Beetroot Leaf Miner

leaf miner affects the whole plant pull all leaves off a new crop and treat with liquid derris . clear all rubbish up this year and bin,,,, treat ground with a spray of  milky white jeyes fluid in a sprayer  

Beetroots need quite a lot of water to grow to a good size and be nice and tender.  I know it's been quite a wet summer but if your soil is free draining they may have been too dry (mine were   )

You need quite an infestation to account alone for such a bad growth rate. In future try to pick off infected leaves as soon as you can and destroy.
When did you sow? Did you thin them? And water as said, all such basic things might matter.  

try april end  

No-just take of the worst leaves and have a rummage in them in case you can find a maggot or pupae to murder actually in the leaf!-at least you`ll feel better. The roots should not be effected by this pest and unless you`ve got rows and rows of them you could examine each plant leaf system and rub off any fresh eggs etc.-this menace is  a pain.
not so bad on beets but nasty on spinach because that` s the bit you eat and as far as I know there is no fix...!

ps-check your spinach too!

I`ve been experimenting by raising all my Swiss Chard (were in pots) onto high shelves (1M) as these too are on their treats list -so far so good