Potty, from the yellow shading on the top of the leaf, I'm wondering if the underside shows fungal rings too? Turn it over and inspect, and if it does, I'd cut off the whole leaf. If the leaf only has that lesion on top, then I'd cut a half-circle out of the side of the leaf. I have a phal with the same infection, but on too many leaves to cut it out. I dusted it with cinnamon. It looks bad, but it has since put out 2 flower spikes, so we'll see...