Help, Beetroot issue

Beet is is probably not spaced properly but space is limited and they are in a planter.
From the images some leaves are dying so obviously some sort of infection, anyone know what?
Should I just pull them now or leave as the beets I would prefer to be a bit larger.

 

Looks like leaf miners.
Hold a leaf up to the light and you might be able to see the insect inside the leaf .
They often grow out of it if it doesn't get too bad.