Ok to cut off a phal leaf?

[greenpassion]

Hi all. I have a small collection of phals, (22) and one of these has a leaf that is off color, redish with marks along it's underside. I would take a pic but the camera on my phone keeps saying 'camera failed' each time I try. Anyway, I am going to treat with physan 20, but I also feel the leaf should be removed. I don't think it's insects, I suspect it's a fungus or some other disease. My question is can I just cut it off, or will this harm the plant. It's not a new leaf-it's one of the large ones close to the bark. Other than this leaf issue, the plant is very happy. Lots of new root growth and recently a new leaf. I grow these in a bark mix under led lights, on a heat mat. I feed MSU fertilizer for well water. Of the 10 on this shelf under lights, 7 are spiked and bloomed. I've just never had to cut a leaf off a phal before, and need some reassurance