Rust has been a problem for many plumeria growers in recent years. I had none until last summer. I suspect I got it from a cutting or plant I got in the mail. For a long time it was confined to the front yard, which is surprising since the yard is only 80 x 100. Finally it hopped the fence and the plumies in the back yard became infected as well. After reading some message boards suggestions, I tried Bayer Advanced fungus control (which is made for lawns) as well as a systemic fungicide. They both have the active ingredient Bayleton, and I got them from Ace Hardware in Largo, Florida.
First I cut all the leaves off, to prevent reinfestation, it was March so there weren’t many to cut. I sprayed the branches with the fungicide spray. Then I sprinkled it around the base of each plant. I’m a bit paranoid, so I treated again in May. When I get new plants I sprinkle them as well. It is now July, and I am happy to say I am still rust free.