1 disease is fine for trash, but single targets should really have 2 diseases on, especially since you'll be using Obliterate rather than Howling Blast.
Glyph of Disease isn't any good for Frost. If you have Icy Talons, you'll be using IT to refresh it. If you're using Glyph of Howling Blast, you'll be using that to reapply your diseases. You'll get much more benefit (and threat) by using a different glyph.