Hello!

Fire blight can be difficult to control, but its severity also varies strongly with spring weather conditions (with cooler, wetter conditions typically favoring extensive infections).  

Antibiotics are used as a preventative measure when the tree is flowering, applied every three to four days as a spray.  If the tree has symptoms of fire blight, they are not recommended--they don't work well to control active infections in branches and overuse in this situation can lead to antibiotic resistance of both the pathogen and other environmental bacteria.  Some antibiotic products are not labeled for ornamental trees or non-orchard settings and so could not be used in a home landscape in any case.  The product label will specify use requirements.

If your tree was impacted by fire blight this past year, pruning out visible cankers over the winter at least 18" below the visible canker margin and spraying copper-based fungicides/disinfestants in the spring both before and during bud-break can help limit the spread of the disease and are both more accessible and more effective options for home landscapes.  Several copper-sulfate and copper-ammonium products are labeled for this use; be sure to read an follow all label directions for mixing and application--the label is the law.

A tree-care professional could also apply growth regulators to your apple tree, which can harden twigs and make them less susceptible to infections.  Minimizing over-watering and excess nitrogen fertilizer will also help to make the tree more resistant to bacterial growth.

I hope this is helpful.

Cordially,

John

Antibiotics are not a recommended treatment for residential trees--there is no solid evidence that they work.

When they are used, typically in orchards, it is as a (preventative) spray.

If your tree has been impacted by fireblight in the past, pruning out active cankers and using a copper-based fungicide and/or growth regulators (if absolutely needed) are more reliable ways to control the disease progression.

Depending on weather conditions, they disease may improve on its own, with no treatment.  

I hope this helps!

John

You bet, let us know if you have other questions!

John