Winter Dormancy

• Some yellowing of plumeria leaves is a normal yearly occurrence for deciduous varieties of this flowering tree. This happens even in truly tropical climates, where winter temperatures are only slightly lower than in summer. Leaves will begin to yellow in fall, soon turn brown, and then drop.
  
  

Yellow Rust

• Pinhead spots of a yellow fungus can cover plumeria leaves and cause them to die and later drop from the tree. The pathogen that causes this disease is a fungus called Coleosporium plumeriae. It causes powdery, elliptical spores to form, primarily on the undersides of leaves: it does not attack stems or flowers. However, in advanced cases, trees can lose all their leaves. Fungicides such as Eagle 40WP can help to control this disease.
  
  

Insect Pests

• Several insects can attack plumeria trees and cause leaves to yellow in severe infestations. These include the long-horned beetle or plumeria stem borer, thrips, blossom midge, whiteflies and mites. Predatory insects often eliminate many of these insects, according to University of Hawaii Extension, but when they don't, tree leaves can turn yellow and drop.