How do you keep mold and mildew from forming in a walk in wine cellar

dehumidifier and Damp Rid

Sounds like you need a dehumidfier and a fan to increase air circulation, and then treat the walk-in with Wet & Forget Inside. What are the walls made of? if they are cinderblocks, I would seal them with Dryloc paint.

What I read on-line is that you want 55 degrees and 50%-70% relative humidity. If you are maintaining these conditions and still getting unwanted growth I would recommend a very modest air circulation within the room. You should then not have any more mold issues.

Get a dehumidifier!

You probably are going to need more trouble shooting than you can get here. But I would add in a de-humidifier to control the humidity to the level where you want it. Once mold is in the room, you will probably need professional help to remove it. The spores are everywhere.

I would definitely use Damp Rid in a few spots.

Containers of REGULAR CHARCOAL BRIQUETTES,no easy light kind.