Dendrobium losing too many leaves

[tedpoe]

I have several dendrobiums divided from an original I bought years ago. Recently, two of them started losing all of their leaves. No conditions have changed. I water (and drain) weekly; they're happy in a south-facing window with a light-filtering shade to protect them from direct summer light. Looking through references, I can't identify the symptom. Slowly, over the last few months, their leaves have individually turned a dusty silver/white, then dried out and fallen. The stems are firm and the roots appear normal.

Attached is a photo showing the texture of the leaves, but the flash masks the green with the fine silvery color.

Any ideas from anyone would be appreciated.

Thanks,
Ted