One problem showing up in landscapes is a canker disease causing wilt and dieback on rhododendron, caused by the fungus Botryosphaeria dothidea.
The first indication of a problem usually is the appearance of individual stems with drooping, wilted-looking leaves that are rolled inward. The wilt symptom may spread until the whole plant is affected. Closer examination of affected stems reveals a reddish-brown to black sunken canker that girdles the stem. Cankers develop slowly and may appear a month or two after infection.
This disease is more likely to occur on plants suffering from heat and drought stress or winter injury. Thus, recent hot, dry weather may have stressed Rhododendrons creating conditions favorable for canker-causing fungi such as Botryosphaeria.
An integrated approach to control most canker diseases, including Botryosphaeria canker, begins with the selection of disease-free planting material. Be sure to choose top quality material from a reputable dealer so that the disease is not moved into the landscape. Always inspect plant material thoroughly before planting. Most healthy, vigorous plants are resistant to Botryosphaeria canker. Environmental stress, however, can readily predispose plants to attack. Healthy plants can resist infection and will slow or prevent spread of the disease throughout the branch. When planting new trees and shrubs, choose a site that is suitable to the horticultural requirements of the species. For example, planting sun-loving plants in shady locations or placing plants outside their natural range can predispose these plants to canker disease. With older, established trees, maintain or improve plant vigor with proper pruning, fertilization, and irrigation. Since drought stress predisposes trees to canker development, watering trees during times of drought is particularly important.
Since B. dothidea infects stressed plants through existing openings, it is important to protect plants by carefully avoiding all unnecessary wounding. Closely monitor and control insects, mites, and disease problems. Through careful monitoring and early detection, Botryosphaeria canker can sometimes be eradicated before a significant reduction in the aesthetic value of the plant occurs.
Branches with symptoms of canker should be promptly pruned during dry weather at least 6 to 8 inches below affected tissue. If possible, remove the branch by properly cutting the limb at the branch collar, not flush to the trunk. To prevent the spread of this disease on pruning tools, surface sterilize tools between cuts with 70 percent denatured alcohol, Lysol, or 10 percent bleach. Since the fungus can persist in dead plant material, branches cut from diseased trees should be taken from the site. Fungicides or wound paints have not proven to be an effective control of most canker diseases and are not recommended.
Botryosphaeria canker is a threat to more than just rhododenron. A few of the many common susceptible landscape plants include apple, birch, blueberry, brambles, crabapple, dogwood, holly, juniper, magnolia, maple, oak, redbud, rose, tulip tree, viburnum.
Kelly R. Jackson is the Christian County Extension Agent for horticulture. He can be reached at 270-886-6328 or visit Christian County Horticulture online at www.christiancountyextension.com.