Mildew. Often made worse by dryness at the roots.

Thanks for the diagnosis, Owdboggy; is it enough to ensure that pots don’t dry out?

Differences of opinion on this. I say yes, but.........
You could try the Milk and water spray which some people use.
9 parts water to 1 part Milk, sprayed on the leaves. Never tried it myself so no idea if it is effective.

This is obviously a different type of mildew from the powdery mildew that appears on the leaves of courgettes and Japanese anemones, to name two of my blighted plants.

It might have been red spider mite infestation if the discoloration is in the leaf tissue, not sitting on top of the leaf like mildew - they'll be gone with cooler conditions and damp weather, but if you had them earlier in the year, the white spots on the mint could have been caused by them - note red spider mite is difficult to see, they look yellow earlier in the year anyway and its probably easier to see bits of webbing on close inspection rather than the mites themselves. They also may affect other herbs if the weather is hot and dry for quite a while. Some info in the link below

http://www.herbexpert.co.uk/controllingpests.html

Hello Bamboo, and thank you for your comment. I’m familiar with red spider mite, having had an infestation on a creeping fig brought indoors last winter. I didn’t notice any of the usual signs in this case. A supermarket pot of thyme that has spent most of the year next to the kitchen window (ie, has never been out with the other herbs) also developed the mottled leaf appearance, but on this plant there has been no return to normal colouration. I’ve noticed pinprick sized brown dots on stems and the surrounding surfaces, as though something very tiny is producing droppings. These probably won’t show up in the photo, but l’ll try to capture them.