Mottled Dahlia Leaf?

Wonder what's caused this yellowing on this dahlia leaf? Haven't seen it before.

MAGNESIUM DEFICIENCY?
Yellowing between the leaf veins is often a sign of magnesium deficiency, so could be this in an early stage. Doesn't seem likely, but given a dose of trace elements just in case.

VIRUS?
Mottling is often associated with virus disease. But this is also unlikely, as I haven't seen it before, and haven't brought any new plants in. But could have been there all along and is now showing up because of recent stress (plants have never been watered, apart from rainwater)

THRIPS?
Seems strange that it is only now noticeable after much rain and thundery hot weather. Might be thrips (thunder flies) damage, though haven't seen any. Another mystery. Will have to wait see what develops.