Circular or oval, usually reddish, blister gall...Parallelodiplosis subtruncata (Felt)
...
P. subtruncata. This species is in every active blister gall I have seen, so is evidently the gall maker. The galls are about 4 mm in diameter and are usually on the leaf laminas but may also occur on the veins and petiole...Hosts: C. florida, C. racemosa, C. stolonifera...

[Elsewhere discusses Lasioptera corni Felt, which was reported once from the same galls - concludes that it was likely not the gall inducer]

- Raymond J. Gagne: (1989) The plant-feeding gall midges of North America^©