Like Nica said, turnips are probably Peach's best weapon against the projectile-less Marth.
If the Marth is aggressive in his approaches, a well-placed turnip throws a monkey wrench into the works. He's forced to find a way around the turnip, narrowing his options, and all the better if he takes the hit too. If he jumps out of an all-out run, you're in shield-grab heaven. Enjoy your new throw toy. If you nail a turnip throw properly against an air approach, he'll run into the 'nip no matter what he does. This buys you a few frames of hit stun to close the gap and do your thing or escape if need be.
Turnips are equally strong when approaching Marth. DJC Turnip Throw is my favorite way to penetrate his defense. If he moves or hits the turnip with an attack, keep your distance and ready another. If he shields, spot-dodges, or (better) uses Counter, run right in and take your prize. Wait a moment if he Counters the turnip or you'll set yourself up for disaster. Spacing is everything - stay close enough to keep the pressure on, but not so close that you're vulnerable. If Marth starts a counter-approach of his own, you'll want the time to deal with it.
P.S. Anyone else notice how quiet it's been in here lately?