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?

Is it transferable to other plants? I grow vegetables although the only
plants growing close to this were Easter Ledger - bistort. We don't eat
their leaves at this time of year.

Not very good pic at:

http://tinypic.com/view.php?pic=2ij4ze1&s=3

TIA

Mary