It may be from misting, but to me it actually looks it could be several things:
1. Burn from minerals in your water maybe? Although that usually occurs towards the tips of the leaves...
2. Maybe systemic fungus from the media around the roots? Check the roots, how are they?
If you're misting everyday, and they don't have enough air movement, you could be creating an environment for fungus/mold/bacteria to grow.