Diplodia tumefaciens (Shear) Zalasky (Macrophorna tumefaciens) has been proven capable of producing galls on aspen (Kaufert 1937). Branch galls do not appear to be as numerous as the trunk rough bark (fig. 11A). The fungus gains entrance into the bark through lenticels but does not penetrate deeper than the outer layer of the cortex, because it apparently stimulates the formation of a protective periderm. As the fungus invades the layer of periderm, a new phellogen develops and a new layer of periderm forms. This fungus invasion and formation of a protective barrier against the pathogen continues for years, resulting in the formation of rough bark and branch galls. Zalasky (1964) reported fungus penetration of intact or broken cuticle and epidermis of aspen and black poplar (Populus nigra), and its occurrence on other poplars.

Diplodia tumefaciens
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These large, dark brown to black eruptions of outer bark tissue can be 35 cm in diameter and occur one after the other in a vertical row up the trunk.