Answer: You can't really cure it, but you can control it to some extent. To successfully control fire blight, all infected twigs, branches and cankers should be removed from the trees. Prune well back--6-8"--from evidence of infection. That is, if you see signs of infection on the last 6" of the branch, prune off 12-14" to be sure you are removing all infected material.
Since the bacteria that cause the disease can be spread during the pruning process, be sure to disinfect your pruning tools after each cut. Use one part household bleach to nine parts water, or dip your cutting tools in pure alcohol after each cut. After you've removed the infected plant parts you can help protect the remainder of the tree by spraying with a lime-sulfur (Bordeaux) dormant spray. Also, look around your property for infected wild trees, and prune or cut them as well.
A very weak (about 0.5%) Bordeaux mixture or other copper fungicide applied several times as blossoms open can reduce new infections, but will not eliminate all new infections nor those already existing in wood. The number of applications needed depends on the blooming period. Once blossoms begin to open, make the first application when the average temperature (average of the maximum and minimum temperatures for a 24-hour period) exceeds 60?F. Apply at 4- to 5-day intervals during periods of high humidity and until late bloom is over. For pear trees, this may mean five to twelve applications per season. Copper fungicides may cause a russeting or scarring of the fruit surface. The risk of this damage begins during bloom and increases as fruits enlarge.
Best wishes with your pear trees.
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