Help with disease/pest/deficiency indentification

Hi all

I'm fairly new growing own vegetables and I would appreciate some advice with my courgette plants.

I started the plants from seed, transferred to pots them and then planted in my garden. Shortly after planting in the garden, the veins in the leaves had a white 'outline'. This has become progressively worse: see attached photos.

The courgettes themselves seem OK.

Can anybody tell what the problem is and how to fix it?

Thanks in advance.

 

Never grown courgettes but looks like mildew to me too.