomly over the lesion surface. Spores from these structures are washed onto new leaf tissue during periods of wet weather or sprinkler irrigation. If cool wet weather persists leaves may become severely blighted.
Twigs and branches are also susceptible to infection. Young shoots can become blighted and cankers (localized sunken discolored areas with cracked bark around the margin) form on larger branches where blighted shoots are attached. As a result of twig dieback, succulent shoots, suckers, proliferate on the lower trunk and main branches of affected trees. These branches are very prone to infections which may progress into the main stem.
PREVENTION & CONTROL
1. Keep trees as healthy as possible. A healthy vigorous dogwood is better able to withstand infection from anthracnose than a weakened tree growing under stress conditions. Maintain tree health through proper watering, mulching, and fertilization. Water during drought periods, but do not use overhead irrigation since this may increase the potential for disease infection and spread.
2. Mulch. Apply 4 inches (10 cm) of mulch around the dogwood, starting 3 to 4 inches (7.5 to 10 cm) away from the trunk and extending out to the drip line. Mulching can help maintain uniform soil moisture as well as help protect trunks from mechanical injury such as mowers and weedeaters. In the late spring and early summer feed moderately with a fertilizer that is low in nitrogen and high in phosphorus. This will help keep new growth moderate. Rapidly growing, succulent twigs which have been stimulated by excessive nitrogen are more susceptible to anthracnose infection.
3. Plant in an ideal location which is one that gets full morning sun and afternoon shade and has well-drained soil.
4. Practice good sanitation. This is especially important for trees infected with anthracnose. Prune out and destroy dead and dying twigs and branches and rake up fallen leaves to help reduce potential sources of spores and improve tree appearance. It is also a good idea to prune out water sprouts which develop on the trunk or main scaffolding limbs since they are very susceptible to infection from anthracnose. Prune only under dry conditions and sterilize pruners with alcohol or bleach water between pruning cuts. Prune mature dogwoods so they get good light and air movement.
5. Plant resistant varieties. Cornus kousa and Cornus x rutgersensis ('Aurora', 'Constellation', 'Stellar Pink').
Do not transplant flowering dogwoods from forested areas to your yard; this can result in further spread of the disease.
1. Effective control is possible only if the disease is detected before extensive dieback occurs. Once the disease attacks, wait until winter to prune back canker-ridden branches or twigs to healthy growth. Prune only under dry conditions and sterilize pruners with alcohol or bleach water between pruning cuts. Remove this infected material from your property, don't burn it, and don't let infected leaves fall to the ground.
2. Avoid high nitrogen fertilizers which stimulate succulent branching. Trees with poor vigor may be bolstered by applying a balanced fertilizer in early spring.
3. Fungicides should be used only to supplement good tree care as mentioned above. Apply neem oil or Soap-Shield Liquid Copper Fungicide.
Soap-Shield is an awesome new product that is easy to use and while other copper fungicides work only on wet leaf surfaces, Soap-Shield works on both wet or dry surfaces. It won't clog your sprayer, and is incredibly effective while using much lower concentrations of copper than other copper-based products.
Use fungicides during leaf bud break and growth in the spring, at 10-14 day intervals. If conditions are favorable for disease development later in the growing season, additional fungicide applications may be beneficial.