What does the back side of the leaf look like. If it what they call "rust" it is actually a fungus and the back side should have spores on it.
Do you mist your plant?
What is your Humidity? If you do not know the humidity in the room, what is the humidity where you live?
Thanks for the reply!
The back of the leaves looks fine. No spots or discoloration at all. They look perfectly normal.
I generally do not mist the plant - the only exception being to foliar feed with the calcium solution I described above.
Humidity is generally between 20% and 35% - never ever more than 35%
If it were a fungus, shouldn't I be able to rub some of it off?