Answer: Based on your description it is possible that there is a mealybug problem or possibly a scale problem. You can try to treat these with commercially formulated insecticidal soap or with a horticultural oil spray. Be sure to carefully read and follow all of the label directions and treat all surfaces of the plant including the undersides of the leaves.
Mandevilla does best in full sun and in a location with good air circulation.
Mandevilla needs an evenly moist soil when it is actively growing. It should be kept moist like a wrung out sponge, not sopping wet and not dried out.
It also needs a rich soil to maintain its vigorous summer growth. You could top dress the soil in the container with good quality compost, then use a slow release granular fertilizer per the label directions.
If you find that the soil is drying out very quickly, such as when the plant has outgrown its container, you could repot into a larger pot. Use a good quality soil mix formulated for container plants. Make sure there is a working drainage hole in the bottom of the container to allow excess water to escape.
Finally, remove any discolored foliage and dispose of it in the trash to try to limit reinfection.
You may also want to consult with your local county extension to obtain a more specific diagnosis 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 for maximum results.
I hope this helps!
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