The Monterey Cypress (Cupressus macrocarpa) and Leyland Cypress (x Cupressocyparis leylandii) are particularly susceptible to Cypress Canker, although more than 25 other conifers are affected, including Castlewellan Gold, Leyton's Green, Naylor's Blue, Swane's Golden, Pencil Pines and the Arizona and Lawson Cypresses.
Cypress Canker is caused by several species of fungi (Seiridium cardinale, S. unicome and S. cupressi) whose spores enter the plants through natural fissures in the bark or through injuries caused by gardening equipment or falling branches. The fungus interferes with the sap-conducting system, eventually causing death of the branch or main trunk above the wound. Older trees are usually more susceptible but any tree is susceptible if stressed, for example, by drought or poor nutrition.
Branches die rapidly, yellowing almost overnight as the foliage is starved of sap. If untreated, the disease will eventually lead to death of the whole plant.
Sunken cankers, with a reddish tinge, form at the entry point of the fungus, and resin often exudes from the edges of the cankers or through cracks in the bark. Individual cankers are long and thin and may be numerous along a branch. Spore-producing structures of the fungus can be seen on the bark surface as small, circular, black dots.
Spores are carried on rain splashes or by insects, birds and other animals. New infections develop when spores are washed down the tree or splashed from tree to tree by rain or overhead irrigation. They can also be transferred from plant to plant on pruning tools, or through the transport of infected cuttings or plants.
Preventative measures such as regular feeding and watering and reducing the chances of wounding, are advisable. Any infected branches can be removed by judicious pruning to prevent infection spreading to the main stems. All tools should be disinfected before and after use with alcohol or dilute bleach.
Severely diseased plants should be removed and destroyed. No fungicides are effective in controlling the disease once infection has occurred.