Scary looking lesions on Dendrobiums?

[Orchidtinkerer]

Hi Pynn,
I thought of temps initially too but the substrate looks suspiciously bad on your picture.
To me most of my problems get fixed by fixing the rootzone first. It really can't hurt at this stage.
There are probably mould spored growing down there, it looks rotted so worth airing and throwing all that old stuff out, cut away any affected leaves with sterilized scissors and give it gentle ventilation to prevent any more growths forming as easily

Ps: It is a den phal so temps should be 20 C minimum, it needs to be kept warm