The spreading pattern of the necrosis does looks like a disease, but I'm not sure. Bacterial leaf scorch (Xylella fastidiosa) can look similar, though apples (Malus domestica) are not considered a likely host. Necrosis starting on the leaves would be atypical of Fireblight (Erwinia amylovora) but not impossible.
You might want to err on the side of caution and sanitarily dispose of any infected leaves, in the hope of limiting further spread. Remove all infected leaves and discard them in your municipal green bin where they will likely be hot-composted. I wouldn't be too concerned about partly or completely defoliating the tree, since it is almost Fall anyway.
I would inquire with your local agricultural inspection office. They should be aware of likely diseases in your area. If you find out anything interesting, please do post your findings here.