Good morning!

Unfortunately there is no treatment for fire blight once it is in the tree. You can prune out the damaged branches which can help prevent it from spreading. 

I would suggest trimming any of the damaged or dead pieces out asap. Usually you want to prune trees in the winter but when there is a disease it is best to prune that out right away.

When pruning out fire blight make sure to use clean tools and sterilize inbetween each use. I always spray my tools with rubbing alcohol to ensure there is not contamination from pruning one tree to the next.

When you prune - make your cut at the next healthy node or point of growth on the branch. 

Here is some more information on fire blight that includes some pruning tips.

Thanks for reaching out!

Rachel