Mystery Leaf Damage

[sadie]

I have a couple of phals which have some really odd leaf damage. I've noticed it just in the last couple of weeks--a month at most. Before that, they were green and thriving. My first thought was spider mites--but there's no tell-tale redness when I wipe my finger on the underside of the leaves. I'd guess lack of water, but the leaves are still rigid, not at all floppy. As you can see, the leaves are getting increasingly yellow but not in a way that makes me think they are getting burned.

I have already sprayed all my orchids this week with insecticide/miticide just in case, but I am worried that isn't the problem. It has me worried because the first plant with these symptoms has already lost a bottom leaf--yellowed and dropped off.

Any suggestions?