A white mould that affects Primroses, causing brown leaf spots. Whitish tufts of conidia are present mainly on the leaf undersides.  The conidia are long and parallel-sided, and formed in chains.

Ramularia interstitialis also affects Primula species.  It causes pale green or yellowish leaf-spots and whitish caespituli; conidia are oval, formed singly

host/microscopy

Photograph the leaf spots and conidia on the host.  Microscopic examination of conidia is needed.   Ensure you have noted the host-plant species in your comments

on Primula species - including primrose, cowslips and garden 'polyanthas'