Rust on a leaf

I am absolutely fascinated by the rust that grows on certain leaves - this is a supermacro shot of this fungus and it was growing on the leaves of a Greene’s Mountain-Ash tree (Sorbus scopulina), at the Bragg Creek Natural Area. As Greene’s Mountain-Ash belongs to the Rose family, I think this is maybe Cedar-apple rust. If you pass a tree/shrub that has orange rust spots of the uppserside of the leaves, turn a leaf over and take a look. You might see these very tiny "tentacles". You just never know what might be lurking under those leaves, LOL. The whole process is complicated to explain - maybe I'll add a link or some info later if I remember, but now I have to get myself ready to go to a Volunteer Thank-you BBQ for the Weaselhead Society in just over an hour's time.