The flowering dogwood (Cornus florida) is a popular deciduous tree native to much of the eastern United States. Growing 15 to 25 feet high in most landscapes, this tree is prized for its attractive blooms in early spring, dark green leaves in the summer, and striking foliage that changes from red to a deep purple in the fall. Although dogwood trees are susceptible to disease, taking simple measures--such as choosing healthy nursery trees, planting in well-drained, lightly shaded areas, watering adequately and preventing damage from lawn equipment and pest--can help prevent most diseases from occurring.
Also known as lower branch dieback, dogwood anthracnose is caused by a fungus that begins as small purple circular spots that evolve into asymmetrical blotches on tree leaves. Spots and tan-colored blotches can grow and eradicate the leaves, eventually spreading to the tree limbs and trunk. Fungicides can be used to control symptoms, but once the infection occurs there is no cure.
Often confused with dogwood anthracnose, spot anthracnose is also caused by a fungus that thrives in cool, rainy weather. The fungus can distort flowers and discolor leaves much like dogwood anthracnose, though enough of the green leaves usually survive to keep the fungus from killing the tree. Fungicides such as propiconazol can be used in early spring to prevent reinfection of dogwood trees the following year.
Powdery mildew is caused by a surface fungus infection that causes small, dark-red blotches on the top of healthy-appearing green leaves. Unlike many dogwood diseases, powdery mildew typically appears in fairly dry, yet humid weather conditions. Excessive nitrogen fertilizer, heavy pruning and over-watering can all encourage the growth of this fungus. Propiconazol, which can also be used to control anthracnose, will help restrict powdery mildew growth.
Crown and Trunk Canker
Canker is often found in transplanted dogwood trees placed in poorly drained soil. Leaves grow small and less green than usual, and twigs and large branches affected by the fungus swivel and die. Canker can also be found around the base of the tree invading and discoloring the bark and sapwood. Diseased bark should be cut away and removed from the area completely once it appears. Once canker completely extends around the base of a tree, the dogwood usually dies.
Flower and Leaf Blight
Infecting flowers at the end of their blooming cycle, blight causes lopsided, wrinkled brown patches near the point where flowers develop. Once the infected flower parts fall on young green leaves, additional infection occurs. Thiophanate-methyl or iprodione sprays can be used during blooming to prevent the disease from occurring during periods of excessively wet weather.