Red spider mites
A few blogs ago when I pictured my first baby aubergine I was having a few problems. The leaves were turning a mottled sandy colour. Well, the little endive and winter lettuce seedlings started turning the same colour, on very close examination...the underside of the leaves were covered in tiny, tiny little critters. Less than a milimetre each and a sandy colour.
Apparently they don't like damp conditions and they thrive in hot dry greenhouses.... Out with the watering can then.... twenty times each day..... I don't think so.