Every few years, we get a lot of Fire Blight in ornamental pears in Denton County. The disease is caused by a bacteria and enters into the tree’s tissues via open flowers. So, the conditions have to be just right for a tree to get infected flowers open, high humidity, and the presence of the bacterial spores. Pollinating insects carry the disease to the flowers.
The disease goes dormant once warm weather hits (above 90 degrees) and will lay dormant until the next spring. If conditions are right, the bacteria ooze out of infected limbs and is picked up by pollinating insects and the cycle starts over again. The disease will continue to travel down infected limbs until the weather turns warm.
The good news is that the Ornamental pears (Bradford, Cleveland, Callery, etc.) are somewhat resistant to the disease. Fruiting pears are not and infected trees will lose limbs and maybe the whole tree will die eventually.
The treatment is to prune out all of the dead areas down about 6 in below the infected tissues. Disinfect the pruning tool with Lysol or 10% chlorine bleach solution between each cut to keep from spreading the disease. Spraying is not really practical since it has to be timed with the flower opening. Spraying is not curative, the infected limbs must be pruned out. There is an injection into the tree that is partially curative, but even that is not complete. The injection is a better treatment than spraying as the material gets into the tree and is effective for longer.
I have seen heavily infected ornamental pears live through this quite well. So, if you decide to not do anything, your tree may be just all right.