Mandevilla leaf drop
My mandevilla leaves turned yellow and dropped off. This has happened slowly over two week period. Some leaves still remain. The one night that we did have a significant cold temp I did cover the plants.
Certified GKH Gardening Expert
Leaf yellowing is due to stress of some kind. This article should help you determine what the cause may be, likely the cooler temps: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/yellow-leaves-on-mandevilla.htm