It appears as though you have Cercospora leaf spot or Cerocspora beticola. This is a soil born fungus that only affects Beets andSwiss Chard.  It starts as small angular lesions that spread if left unattended, to contort the leaf as you show it is your pictures.  This fungus can be treated with a fungicide called Chlorothalonil as an additive by-product.  If treated soon enough you can strip the outer leaves off and use the inner leaves as they develop without the lesions.  You mentioned a continuous problem: if you are growing it in the same spot every year you might try changing the location of your new plants in the garden. (i.e.: crop rotation)  Thank you for using the Ask an Expert System.