I noticed this black splotch over a week ago, and several other splotches scattered all over the other leaves. The black spot is sunken in, and feels paper-thin to the touch.

This picture is a bit "outdated"--right now, the middle leaf in the picture is almost 2/3rds yellow. The yellowing has extended all the way to the base of the leaf.

Here, you can see the slight yellowing and the developing brown spots. Oh btw, there are no insects. The little white pieces are dead leaves from my asparagus fern.

I've never had this happen. This is, in fact, the healthiest phal that I raised from a batch of seedlings a couple years ago. It currently has a spike with 30 buds and another spike developing, along with tons of good roots. I was very surprised when I saw the leaf condition. One guess is sunburn. It does receive some direct morning sun--I know, against regular advice--but it's been receiving this kind of sunlight for years. Another guess is fungus or bacterial rot...I'm pretty stumped.

What do you think is the problem? Thanks!