Rust Fungus is caused by Coleosporium dominguense and Coleosporium plumeriae
The plumeria cultivars most susceptible to this fungus are the Plumeria rubra types and the Plumeria obtusa. This fungus manifests itself as red-orange pustules on the backsides of leaves. It presence can always be determined be the appearance of yellow, orange or reddish-brown powdery pustules on the leaves, stems, or buds of the infected plant. The spores produced in these pustules are carried by splashing rain or air currents to near-by healthy plants where new infections will occur. Your first line of attack should be to cut off affected leaves. Do not add them to a compost pile because the disease can spread. The Plumeria Society of America recommends using a broad spectrum fungicide—those products containing bayleton, benomyl or oxycarboxin are appropriate. Cutting down tall weeds around plumeria trees helps to improve air circulation and will reduce the humidity this pathogen needs to survive. Also, when you plant plumeria trees, be sure to leave plenty of space between trees.
Rust Fungus does not kill Plumeria, but can rapidly de-foliate an entire tree.
Most plumeria cultivars grown are susceptible to the pathogen and have numerous powdery spore masses on the underside of leaves. Leaves can turn brown and fall from the plant as early as two months after the springtime flush of new leaves is infected by the fungus.
How to Control Rust Fungus
Rust fungus will over-winter on infected plants.
Additional Ideas for Controlling Rust Fungus
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.
Additional reading form University of Hawaii pd-61-1