( "Where does the blight in the canker come from? From the air straight to the canker site ")
Yes , the air .
In the spring , when temps warm , and sap flows,
The bacteria oozes out, forming long strands that are blown on the wind, or carried by insects, rain can carry it down the tree . Insects , pruning , hail storms , etc cause wounds.
If these land on a wound , or flower, they can gain entrance to the plant. Causing infection.
Fire blight is native here, living on wild and cultivated rosacious hosts.
Where you see the canker is where it infected the host, it dose not move undetected inside the plant.
Generally recommended to cut a foot below infections.
Most would cut infections off, rather than trimming.
But judicious trimming may work.
I believe a high nitrogen status of the tree, is responsible for most blight, trees that are growing slow ( low nitrogen) seldom get serious blight,
Trees that are growing excessively ( high nitrogen) often get serious fire blight. And varietys vary ,as you know.
Here ,I only push trees with nitrogen, the first ~ 3 years , just to get them bigger than a deer. Then slow them down to get fruit. Seldom have fire blight on my older trees.
I met a commercial pear grower in northern Ohio, that used a small propane torch to stop infections during the spring.
He would burn the edges of the cankers with the torch until the bark boiled , stoping it. Cut out the bad ,in dormant pruning. Sounded bad , but he had a good pear orchard.
Fight fire with fire , I remember him saying 
This is exactly what I needed to know, thank you. For years I had that same understanding then talked myself out of it. LOL.
Well that is mostly accurate,
Often it starts at the tip of a branch, and keeps moving down.
But you can see where itās been.
So are there three points of attack for blight?
- Blossom blight that moves down the branch
- Twig blight that moves down the branch
- Blight that lands directly on bark, forms a canker on that spot and does not infect the branch above or below the canker.
Itās all the same kind, just a matter of where it gained entry.
Read a interesting paper, ( canāt remember source)
( A university , Plant pathologist study)
They were spraying a benign /beneficial ,bacteria that colonized the wounds , out competing the fire blight for nutrition.
And stopping / preventing it.
Sorry donāt remember details.
This is likely what happens naturally when some of those cankers just stop on their own ?
Hmm, sounds reminiscent of Michael Phillips āHolisticā approachā¦
When pruning out fireblight, can I tell from the inside color of the wood whether Iāve pruned low enough? In other words, does the cut cross section of blighted wood have a distinctive color? If so that would really help vs guessing how far the blight has travelled.
Fireblight has come back every time ive seen discolored wood after pruning. It also has come back some of the times that i have not seen discolored wood. Ive tried cutting out cankers from the main trunk and ive also left cankers on the trunk. Each time, FB has eventually returned and caused problems. I have cut back six trees to 3.5 ft trunks and only one has come back free of FB. The times i have left an ugly stub, the FB has moved from the stub into bigger wood and set up shop there. These troubles could be due to my hot humid climate.
Thanks for your report. Where are you located? Iām in Maryland 7B. Have you seen a tree wall off blight without you cutting it out? Were your ugly stubs in at least two year old wood?
Thats a good point on the 2 yr wood. Ive done it on 2-3 yr wood and on 1 yr wood with the same result. Im guessing part of the problem could have been that in some cases i did not notice the blight until it had moved too far down the branch and into the trunk. Sometimes i see no signs of bligjt as it moves inside a branch over the summer then i see some ooze in the fall. And i inspect VERY carefully all summer but still seem to miss it. This why i gave up and spray strep, which has eliminated FB the last 2 years for me. Or it could just be good luck, but doubt it because the local callery pears were pretty torn up with it both years.
Agree strep worked the one time I tried it. I donāt like using the sprayer, gets heavy, etc. I may give up on apples here. Are you in the South? You apparently donāt want to give your location, makes it hard for me to extrapolate your results.
Iām in Alabama 7b/8a. Im also using geneva rootstocks which may be resistant, but the flowers and young wood ended up too close to the trunk, which didnt help with FB.
Hereās a 2017 Fireblight article from Penn State by plant pathology professor Kari Peter, carrying on Dr. Steinerās approach to pruning: file:///Users/Steve/Documents/apples/FIRE%20BLIGHT/EXC%20KARI%20PETER%20ON%20MANAGE%20BLIGHTApple%20Disease%20-%20Fire%20Blight.webarchive