Could be bacterial (acidovorax), fungal (guignardia) or a virus.
First, isolate it from your other plants, then, short of testing, the only thing I would recommend would be treatment with a copper-based product, as that would be likely to treat the first two. If new growth comes out clear, you're good to go.
I have never used powdered copper, would not know how to apply. Personally, I'd just isolate the plant away from your other ones, and see what the new growth looks like. I strongly suspect that it is bacterial or fungal. Cyms get ugly leaves quite easily. One definitely can't identify virus by appearance. The only way is by testing.
I see. I didn't know that about Cyms actually. I only recently started growing them again and I guess I've been lucky with the few plants I got before this one.
Thanks for your help! I'll keep it isolated until the new growth appears and keep my fingers crossed.