csystudio!!
That is indeed rust. The wet season here in Florida usually sees it proliferating. I spend half an hour in the morning picking off unsightly leaves while I stroll among my plumeria and admire their blooms. In other words: I don't sweat the rust. It doesn't harm the plants, and there is a theory that says it helps plumeria defoliate before the storm season...
But there are various remedies available if it bothers you. You have been given good advice already.
