Fire blight is caused by the bacterium
Erwinia amylovora, as yet there is no general agreement as to why this fairly common bacterium should in certain trees develop into a pathogen capable of killing the tree. It is found on the bark of the trees entering into the internal tissues through lenticles and other natural openings as well as injury sites. Epidemics seem to be triggered by temperature, humidity and linked to tree growth stages. It attacks blossoms, new growth shoots and scaffolds. Its also seems to be Rosaceous plants that are most susceptable.
In commercial apple orchards there are a range of control measures used to manage fire blight, not least the planting of resistant species. The use of both copper and sulfur based sprays is common, also a certified antibiotic streptomycin sulfate is also used though the potential negative impacts of using such products I think out weigh any minor benefits.(Bacterial resistance)
I would agree with most of the points outlined as treatment options but would add that having both the soil and foliage assayed by a lab like the Soil Foodweb Inc could be worth it for a tree of significant value to you. The foilar application of a suitable compost teas together with seaweed extracts and organic silica has been shown in individual cases to be wothwhile.
Just remember that the cycle of pathogens like fire blight are connected to the same processes outlined by Shigo and others, excesive fertilisation of trees in impoverished compacted soils, poor pruning practices, inappropriate species selection for the region they're growing in (excessive humidity/irrigation practices) One of the triggers for fire blight does appear to be sudden moisture increases linked to excess soil nitrogen availability.
If you're interested in the Soil Foodweb visit them here and contact them for futher info re your possible fire blight problem
http://www.soilfoodweb.com/03_about_us/approach.html