You are using too many chemicals way too often.
Dendrobium nobile likes water and then dry out slightly. The pot you have selected is too large. Dendrobiums like to be under potted, tight at the roots. I think that there is an excellent chance that you are causing the problems you are seeing. When you purchase a new plant, it is stressed. From grower to you causes stress, you spray it with chemicals causing more stress. It is over potted or even repotted, more stress!
Just chill for a while, let the plant adapt to your conditions. But when Dendrobiums are stressed those little black spots develop.
Meanwhile, welcome to our Orchid Forums. Go back through some of our threads. I posted one last June or July on how BigBill repots Dendrobiums. This might be helpful.
Read through our other forums too, they are full of good information.
