Blister blight, caused by the fungus Exobasidium sp., has been sporadically reported [in British Columbia] ...

This disease manifests its symptoms on current-season leaves and developing berries. Leaf symptoms become obvious in late spring and early summer, in the form of large (one to several cm in diameter) convex blisters which are at first all one color, but later turn pale pinkish on the concave underside, finally browning. When several blisters occur on the same leaf, the leaf becomes twisted and distorted. Developing berries also turn prematurely reddish and swell to several times their natural size. The ... surface of the berries and the undersides of the leaf blisters [are] covered with a thin layer of fungal tissue bearing a palisade of basidia. The basidia in turn produce basidiospores that are forcibly ejected and airborne during moist weather conditions.

[monochrome photos page 4]

- Hunt, R.S., B. Callan and A. Funk: (1992) Common Pests of Arbutus in British Columbia^©