Dennis has been very helpful on clarifying the question I had about shoot blight but I have had experience that casts some doubt on what he said about fireblight necessarily being a ballooning problem once established in an orchard. I manage several sites where I have had strikes on a single or a few trees, sometimes for several seasons, where the damage has been minor and never spread, even to nearby trees of the same cultivar.
I suspect that fireblight behaves somewhat differently in different regions.