I've seen those white patches on ash before. I never have attributed it to any particular disease, but to growing conditions or tree vigor conditions. I could be wrong on that.
Usually, the ash is growing off site, somewhat. They need a pretty good site, but will grow on lesser quality sites. That stresses the tree.
Another indicator pointing to stress in your pictures are the swollen knots. They usually indicate heartrot. That could come from old logging or fire damage.
I've also seen patches similar to that on white oak. That usually indicates worm infestation. Glass worm is prevelent in some ash, and I've seen very little ash with white patches turn out to be veneer quality. Glass worm will reduce ash and white oak to sawlog quality.
