Common rust disease causes premature leaf drop of frangipani (plumeria) plants.
A rust (fungus) disease, Coleosporium plumeria, defoliates many varieties every year.
Top photo: The rust disease coats the underside of the leaf with orange pustules and causes leaves to turn yellow, then brown and rapidly defoliate in September-October. Bottom photo: Powdery mildew of ‘Bridal Bouquet’ plumeria.