In message , K
writes
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.


Some rusts have wide host ranges; others have narrow host ranges.

Hollyhook rust, for example, attacks a variety of mallows with various
degrees of severity. Hollyhocks, common mallow and it's near relatives,
and musk mallows are the worse sufferers. Shrubby Lavateras aren't much
affected.

Some rusts have alternate hosts - if I recall correctly one rust attacks
wheat and barberry.

I'd be surprised if a rust of Euphorbia infected other genera.
--
Stewart Robert Hinsley