Blister canker is a disease caused by the fungus Biscogniauxia marginata (formerly known as Nummularia discrete). This fungus attacks mainly apple and crabapple but can also infect pear and mountain ash. Blister canker is a major apple disease east of the Rocky Mountains, especially in the Upper Mississippi and Lower Missouri River valleys, where millions of apple trees have been killed. Characteristics of this disease include old cankers up to 3 feet long on dead wood The cankers are mottled with living wood and dotted with large numbers of round cushions of fungal stromata that resemble nail-heads. These "nail-heads" gives this canker a blistered appearance. (Fig. 1) Fungal structures that are inside these stromata can spread to nearby plants and cause new infections. Cankers can be hard to tell apart from other canker symptoms at early stages. Also, in advance stages, dead areas are usually somewhat depressed, due to a shrinking of the bark. The bark becomes dry, brittle and irregular patches may fall, exposing the dead wood.
This fungus attacks plants in stress and starts infections in places where trees have been physically injured, usually by careless pruning. Wherever the mycelium penetrates the bark the tissue is killed and disintegrates. The spread of the fungus beneath the bark is not uniform and some healthy areas may remain un-infected, which explains the mottled appearance of young cankers. Since the fungus can spread from and live in dead wood, part of the management of blister canker includes removal of infected plant material. In addition it's important to shape trees early to prevent large pruning wounds on older trees.
Blister canker on mountain ash. Photo by Fanny Iriarte.
Blister canker. Note the characteristic round cushions of fungal stromata breaking open through dry/brittle bark. Photo by Fanny Iriarte.
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