Mary Fisher writes
Just now I pulled up a 4" petty spurge (a frequent and several-times-daily
activity although I usually get them when they're smaller).

I noticed that on the underside of all the leaves were orange/brown slightly
raised and crowded together spots.

Any idea?

Rust.

Is it transferable to other plants?

Dunno - I'm pretty sure there's more than one type of rust. I don't
know how species-specific any of them are.

--
Kay