Answer: Unfortunately, based on your description I am not able to diagnose the problem specifically. Mandevilla is generally pest and disease free, although I have found some reference to bacterial disease problems that can sometimes occur if the foliage is left wet overnight.
I would suggest you consult with your local Rutgers cooperative extension to obtain a definite identification of the problem and based on knowing that, decide what to do. If a chemical control is needed, they will have the most up to date information on what to use and how/when is best to use it.
In the meantime, avoid wetting the foliage when you water, and keep the plant in a very sunny location with good air circulation. Remove the affected foliage and dispose of it in the trash to limit reinfection. I'm sorry I can't be more specific for you.
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