Last year these rust spots appeared on one lower branch of this five year old tree. A fellow at the nursery were it was purchased suggested that I water more which I did. This year however the rust spots have appeared on almost all leaves on all branches. It seems systemic. In spring and early summer the tree appears healthy, but the spots develop mid season. Other plants growing in the same area (clumping bamboo, hostas and heuchera) do not develop these spots and are healthy. There are four other species of maples in the yard in close proximity that do not develop these spots. Is this some type of fungus? If so can it be dealt with, I am concerned that it may spread to the other trees.
This is what's known as Tar Spot of Acer (or Maple) and yes, it's a fungal infection. It is not related to dryness at the root, so your nursery guy either didn't know what it was, or preferred not to say. In the UK, there's no effective fungicide available for treating this. It will not kill the tree, just makes it look unsightly. You may reduce the effects by rigorously sweeping up or collecting fallen leaves, leaving none on the ground over winter, to reduce overwintering spores. More information here https://www.rhs.org.uk/advice/profile?PID=750