Mildew, to which Oak trees are rather prone. All you can do is to soak them with a systemic fungicide and keep the babies in a windy spot.
Oaks are not happy about being transplanted, especially if there is any damage to the tap root so pot on frequently and carefully and plant out in their permanent home as soon as they are a reasonable size.

Thanks Owdboggy

Some years can be worse than others too. As said, both our native species get this mildew, especially the second flush of growth (lammas growth) in mid summer.