The fungus Taphrina populina causes galls to form on the leaves of Black Poplars and its hybrids. The galls are raised bulges on the upper leaf surface, concave below and lined with bright orange/yellow erect asci.

The galls of the mite Phyllocoptes populi look very similar on the leaf upperside, but are on Aspen and have an erineum underneath.

host

Photograph the gall on the host plant, both upper and underside of leaf.   Ensure you have identified and noted the species of host

Black Poplar and its hybrids.