|Spot anthracnose on dogwood (Cornus florida) petals. |
Joey Williamson, ©2009 HGIC, Clemson Extension.
Clemson University Cooperative Extension
The flowering dogwood (Cornus florida) is a small, deciduous ornamental tree that is native throughout the eastern United States. Although dogwoods are well adapted to South Carolina, they can be affected by many pests and diseases. Maintaining healthy dogwood trees by following the recommended cultural practices is the first line of defense in reducing any of these problems. More information about growing dogwoods is available in HGIC 1010, Dogwood.
Powdery Mildew: Erysiphe pulchra (formerly Microsphaera pulchra) is the fungus that attacks leaf surfaces and tender shoots and causes powdery mildew. New growth is covered with a fine, white, powdery coating, typically on the upper surfaces of the leaves. Infected leaves exhibit marginal leaf scorch, dead patches, reddish discoloration, yellowing and premature defoliation. Spores are spread by wind to surrounding dogwood plants. Powdery mildew is most common in dense, shady areas where the air circulation is poor. Warm, dry days and cool, damp nights favor disease development.
Prevention & Treatment: Most powdery mildews of landscape trees occur late in the summer and are therefore of little consequence. Infection that begins early in the season can be devastating, and the use of fungicides may be warranted. Cultural controls should be the first line of defense. Begin by raking up and destroying all fallen leaves. Prune out dead and infected branches and twigs. Improve air circulation and sunlight penetration around the tree by removing overhanging branches and crowding vegetation.
Resistant species and cultivars are available and should be considered for new plantings. Cultivars of the oriental dogwood Cornus kousa (such as 'Milky Way', 'Milky Way Select', and 'National') and many of the Cornus florida x Cornus kousa hybrids (such as 'Aurora', 'Constellation', 'Celestial', and 'Stellar Pink') are generally resistant to powdery mildew. The flowering dogwood (Cornus florida) cultivars 'Appalachian Joy', 'Appalachian Blush ', 'Appalachian Snow' and 'Appalachian Mist' are very resistant to powdery mildew. 'Cherokee Brave', 'Springtime', and 'Pygmy' have partial resistance. All other flowering dogwoods (C. florida) are susceptible. Red-twig dogwood (C. sericea) is very susceptible to powdery mildew.
If disease is severe enough to warrant the use of fungicides, be sure that the dogwood is a valuable specimen and the spray equipment can provide good coverage. For fungicides to be effective, they must be applied as soon as symptoms are noticed. Very effective fungicides for dogwood powdery mildew control include myclobutanil and propiconazole. Some control can also be obtained with triadimefon, thiophanate methyl, sulfur, or copper fungicides. Product labels will provide information on how often to spray. The first four fungicides listed have systemic properties and can be sprayed less often than sulfur or copper fungicides. When powdery mildew persists and sprays are repeated, it is recommended to alternate fungicides to decrease the chance of fungi developing resistance.
Spot Anthracnose: This disease is caused by the fungus Elsinoe corni, one of the most common leaf diseases of flowering dogwoods. The flower bracts are usually attacked first and then the leaves, young shoots and fruit of dogwoods, primarily during wet spring weather. Symptoms are small (⅛ inch), tan spots with reddish-purple borders. When infection is severe, these spots can cause flower bracts and leaves to become wrinkled and distorted. As further infections occur, individual spots eventually merge to form larger spots. The centers may drop out. This fungus survives from year to year on infected twigs, fruits and other tissues. Frequent rains or extended periods of high humidity are needed for disease development. When dry weather follows bud swell and bloom, the symptoms are rarely seen on the flower bracts. If spotting does not appear on the bracts, the disease may not be severe on the leaves.
Prevention & Treatment: In most cases this disease doesn't result in significant damage, but severe and repeat infections each year can significantly weaken a tree. Thin the canopy to increase air movement. Planting species and cultivars with some degree of disease resistance is an excellent option for managing this problem in the landscape. The disease-tolerant and resistant varieties include:
· Kousa dogwood (Cornus kousa) 'National', 'Milky Way Select'
· Flowering dogwood (C. florida) 'Cherokee Brave', 'Cherokee Chief', 'Welch's Bay Beauty', 'Cherokee Princess' and 'Springtime'
· Rutger's Hybrid - 'Stellar Pink'
The worst spot anthracnose has been reported on Cornus florida 'Rainbow' and 'Cherokee Daybreak '.
If spotting becomes severe, fungicides can be used in the spring starting at bud break and continuing until leaves are fully expanded. Fungicides available for use include chlorothalonil, mancozeb, propiconazole, thiophanate-methyl or copper fungicides. Fungicides for spot anthracnose will also control dogwood anthracnose (canker anthracnose). Apply all chemicals according to directions on the label.
Dogwood Anthracnose (Discula Anthracnose): This is a relatively new disease of dogwood in South Carolina, and it is caused by the fungus Discula destructiva. Dogwood anthracnose is most severe only in areas of the state that are higher than 2000 feet. A few cases have been reported at lower elevations where dogwoods are grown in very cool, moist, shady locations. It is a serious disease capable of killing large numbers of trees and most Cornus species can become infected. The first symptoms that appear in the spring are spots on the leaves and flower bracts. Leaf symptoms develop first in the lower canopy and progress up the tree. Infected leaves have tan spots with purple edges, dry brown margins or large blotches on them. 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. Diseased trees produce numerous epicormic shoots or "water sprouts" on the lower trunk and lower limbs, which soon become infected.
- During hot, dry summer weather, prune and dispose of all dead or cankered twigs and limbs. Remove all "water sprouts." Rake and remove fallen leaves. Do not leave dead leaves attached to the tree. Improve air circulation and light penetration by removing understory plants and crowding vegetation.
- Avoid high applications of nitrogen fertilizer, since this can promote very succulent (susceptible) new shoots. Maintain healthy dogwoods by following recommended cultural practices.
- Avoid transplanting dogwood seedlings from the woods as these plants may harbor the fungus.
- Fungicide sprays to protect the new leaves and shoots need to begin at bud break in early spring. Fungicides for spot anthracnose will also help to control dogwood anthracnose. These include: chlorothalonil, mancozeb, propiconazole, thiophanate-methyl or copper fungicides (see Table 1 for specific products). Maintain a protective covering of fungicide when new growth is present. Apply all chemicals according to directions on the label.
Although insects often damage dogwood trees, the damage is usually minor. If the tree is planted in full sun with limited water or under other stress, the damage can be serious. Most insect damage occurs on the trunk and branches of dogwoods. Commonly occurring insect pests of dogwood include the dogwood borer, dogwood club-gall midge and scales.
HGIC 2003; Printer Friendly Version (PDF)