Stewart Robert Hinsley wrote:

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.

Talking of rust, I seem to be having a major problem with it this year,
so far affecting peas (mange tout) garlic, hollyhocks and rhubarb.

Is it transferable amongst these crops/plants, and what causes it, and
is there anything I can do to prevent/treat it?