On any other plant, I'd say it looks like iron chlorosis. I'm not sure about apples and chlorosis, whether they are likely to suffer from it or not. If that's what it is, it can be corrected by applying fertilizer that contains iron chelates or foliar spray with iron made for that purpose. Iron chlorosis is an iron deficiency most often seen in soils that are too alkaline for iron to be " unlocked " from the soil and be available to plants.
I'm not sure what ailments apples get that differ from other " landscape " plants. Maybe Maya can jump on this ??