I'd be interested in more information on this too. I suppose it's possible, but I can't imagine it's a big problem. I've used oak leaves from a tree in my back yard (mixed with pine needles, maple leaves and others) over top of a deep layer of soil for the last six years and have never had that issue pop up. I live in the heart of T.c.carolina territory and the surrounding forests that they inhabit are thick with oak trees. I pick up (and photograph) a lot of wild turtles from the area during the warm months and have never noticed one with what looks like a shell problem caused by a fungal infection. I guess I'll start looking for evidence of this from now on.