Help with dendrobium.

[NYCorchidman]

Nobiles are pretty much disease free given the right culture. Strong light and lots of water is all they need.
I wouldn't spray healthy plant for preventative measure as doing so is one, not necessary, two can weaken the plant.

Your second question is for a while new thread. lol

Spraying sick plant takes at least a few weeks. You want to spray at recommended frequency, and then you want to make sure sick plants are healthy again.