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!