We have been fighting fire blight on our apple trees for years. Once it gets into the trees it is very hard to fight off. There are only two ways that I can find.
One, cut off all branches with withered leaves. The cut has to be 6 inches below the infection and you must dip the blades of the clippers in a bleach and water mixture between each cut. This prevents the blades from spreading the infection.
Two, spray with Fertilome. It only helps to spray right when the buds on the tree emerge in spring until they flower. I sprayed every 3 to 4 days at that time. And still I get more blight on my branches.
This year, in addition to spraying, I will be adding some fertilizer. The theory being that a strong plant can fight off disease easier. Of course it could be that a healthy plant will push the blight throughout the tree.
Lee helped by using a heavy pole to make a hole deep enough to drop in the fertilizing stake. You can pound them in, but if the soil is too hard, the stake will break. You place them around the tree at the drip line, or as far out as the branches grow, If you place them too close to the tree, you can burn it with the concentrated fertilizer.
There is no downside to this. I help the trees or the fire blight consumes the trees. If that happens I will I tear them out and quit fighting.
It is very discouraging. We have had these trees for about 5 years and they are just getting to the point where they should be producing a significant number of apples. I no longer complain about the hight cost of fruit and vegetables.