Powdery mildew on courgettes

I also got exactly the same symptoms on the leaves of my courgettes (variety 'Defender F1'). The cause of these silvered, angular patches on the leaves which are more or less defined by the veins is non-parasitic; in other words this abnormality is not caused by a fungus, bacterium, virus, insect or other pest. This is a 'physiological blotch' and is caused when parts of the outside layer of the leaf (epidermis) are detached, letting layers of air to enter the area between the outside (epidermis) and inside (parenchyma) of the leaf, resulting in this silvery appearance. It is only found in certain varieties of courgettes.

Brill! thanks for that - I have often wondered what it was but never really worried about it as the plants are heavy croppers. Still, useful to know it's nothing to worry about