When the whole leaf is affected, the infection may then proceed through the petiole of the leaf into the stem causing stem cankers. Infection of the stems and shoots may also occur directly. Cankers are generally tan on healthy gray background and produce the same reproductive structures on their surface as are found on the undersides of leaves. Over time, infection of twigs and shoots by these cankers kills branches, usually beginning with those low on the tree and moving upward (Fig. 2). The tree may attempt to compensate by sending out sprouts from the trunk, but the fungus easily infects them. Infection of the sprouts usually spreads quickly to the trunk and causes severe cankers with split or buckled bark. Affected trees may die within 1-3 years; saplings may die in the same year they are infected.
Reproductive structures of D. destructiva form underneath leaf spots and on the surface of twig cankers. Huge amounts of asexual spores are formed inside and, in the spring, ooze out in slimy beige clusters. Local dispersal of the spores occurs by splashing rain while long distance dispersal may also occur via insects and birds. Transportation of diseased stock into new areas spreads the disease as well, especially into areas where dogwoods are not native. Spores of the fungus land on shoots or leaves penetrating them directly causing the quick death of the plant tissue due to the production of several toxins by the fungus.
A combination of several practices is recommended for managing this disease. Healthy trees are much more able to cope with disease than stressed trees. Keep trees stress free by applying 3-4 inches of mulch around the base (but keep the mulch off the trunk), watering during dry periods, and fertilizing moderately. Since shady moist conditions favor the development of this disease, avoid overhead irrigation and plant trees in sunny locations when possible. It is also advisable to avoid fertilizers with high nitrogen content as this promotes the rapid growth of succulent shoots that are extremely susceptible to disease. Avoid mechanical damage to any part of the tree especially by mowers or other equipment. Keep the inoculum levels low by raking and removing leaves in the fall, pruning diseased branches, and pulling adherent dead leaves from the tree. Prune any water sprouts that grow from the trunk. If possible, prune during dry, hot weather and disinfect the pruners blades in a solution of 70 percent rubbing alcohol between cuts.
Fungicides can be applied in the spring, starting at bud break and continuing every 10-14 days until the leaves are fully expanded. Further sprays may be necessary if the summer is unusually wet. Homeowners or professional applicators in New York State should use a fungicide containing one of the following active ingredients: chlorothalonil, myclobutanil, or propiconazole. Propiconazole is a systemic fungicide and may require less frequent applications. Other pesticides may also be available for commercial or professional use. Please refer to the appropriate pest management guidelines, or contact your local Cooperative Extension Office for more information on currently registered products, and be certain any formulation(s) of pesticide(s) you purchase are registered for the intended use. Consult the labels for application information and directions. An application in the fall after the leaves have begun to turn but before they have dropped may also be performed.
To avoid or lower the risk of dealing with this disease, consider using resistant varieties. The white flowering Kousa Dogwoods, Cornus kousa have shown good resistance and require less input to maintain a healthy tree. A number of crosses between C. kousa and C. florida have been made in attempts to produce the flowering characteristics of the Flowering Dogwood with the resistance of the Kousa Dogwood. These cultivars are available on the market and are known as the 'Stellar' Hybrid series, 'Aurora', 'Celestial', 'Constellation', Ruth Ellen', 'Stardust', and 'Stellar Pink'. A resistant Flowering Dogwood cultivar named 'Appalachian Spring' has also been developed from a living tree in an otherwise devastated Maryland forest and may soon be available.
Source: Cornell University