White patches on Brassica leaves

[digon]

This is my first attempt at growing veg. I planted out some seedling cabbage (3 varieties) ,Cauliflower and PSB in September. During the past couple of weeks about half of them have had white blotches appear on the leaves. The blotches look like bleach stains and don't seem to be powdery or fury.
Does anybody have any ideas what this is and what to do to stop it affecting the remaining plants?

[Two_Sheds]

See the little trails? That's leaf miner damage. Pick off & compost the affected leaves, more will grow

[digon]

Thanks TS.
I spent hours googling for white blotched cabbage leaves etc and came up with zilch.

I guess debris netting is too course to stop these flies or maybe they where already on the ground when I put the netting over.

[Two_Sheds]

You can't really prevent them, just pick off the damage. They attack many different plants