In early Spring, the rust produces bright orange aecia on the leaves (underside) and petioles of the alternate hosts, Lesser Celandine Ficaria verna and Meadow Buttercup Ranunculus acris; occasionally on other species of Ranunculus.  The rust is a gall-causer in the aecial stage.  

The primary hosts bearing the uredinia and telia are various grasses, including Cynosurus cristatus and Dactylis glomeratus. 

Uromyces rumicis produces aecia on Lesser Celandine.  The primary host are Docks (Rumex), bearing uredinia and telia.  The aeciospores are said to have 'refractive granules' in the walls, whereas U dactylidis aeciospores lack these granules.  

Puccinia magnusiana produces aecia on some species of Ranunculus (R repens, R flammula, R lingua, R bulbosus.)  The aeciospores are finely warted. 

host/microscopy

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.   Microscopic examination or confirmation by an expert before submission to NatureSpot may be needed.   If it is on Celandine and has not been examined microscopically, it should be recorded as the genus Uromyces.

Lesser Celandine, Meadow Buttercup and some Ranunculus species.

Spring and summer