Black or sooty mold on Plumeria leaves is often a secondary symptom of aphid or scale insect infestations. These pests excrete honeydew, which serves as a medium for the growth of black mold. Treat the underlying pest issue to eliminate honeydew and, in turn, prevent black mold. Use insecticidal soaps or horticultural oils to control aphids or scale insects. Encourage beneficial insects to help keep pest populations in check.