Hi Michael-
I'm wondering if that's not actually scab, but rather leaf blister mite damage. I had something very similar last year and Jellyman diagnosed it as that, and I believe he was right. In my case, it was on my William's Pride which also very scab resistant. With the mite damage, it seems that the damage is localized within the veins (I think they're veins? not a biologist) in almost a polygonal fashion. With scab, it seems like the scab legions are not bound by those veins. See the pic and Jellyman's diagnosis below and see if you agree. Have you sprayed any oil lately?
-Glenn

Here is a link that might be useful: Apple Mite Damage

Hi Glenn: the photo in the "Blister mite leaf damage" link appears to me to show bumps protruding from the under side of the leaf. My washed out photo shows caved in cells on the underside of the leaf so I suppose it isn't BM damage. Maybe I'll go out tommorrow and get some better shots and glass the leaves with my loupe.

Michael

Not mites. Not scab. Instead, a temporary environmental glitch.

Is it a young tree?

And is it well rooted in?

To determine the latter, grasp the trunk and rock the tree while looking at the ground at the base of the trunk. If the soil moves, even just a little bit, the tree isn't well rooted in.

Greetings again: upon closer examination with a 14x loupe I can barely make out what appears to be reddish brown critters living along mostly the midrib vein exclusively on top of the leaf. Some of the critters are also on lesser veins. I suspect mites because they certainly wouldn't be unheard of, and because their color and extremely small size rules out many other insects. I don't have my spray records in front of me but I think that tree was sprayed once with malathion within the last 3 weeks which could leave me with a mite problem. The newest leaves on the tree appear unaffected also indicating the problem began earlier in the season.

Michael