Answer: Mandevilla likes heat and sun, it needs full direct sun all day and an evenly moist soil, meaning damp but not sopping wet or saturated. It needs a spot with good air circulation. It also needs regular fertilization during the summer with a water soluble fertilizer such as 10-10-10 and/or a slow release fertilizer. Read and follow the label instructions.
Based on your description, I am not sure if your plant might have a fungal or bacterial infection causing the browning, or if it is just in need of more light, better air circulation and more water. It may also have been exposed to too much cold during some of the cold nights recently. If you can adjust the growing conditions a bit, perhaps it will improve. If it continues to darken, you might want to consult with your retailer or with your county extension. Good luck with your plant!
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