Is that cherry rust?

I'm no expert, but I've been thinking some type of rust for the last couple of hours as well. But I have been unable to find a picture that looks exactly like yours. Rust is very treatable with fungicide, but I suspect it is too late to have any positive effect this year.

Thanks for the responses. I checked the plums closer last night and indeed they were affected as well. I never use chemical sprays ever - but last night doused everything good with Malathion which killed all bugs as it appears that is what caused it. I will be keeping a close eye on things and see if that stops the spread.

Bugs don't cause rust - that is a fungal disease. So spraying with an insecticide, and especially one as toxic as malathion, is a pretty bad idea. It won't do squat for the rust and I personally would not want to eat any fruit that came from a malathion sprayed tree.