These are Aspen gall. Diplodia Tumefaciens produces galls when infecting aspen branches and rough bark when infecting large stems. The gall does not kill trees, but heavy infection does make trees unsightly.

Spores likely gain entrance through wounds or natural openings such as lenticels. In the case of Diplodia tumefaciens, penetration is no deeper than the outer layer of the cortex, because it apparently stimulates the formation of a protective periderm. 

The sequence of fungal invasion and formation of a protective barrier against the pathogen continues for years, resulting in the formation of rough bark and galls.

There is no known controls for these type of disease. Wound avoidance, can include maintaining high tree vigor and removal of heavily infected trees are the control guidelines.

Best,

Marvin