Hello everyone,

My Begonia 'Gene Daniels' has been getting some strange spots on its leaves that I haven't been able to diagnose or treat.

They manifest as lighter grainier areas on the leaf that gradually spread out from the main veins. Eventually they spread to the whole leaf.

Younger leaves are initially unaffected, but succumb with time

Weirdly the external texture of the leaf doesn't seem affected at all. Asides from their color change, the tissue in the patches is not sunken in, dry, waterlogged, covered with any sort of residue, etc. It appears superficially just as healthy as unaffected tissue, but discolored. I've checked for any mites or other pests and didn't find any (with my naked eye at least) and the symptoms don't seem to match the round spots caused by mildew and other fungal issues.

Dos anyone know what this is and how to treat it? I really don't want to cull this plant but it is extremely unsightly currently.