If you tell the readers where you are in the world and where you have the plant, you will get better help.

No good spraying with fungicides etc. If your orchid growing conditions are not good.

I'm from Southern California in zone 10a. The plant is in front of a southern facing window getting bright indirect light. The rest of the plant looks fine, but that one leaf is the only one showing any yellowing.

It might be an old leaf which will dry up and fall off. It doesn't look like fungus to me.

Just leave it alone and if it dies, you can remove it. That's normal. Phals lose their old leaves.

Jane

That's what I was thinking, but I just wanted to make sure since the brown spot started small and has become larger and then the leaf started to yellow from the spot outward.

Could be an old sunburn spot, but if it bothers you, take the leaf off. Doesn't look fungal