Eriophyes ulmi
The galls are intermediate in form between those of the hard and soft maples [ie, Vasates aceriscrumena and Vasates quadripedes] and somewhat balloon-shaped. The interior wall is ridged vertically. The galls usually occur on the upper side of the leaves. Occurs on soft elm (Ulmus americana) and Rock Elm (Ulmus racemosa [thomasii]). Very common.
”- TD Jarvis: (1906) Insect Galls of Ontario©
Reference: https://www.biodiversitylibrary.org/page/40931581#page/311/mode/1up