Rust is a group of fungi that attack many plants. Each fungi attacks a specific plant. This occurs from too little sun and too much water.
The best fix is to give the plant more sun and drier air. That’s not so easy outside and not during the occasional cold, wet spells we get in Houston.
The next option is to use a fungicide. You can find them at any place that sells plant supplies.
As long as the wet, cold spell does not last too long, the fungus should not hurt the plant, just discolor it.