When humidity and temperatures are high, frangipani leaves are often attacked by a rust fungus that produces massive numbers of yellow-orange spores on the undersides of the leaves. The affected leaves turn yellow and brown and drop from the plant prematurely. In order to reduce the spread of the disease, badly infected leaves should be removed from the tree and fallen leaves should be raked and discarded.

 Additional Resource 

http://www.doacs.state.fl.us/pi/enpp/pathology/pathcirc/pp319.pdf