The bacterium Erwinia amylovora, commonly called "fire blight," is a destructive disease that is very prevalent in pear trees. It causes leaves and flowers to turn black and often destroys limbs or even entire trees. First symptoms in the form of cankers are visible long before leaves and young fruit begin to shrivel and blacken, but often these cankers go unnoticed, allowing the disease to spread and become more severe.
Cankers on branches and trunks can be identified by the watery, light tan bacterial ooze they emit that leaves dark streaks on the tree. Fire blight most commonly infects open flowers, turning them a signature black color on pear trees. Infected flowers and leaves cling to branches, giving them a scorched look, which is why the disease is called fire blight.
In order to manage fire blight, trees should not be fertilized or heavily pruned. This will promote growth of the disease. Apply antibacterial blossom sprays to control the disease, but avoid doing so in wet weather. Though this will not eliminate the disease, it prevents new infection. Diseased wood should be removed and destroyed, and this should be done in summer or winter when the bacteria are not spreading through the tree. If the infection is rapidly advancing, however, the wood should be removed immediately, no matter the season.