Sooty mold is actually a symptom of an aphid, whitefly or thrip infestation. These tiny insects feed on plumeria leaf and stem juices and secrete a sticky, sugary liquid called honeydew. Clusters of the insects may be visible on the undersides of infested leaves. Black sooty mold grows in the honeydew on plumeria leaves and stems. The mold can interfere with photosynthesis as it coats the leaves, and can cause stunted growth and reduced plant vigor.
Control
Plumeria trees in sunny, well-ventilated locations are less susceptible to mold infections. Fungicides, including mycobutanil, control plumeria rust, according to the University of Hawaii at Manoa Cooperative Extension. Dispose of fallen rust-infected leaves, and spray the ground under the tree in the winter. Apply copper fungicide, neem oil or horticultural oils during early stages of powdery mildew infection, and remove any diseased leaves and stems. To prevent sooty mold, inspect stems and the undersides of leaves for insects, and remove the insects by hand or with insecticidal soap or a forceful stream of water. Carefully follow pesticide label directions and precautions.