You might start at the link below until someone else comes along that knows more.
The link is for diseases not pests but it might help...

Here is a link that might be useful: asp net

Perhaps not a disease/pest at all, but instead edema? Sometimes spelled oedema.

Thanks gobluedjm and jean001a, much appreciated!

I'm glad no one thinks it's a pest! I had sprayed the leaves a couple of times before and the marks are still there, although they can easily be dislodged with a little surgery using a sharp kitchen knife though.

I will look at the site goblue and look up some photos of edema.

Thanks again:)
Billy Rae