I think that if extender patties were the "cause" of the resistance those
who are using that methodology would be the ones with the biggest AFB
resistance problems.

While there is no doubt that low level exposure to an antibiotic and lead to
a population of bacteria that do not respond to that treatment there are
other possible answers. One of the key points made a while ago was the
debate over the term SAFB (South American Foul Brood). Like many of the
other problems we have seen over the years the cause may be transportation
of the new strain into an area where it causes damage due to lake of
resistance in the native stock. The resistant AFB could have come from
several places but once it hit North America it became a problem. Maybe a
case where AHB has less of a problem with that strain than the EHB does.

I guess this amounts to another load of speculation. At present I do not
treat for AFB or EFB. I have not needed to, yet. My feelings are mixed on
this but for me (and others have different needs) I think I would rather
spend my money on new frames and a package of fresh bees than on a lot of
chemicals. Of course if I have an entire yard showing AFB I may change my
mind in a hurry!