Fireblight is a bacteria that is most noticeable during the spring when the temperature begins to warm and become humid. Cankers, on the branches and trunk, is where the bacteria overwinters. Prune out the cankers, remember to sterilize your pruners after every cut with 1 part bleach to 4 parts water. The bacteria in the cankers are spread by rain or insects to the leaves or flower buds. The 'shepherd's crook' or the blackened curve at the end of branches is the telltale sign that your tree is infected with Fireblight.
Limit fertilizing as this promotes new shoot growth which is the most susceptible to Fireblight. If applying chemical, the timing has to be right. Make sure to check the label.