1-Remove the diseased foliage
This address the disease directly. Its a first aid measure which serves to get the disease out of the picture. Removed leaves should be burned.
2-Treat the tree
Wash it down with a good steady stream of water. There is thread on here about applying a paste to a tree. Take a look around, I suspect there is some useful information.
3-Treat the soil in which the tree is rooted
You have that pond and clay soil. The bottom of the pond would be rich in minerals. Included in compost, it would add nutrients. Apply the compost to the soil around the tree, churn it in.
There is, as yet, no cure for fire blight and the best way to deal with the infection is to remove infected stems and branches cutting no less than 8 inches up from the infected area. Because the bacteria are so easily transmitted, care should be taken in disposing of infected plant material. Either burn or discard in the trash. Do not leave infected material where the bacteria might be spread to surrounding bushes or trees. Care should also be taken with tools which have come into contact with the bacteria. Tools can be sterilized in an alcohol solution (three parts denatured alcohol to one part water). Diluted household bleach can also be used (one part bleach to nine parts water) as long as the tools are wiped dry after disinfecting to prevent corrosion.
A variety of bactericides have been developed to combat fire blight, many of the most common containing streptomycin sulphate. Check with your local garden or landscape centre for details and remember to follow the instructions carefully.
Looking at the plan I offered, there are some adjustments to make. Emerson is accurate that this is not something to frig with. If you have other apple trees nearby which are unaffected, it may be wise to remove and burn (sanitize) the infected trees rather than attempt to save them.
If saving the tree is the objective, infected sections and entire branches need to be removed. The bacteria spread easily, with splashed water being a primary vector. If there are susceptible plants in the area, curtains/tarps would help to contain any water being used to wash down the tree. Also, any tools used in the surgery would need to be sanitized (fire or bleach would be effective) lest the tools spread the disease.
Emerson White wrote:
They weren't very good at sterilizing surgical instruments in olden times, I think that is why they stopped using thymol. Unless you want to get into fractional distillation (and you don't) you are much better with some good ole moonshine and a match, dip the metal cutting edge then light it up.
You're right, thermal methods (notably the autoclave) are a lot more reliable and broad-spectrum, and have a lower marginal cost, to boot. I think the marginal cost was ultimately the biggest reason for the change-over.
I should've been more clear: I was thinking thymol might be worthwhile on parts of the living tree, and perhaps hands etc., where fire would be inappropriate.
I know of several investigators who have been researching biocontrol methods for combating fireblight and it's good to see that some of this work is "bearing fruit" (no pun intended). I do think one needs to consider that pasteurized milk will have killed off any beneficial bacteria, but milk or milk-derived cultures that may still harbor active bacteria may work. From a recent research paper:
"Lactic acid bacteria (LAB) can be a source of biological control agents (BCA) of fire blight disease. Several species of LAB are inhabitants of plants and are currently used as biopreservatives of food because of their antagonistic properties against bacteria, and are considered as generally safe. Candidates to BCA were selected from a large collection of LAB strains obtained from plant environments. Strains were first chosen based on the consistency of the suppressive effect against E. amylovora infections in detached plant organs (flowers, fruits and leaves). Lactobacillus plantarum strains PC40, PM411, TC54 and TC92 were effective against E. amylovora in most of the experiments performed. Besides, strains PM411, TC54 and TC92 had strong antagonistic activity against E. amylovora and also other target bacteria, and presented genes involved in plantaricin biosynthesis (plnJ, plnK, plnL, plnR and plnEF). The strains efficiently colonized pear and apple flowers; they maintained stable populations for at least 1 week under high RH conditions, and survived at low RH conditions. They were effective in preventing fire blight on pear flowers, fruits and leaves, as well as in whole plants and in a semi-field blossom assay. The present study confirms the potential of certain strains of L. plantarum to be used as active ingredient of microbial biopesticides for fire blight control that could be eventually extended to other plant bacterial diseases."
From European J. of Plant Pathology (2013) Volume 137, Issue 3, pp 621-633: http://link.springer.com/article/10.1007/s10658-013-0275-7