A disease that plagues both nurseries and home growers of the camellia is popularly called dieback or canker. The technical name for the fungal disease is anthracnose. Varieties of anthracnose fungi strike numerous trees, shrubs and flowers. The anthracnose that strikes camellia is the fungus Glomerella cingulata.
The normally shiny, deep-green leaves of the camellia turn dull or yellow on diseased shoots. The foliage wilts and turns reddish brown. The dead, twisted leaves remain attached to dead shoots or branches. Lens-shaped cankers form around the base of dead shoots or on the edges of pruning wounds. The cankers enlarge, eventually encircling root collars and the base of the main limbs. The black fruiting bodies of the fungus, the size of a pinhead, appear in a circular pattern on the cankers. Diseased shoots gradually die in one or two years until the entire plant is dead.
Cycle of the Disease
The fungus G. cingulata infects camellias through pruning wounds, graft unions and leaf scars. It is sometimes introduced by diseased cuttings and plants grown in containers. The fungus survives the winter on cankers of diseased plants. After several days of wet spring weather, fruiting bodies on cankers ooze orange-pink masses of spores. Infected pruning tools and splashing water can spread the disease.
Although symptoms are sometimes noticed in a few weeks, shoot dieback does not become apparent for several months, often made worse by the stress of excessive summer heat.
Routinely inspect your camellias for symptoms of dieback. Remove discolored leaves. If there are cankers on the root collar or main trunk, discard the plant. Remove cankers with cuts made in green wood. Discard diseased rooted cuttings and their liners and containers. When pruning make flush cuts that will heal quickly. Clean pruning tools with isopropyl alcohol or other disinfectant.
Fungicides cleared to control anthracnose on camellias contain the active ingredients azoxystrobin or thiophanatemethyl. Check the labels of commercial fungicides for those ingredients. The effectiveness of these fungicides has not been proved. They are best used in combination with good plant care to prevent infections. They are unlikely to restore the health of camellias already suffering from extensive dieback. Apply in a heavy spray, thoroughly soaking the shoots, leaves and trunk. Apply in wet weather if you can.
The common camellia, Camellia japonica, is the least sensitive to dieback. Governor Moulton, Professor Sargent, Rose Emery and Woodville Red are camellias that are partly resistant to dieback.
The C. reticulata cultivars of camellia are highly susceptible to dieback. These include Butterfly Wings, Chrysanthemum Petal, Lila Naff, Mandalay Queen, Mouchang, Shot Silk, Chang's Temple, Pagoda, Professor Tsai, Moutancha, Purple Gown,Tali Queen, Crimson Robe, William Herrich, Cornelian, and Willow Wand. The hybrids of C. reticulata, including Budda, Confucius and Captain Rawes, are also susceptible to dieback and canker.