I appreciate all the suggestions. No scale -- I have looked very
carefully, I do know what they look like & have had them on other
plants but not near this one, & they don't make raised black warty
spots like that. Also there are 2 other phals on the same windowsill
getting the same care, & they are quite happy, & all of them have been
for years. So I'm not sure about environmental factors either. I was
wondering if it could be some sort of infection from an injury to a
leaf.
I'll keep an eye out for the book you mentioned.

Alison