What is this?

[pickledtink]

Can anybody definitely identify this? I thought it was mildew as I often have that problem especially on courgettes but this is a bit different. Sort of like a blistered second skin over some parts of the leaf. When it gets bad it starts to go into brown edged holes in the middle of the patch. I've dusted with sulpher but it doesn't seem to have really helped.

[Carole B.]

I don't know what it is but mine always seem to get it and the plant never seems to suffer from having it so I don't let it bother me!

[sprout]

It might be a feature of the plant? Several of my squash leaves look like this, and I just assumed it was a marbled-leaf variety. Seems to do no harm to the crop