Looks like leaf miner damage. You can Google 'leaf miners' for all the details but beets are one of their favorite targets and there are few effective preventatives other than row covers. They are the larvae of a fly and they burrow in between the layers of the leaf so they are protected from contact poisons. The goal is to prevent the flies having access to the leaves.

Strip and remove the severely damaged leaves. As the plants mature they can increasingly tolerate the miner damage.

Dave