Hmm, this is the first time I've ever seen it on a plumeria but I've only grown them for five or six years and always in containers. I only have this one Plumeria right now; gave @ a dozen away to former neighbors a couple of years ago. I will have to ask them if they've seen rust this year!
~ I'm an old gal who still loves playing in the dirt!
~ Playing in the dirt is my therapy ... and I'm in therapy a lot!