Root rot and stem rot are major plumeria diseases. Soil mixture prepared should have very little water content in it. Stagnant water is the major cause of root decay and results in the leaves falling off. Reduce humidity and keep the place as dry as possible. If you find that your plumeria has already been affected by this disease then the first step is to pull the plant out of the soil, carefully. After that, remove the affected stem and clean this portion with fungicides. After cleaning, keep the plant in a shady region for two weeks and relocate it to a new place.
Another disease affecting plumeria is premature leaf fall. The primary symptom is leaf rust or the appearance of white powder on the nodes of the stem beneath the leaves.
Pathogens are small insects and pest. If they are found, then spray insecticides under the leaves at the earliest.