The fungus Entyloma microsporum galls the leaves and petioles of Ranunculus acris (Meadow Buttercup) and Ranunculus repens (Creeping Buttercup) causing hard yellowish pustule like swellings.  The pustules do not contain dark sooty spores.  The spores are embedded in the host tissue.  Powdery white conidiophores bearing the asexual conidia may be seen emerging from the host-plant's stomata.

Urocystis ranunculi may have similar pustules in its early stages, but has dark spores.  Even when immature these can be seen inside the unopened blisters when held up to the light.

host

Provide photos of gall and host plant; photograph underside and upperside of leaves.  Ensure you have identified and noted the host-species

Buttercups