Dear LL,

Thank you very much for your inquiry. Read the response below from my Garden Nurse, Susan Gruber.

Mandevilla leaves may turn brown and fall off for several reasons including overwatering, watering with chlorinated water, insect infestations and leaf diseases. If the vine seems otherwise healty and is blooming well, I doubt lack of fertility is the problem.
First, be sure the plants potted in a well-drained container with good quality planting mix. They should be kept moist but not wet. Water with chlorine-free water with a small amount of water-soluble liquid fertilizer sold for houseplants (any brand is fine).
Carefully inspect the stems for very small insects such as mealybugs, scale and mites. Some of these may be very tiny and will not move. Leaf diseases usually begin as spots or blotches which get larger over time. Taking a few leaves along to your favorite nursery or is a good way to get help identifying the problem and finding the right product to control the pest if one is present.

Again, thanks for the question and let me know if you have more.

Respectfully

Trey Rogers, The Briggs & Stratton Yard Doctor, YardDoctor.com