Fungal diseases can mar the foliage and bracts of Virginia's state flower, the native flowering dogwood, Cornus florida. The diseases, spot anthracnose and Septoria leaf spot, appear every year to some degree and in most years cause little damage. Discula anthracnose, on the other hand, can eventually kill a tree. Another relatively recently observed disease of dogwood, powdery mildew, can also be very destructive to the overall health of the tree. This publication attempts to distinguish the symptoms of these four major diseases of dogwood.
Symptoms of spot anthracnose, caused by the fungus Elsinoe corni, first appear very early in the spring on the bracts, and later on the foliage. In general, white cultivars of dogwood are more susceptible than pink cultivars. The fungus causes uniformly tiny (less than 1/8" diameter), circular lesions with purple borders and lighter, almost white, centers on bracts and leaves (Figs. 1 and 2). These lesions are easily distinguished from leaf spots caused by other dogwood pathogens because of their small and uniform size. Later in the season, the centers of the spots often fall out, giving the leaf a shotholed appearance. In seasons when environmental conditions are conducive to disease, spots on bracts and foliage may be numerous, and leaves or bracts become puckered or distorted around the spots as the leaves expand. Spots similar to the leaf spots may also form on green branches and fruit; however, the fungus does not cause dieback. In cases of severe infection, buds may fail to open.
In most years control is not necessary; however, if disease was severe the previous year or if a cool, wet spring is predicted, fungicides may be warranted. Spot anthracnose can be controlled preventatively with chlorothalonil (e.g. Daconil 2787), mancozeb (e.g. Fore or Dithane T/O), or thiophanate methyl + mancozeb (e.g. Zyban). Spraying should begin when buds begin to open and be repeated when bracts have fallen, four weeks after bract fall, and in late summer after flower buds have formed.
Avoid planting dogwoods in sites where leaves will remain wet for long periods of time. Sites along streams, lakes, or ponds, or areas where fog tends to collect are prime sites for disease development and should be avoided. The disease is more of a problem in shaded locations; thus, planting in sunny locations can help to prevent disease. Although dogwood is naturally an understory tree, trees that are properly cared for can do well in full sun.
Trees should be mulched to a depth of 2-4 inches to help conserve soil moisture. Place mulch in a donut-shaped ring around the tree instead of piling it against the base of the trunk. Placing mulch in contact with the trunk causes bark to remain moist and become more susceptible to decay. Water trees during drought and fertilize as needed to help trees remain vigorous. Avoid using fertilizers high in nitrogen, however, as they can stimulate succulent growth that is more susceptible to infection by Discula. Water trees with a soaker hose rather than a sprinkler to avoid prolonged leaf wetness.
Remove cankered branches and destroy them as soon as they are noticed. Make cuts well below cankers through healthy wood. Sterilize pruning tools between cuts with rubbing alcohol or a 10% solution of household bleach. Remove any plant debris that falls to the ground from infected trees. Also, avoid collecting and planting seed from wild dogwoods or transplanting dogwood seedlings from the woods because these plant materials may harbor the fungus.
Few cultivars of flowering dogwood have resistance to Discula anthracnose. Breeding programs to identify sources of resistance are ongoing. The 'Spring Grove' and 'Cherokee Sunset' cultivars of flowering dogwood have been reported to have resistance to Discula anthracnose; however, 'Cherokee Sunset' is very susceptible to powdery mildew. Chinese dogwood (Cornus kousa), also called Kousa dogwood, although not immune, has resistance to Discula anthracnose. Many cultivars of Kousa dogwood have high levels of resistance to Discula anthracnose (Table 1). Other cultivars with lower levels of resistance may develop leaf spots but do not develop cankers. Kousa cultivars with high levels of resistance are the best choice for mixed plantings of Kousa and flowering dogwood. If cultivars with less resistance are planted, they could develop leaf spots and serve as a source of fungal inoculum for nearby flowering dogwoods. Contact your nursery owner for the latest information on dogwoods with resistance to Discula anthracnose.
Dogwood cultivars with resistance to Discula anthracnose
(data from N.C. State University, 1996)
|Milky Way Select||Excellent|
|**Cornus florida x C. kousa hybrid.|
Septoria leaf spot, caused by the fungus Septoria cornicola, is a late-season disease of dogwood that is little cause for concern. Angular, dark brown leaf spots with purplish margins, bordered by leaf veins, are typical of this disease (Fig. 7). Leaf spots first appear in early July in Virginia, but leaf spotting does not become severe until later in the summer. Symptoms caused by Septoria could be confused with those of Discula anthracnose; however, Septoria leaf spots are much more uniform in size (up to about 1/4 inch in diameter) and angular. Also, Septoria does not cause a general blighting of the leaf.
Because the most severe symptoms of Septoria leaf spot occur late in the season when leaves are beginning to senesce naturally, no control is recommended. Removing fallen leaves may help reduce the amount of fungal inoculum available for infection the following season.
Powdery mildews comprise a group of related fungal species within the same family. Powdery mildew fungi are common on a wide variety of ornamental species. Symptoms and signs of powdery mildews on different plant species resemble one another; however, most species of powdery mildew fungi infect only one or a few closely related species of host plants. Although powdery mildews are common on other plants, the disease was not common on dogwood in Virginia until 1993. It is possible that the species that is now prevalent on dogwoods (Microsphaeria pulchra) was introduced just prior to that time.
Mycelium of the powdery mildew fungus grows on leaf and bud surfaces, and appears as a white coating on these plant parts (Fig. 8). In the early stages of infection, the white fungal growth may be subtle and difficult to see; however, symptoms of infection can be severe. Leaves may be stunted, reddened, and curled or puckered by mid-season. Affected trees appear water-stressed even if adequate water has been provided. The disease has been shown to cause stunting of roots and, after repeated years of infection, the tree may be stunted overall. Both flowering and Kousa dogwood are susceptible to this disease, although resistant cultivars of both species have been identified.
Resistant cultivars of both flowering and Kousa dogwood are available. Because this disease has become so prevalent in recent years, resistant cultivars are a wise choice for new plantings. Refer to, Powdery Mildew-Resistant Woody Ornamentals (VCE Publication 450-616) for a list of dogwood cultivars resistant to powdery mildew.
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009