e a few of the infected leaves in a plastic baggy to the nursery-not a big box store.
Ask them to confirm hyour suspicion that it is rust.
If it is rust (a fungus) get out your pruning shears and lopers. You need to cut off the infected branches at least 6" back from any sign of infestation. Clean up all leaves and tree debris so it is spotless beneath the tree-fungus spreads by spores. The spores can be shifted from one branch to another by rain, overhead watering, or even the wind.
When the tree goes dormant for the year make sure to clean up carefully and thoroughly.
In the spring when the leaves are beginning to pop spray with a fungicide recommended for ornamental trees-possibly Orthenex.
Continue to keep the area adjacent to the tree clean of all litter.
It's too late in the year to spray for rust now-it won't do any good. Concentrate of hygiene.