Forum Flora Germanica

Ruled out all other options, must be fungal I think. Bottom side no difference compared to healthy ones. One extra 'painted' has only leaf veins left green. Outer twig leafs (looks like a small bit more windy) 'uninfected'.

http://agrozoo.net/jsp/Galery_one_image ... c0c78ace2d

I found a copy image (image stock - not named) the other day of this on net, so it is not something rare, I guess.
I suppose this thing appears only in rainy spring/summer.