My mandevilla plant leaves are rapidly turning yellow and falling off.
Mandevilla Plant Leaves Are Rapidly Turning Yellow and Falling Off
Certified GKH Gardening Expert
There are a number of reasons why the leaves may turn yellow. One might be overwatering. Too much water causes yellowing of the leaves and mandevilla do not like overly wet feet. Another possibility could be too little water. If this is the case, the yellow leaves would be curling up as well. Mandevilla favor humid conditions. If your plant is exposed to the sun, the soil may be too hot. Try misting/spraying the leaves with water in the mornings and keep the soil moist (NOT wet). Mealy bugs can occasionally affect these plants, causing them to become weak and leading to yellow foliage. Finally, plant stress due to a change in location, temp., etc. can lead to yellow leaves.
Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm