The dogwood tree is a popular landscape and home gardening tree, prized for its early spring blooms. The dogwood is highly susceptible to a number of diseases though, many fungal in origin. Identifying the symptoms can help you prevent the disease from spreading.
Diseases in dogwoods can be caused by micronutrient deficiency, fungus, insects and bacterial pathogens. Fungal problems tend to be more prevalent in dogwoods and include dogwood anthracnose, powdery mildew and sooty mold. Bacterial diseases such as crown canker of dogwoods or collar rot are caused by an injury to the tree. Micronutrient chlorosis is caused by micronutrient deficiencies in the soil.
Each disease has its own set of symptoms. Anthracnose develops beige spots with purplish centers on leaves shortly after they appear in May. The underside of the affected leaves display black lesions. The bracts are also affected, bearing reddish brown spots. Leaves will begin to turn brown and droop. The fungus can then spread through the stem, causing cankers. Powdery mildew leaves a white to gray powdery-film on foliage. Leaves become stunted and disfigured. Sooty mold is a black mold-like substance that grows in the in the honeydew left on leaves by aphids and other insects. Crown canker of dogwood's early symptoms include a sparseness of leaves and bracts, a lighter than normal coloring of the leaves in summer and early yellowing and fall of the leaves. During dry periods, the dogwood will show exaggerated wilt. Twigs and branches will begin to die back. A canker develops near the ground on the trunk, in the crown area, killing the bark. Sometimes an ooze seeps from the canker before the lesion is visible and inspection of the bark finds it loose and the wood beneath discolored. Micronutrient chlorosis results in a yellowing or light green coloration of leaves. The leaf veins remain green while the leaf itself changes color. The newer leaves that develop as the season progresses have more pronounced discoloration.
In all cases, the overall health of the tree will begin to fail. With anthracnose, the disease moves through the branches, usually from the bottom up, killing the branches as it goes. Established trees may die within one to three years of infection while saplings usually die the first year. Powdery mildew, in severe cases, may result in premature leaf drop. As nutrients are decreased in affected tissue, the overall health of the tree may begin to decline. Sooty mold blocks the leaf's ability to photosynthesize and produce food. If the insect population is significant and the sooty mold has a good basis to spread throughout the tree, the tree can suffer significant damage or even death. Crown canker of dogwood damages the bark. As the canker expands and the disease progresses, the damaged tree is susceptible to attack from other pests. Micronutrient chlorosis can range from minor symptoms of discolored leaves to premature leaf drop, which can ultimately weaken a d kill the tree.
All trees are more disease resistant when properly cared for. Plant trees in the correct soil type and choose disease-resistant varieties. Provide proper watering and fertilization. Make sure dogwoods get the correct amount of sunshine and protected from winds as their species dictates. Add a layer of mulch around the tree but do not let mulch come in contact with the trunk. The best prevention is to avoid causing damage to the tree. Injuries allow the pathogens and insects access to the woody tissue. Avoid injury with lawn mowers and use care when transplanting.
In the case of crown rot, severe cases cannot be treated. However, small cankers can be removed with a knife, excising several inches around the damaged tissue as a precaution. Paint the open wood with an orange shellac and seal with wound paint. For fungus disease like Anthracose and powdery mildew, fungicides may be beneficial. Apply in the spring before bud break and then every seven to 10 days throughout spring. Additional applications may be needed in the summer if weather is particularly damp. For soil deficiencies, test your soil at home or have an extension office test it. Follow the advice to correct any soil deficiencies. Sooty mold can be washed off the leaves, but insecticides need to be used to control the insects or it will return.