Since we had such a mild winter here in Florida, some plumeria trees never went completely dormant and still have leaves with rust fungus. Normally, leaves will drop for several weeks in the winter and by the time new leaves emerge the rust is gone. But that hasn’t been the case for some trees here in Florida, which have been putting out new leaves that keep getting infected. You want to get a handle on it before it gets out of hand.
First, pick off all of the leaves that are heavily infected. Throw them in the trash along with any that have dropped on the ground. Mix 1 heaping tablespoon of baking soda and 1 tablespoon of Neem oil(or other horticultural oil) in 1 gallon of water, along with a few drops of dish soap (soap helps it stick to the leaves). Using a tank sprayer, spray the entire plumeria plant or tree once a week, especially the new growth. Make sure to spray under the leaves. This should keep the rust under control.
Note: When it gets warmer, do not spray in the heat of the day - Neem can burn foliage. Neem oil is available at Ace Hardware, Home Depot, Lowe’s and other garden centers.