Mandevilla Plant Dropping Leaves. Some varieties die back to ground level at the first frost. Too much or too little water can be reasons for yellow mandevilla leaves.
Try mistingspraying the leaves with water in the mornings and keep the soil moist NOT wet. Check the soil around your plant to see if its too moist and of so allow it to dry some. In most cases the growth of fungi is a sign that conditions are too humid or moist.
Ive repotted them twice in new soil and larger containers over the years in case there were soil issues.
You may also want to consider moving your plant if possible to an area that doesnt receive direct sunlight for extended periods of time. Alice du pont and they have been growing wonderfully all summer outdoors in containers. Several days after bringing it in the leaves are turning spotted brown and dropping off starting at the base I sprayed the plant twice with Ivory soap and water before bringing it in and put insecticidal spikes in the soil. Too much or too little water can be reasons for yellow mandevilla leaves.