This is a disease I labled "white shell" a few years ago. Notice how it starts at the edge of the scute and progresses towards the middle. It occurs because the turtles do not dry out enough and the layers of keratin swell and then split along the edges and that allows bacteria (NOT fungi) to get in and start digesting the keratin. Eventually it will progress into the bones and kill the animal.
To prevent the disease always have a heat lamp (60-75 watt bulb will do) over the sunning area so the turtle can dry out completely most days of the week. Do NOT cover your tank so the humidity is low. The best treatment is natural sun outside but be careful not to cook your turtle. if your turtles frighten easily and flee into the water keep them somewhere during sunning time that they cannot see you or put a visual barrier around their tank.
I have found that the turtles will cure themselves in 1-2 years if fed well and given the opportunity to dry out.
Mark Feldman