Based on the symptoms pictured, Fire Blight is still our best guess, especially given that this year has seen instances of widespread infections due to this spring's weather patterns. (The linked page is for farmers, but still applies to outbreaks in home gardens.) The only management approach at this time would be pruning to remove all symptomatic growth (and then some, since Fire Blight pruning requires the removal of some wood beyond the point of infection as well, since it can be asymptomatic and would otherwise harbor the pathogen). Even if this dieback were the result of another disease, the recourse of pruning it out would be the same, since no fungicide would be able to cure existing points of infection. Plus, the period of time when new infections would arise has likely passed for most (if not all) plant diseases that would cause stem dieback, so preventative treatments wouldn't provide much benefit until the following spring.

Often, the spray regimen needed to protect apples from an array of common pests and diseases includes preventative fungicides for Fire Blight (although bacterial, this pathogen is vulnerable to a couple fungicide ingredients, mainly copper) and other infections, so their use is generally all the protection a tree needs. Our Growing Apple and Pear Trees page has more information about pest and disease prevention options.

Miri