The flowering dogwood (Cornus florida) is the species most people think of when a dogwood is mentioned. Dogwoods prefer moist, well-drained, acidic (5.5 to 6.0 pH) soil that is high in organic matter, and prefers partial shade. There are three main foliar diseases that affect dogwoods during this time of year.
Powdery mildew is a fungus that appears on the leaves and tender shoots. New growth becomes covered in a white coating, usually on the upper surface of the leaves. As the disease progresses, the leaves look scorched along the edges or have dead patches. They also turn yellow and drop off. The disease is common in a dense, shady area with poor air circulation.
Usually, powdery mildew appears late in the year and generally doesn’t pose a big threat to the plant’s health. However, if this disease is present in the spring, it should be treated with a fungicide. Effective fungicides include myclobutanil and propiconazole. Some control can be obtained with thiophanate methyl. Product labels will provide information on how often to spray.
A preventative as well as treatment measure is removing dead or infected plant material from the tree. This practice also improves air circulation, which discourages fungal growth.
Spot anthracnose is one of the more common leaf diseases of flowering dogwoods. Flower bracts are usually attacked first, and then the leaves primarily during wet spring weather. Symptoms are small tan spots with reddish-purple borders. When the infection is severe, spots can cause flower bracts and leaves to become wrinkled and distorted.
This fungus survives from year to year on infected twigs, fruits and other tissues. Frequent rains or extended periods of high humidity promote disease development. In most cases the disease does not result in significant damage, but severe and repeat infections each year can significantly weaken a tree.
To halt the damage, thin the canopy to increase air movement. If spotting becomes severe, fungicides can be used in the spring starting at bud break and continuing until leaves are fully expanded.
Dogwood anthracnose is a relatively new disease. It is most severe only in areas of the state above 2,000 feet. A few cases have been reported at lower elevations where dogwoods are grown in very cool, moist and shady locations. It is a serious disease capable of killing several trees, and most dogwood species can become infected. The first symptoms appear in the spring with spots on the leaves and flower bracts. Infected leaves have tan spots with purple edges, dry brown margins or large blotches throughout the canopy.
Blighted gray or drooping leaves hang on the twigs and are often the first symptoms noticed during cool, wet weather. Infection spreads into the shoots, main branches and trunk, causing brown sunken areas (cankers) to occur. Cankers can girdle and kill individual branches or twigs. Multiple cankers can girdle the main trunk and eventually kill the tree.
A combination of cultural and chemical measures is necessary to control the disease, including planting resistant species and cultivars. Effective control may be possible if the disease is detected before branch dieback begins. During hot, dry summer weather, prune and dispose of dead or cankered twigs and limbs. Remove all water sprouts. Rake and remove fallen leaves. Improve air circulation and light penetration by removing understory plants and crowding vegetation. Avoid high applications of nitrogen fertilizer, as this can promote very succulent (susceptible) new shoots.
Fungicide sprays to protect new leaves and shoots need to be used in early spring. Fungicides for spot anthracnose will help to control dogwood anthracnose.
Raking and pruning to improve air flow will discourage fungal growth. There are also species and cultivars that are resistant to some fungal diseases.