I have a few raised beds planted with a mix of loose leaf salad greens and herbs. They are on the north side of the house planted in miracle grow potting mix. They are doing great, but all the greens (not sure what they all are, the seeds were "mesclun salad mix") have white spots (in the leaf, not on it) and the dill is covered with a thin white powder that does wash off. What could be causing this?

Maybe they aren't getting enough sun? I put the beds in the shade so I could have fresh salad all year. Any ideas?