This canker disease is caused by the fungus Seiridium unicorne . A canker is a definite, localized, usually dry, dead, often discolored, sunken or cracked area surrounded by living tissue.
Cankers form on stems, branches and in branch axils causing dieback of shoots.
Twig dieback has also been observed with a gray discoloration at the point of infection. Diagnostic features include resin oozing from cracks in the bark, dark brown to purplish patches on the bark, multiple sunken cankers with raised margins, and yellow to brown discoloration of foliage above the canker. Spore-producing structures of the fungus appear on the bark surface as small circular black dots. New infections develop when spores lodge in bark cracks and wounds. Therefore, diseased plant parts should be destroyed.