Todd,

No chance of sunburn, I assume. Ruling that out, it might be bacterial or
fungal; our FL hot and humid weather right now creates the perfect breeding
ground for both. I spray with Physan when this is a problem, and it works
like a charm for me. Others use Phyton 27, but I have too many Dendrobiums
to chance that (it contains copper, anathema to my Dends).

Diana