I never pais attention to this as I am used to see it here.
I think that this is not a disease. In fact there are some species of clover that show different patterns and stains on the leaves, like Trifolium repens.
You awoke my curiosity and I have been looking around, but did not get to a right id.
Everywhere, I read how variable they are, so I think I wont botter, and continue with my fungi :D
Delicious milk-cap (29)