dendrobium yellowing leaves with spot

[wilson79]

I bought this orchid before it began blooming from a plants sale at the conservatory at my university. I didnt have any problems with it until the flowers faded and the spike died back (after flowers bloomed for over a month). It is potted loosely in miracle grow orchid mix (all I have been able to find so far in my area) and sits atop an improvised humidity tray. I lost a leaf today, it was yellowing and developed a dark spot and fell off when I touched it. I am new to growing orchids, but I am in love with them. Please help me save this guy!