Sometimes the only way to stop the progress of diseases such as anthracnose, canker and rots, and fireblight is to cut away the diseased tissue. When you remove a diseased limb, make the cut far enough below the afflicted area to leave only healthy tissue at the point of the cut; a point about a foot below the diseased part is usually sufficient.
It's easy to spread disease to healthy tissue via cutting blades. You may accidentally brush your tools against stems or leaves after cutting infected wood; you may even briefly forget that you've been pruning diseased wood and make your next cut into a nearby healthy branch. To avoid transmitting infection, it's crucial to dip your tools in disinfectant before making each new cut. Use full-strength rubbing alcohol or a solution of 9 parts water to 1 part household bleach.
Do not compost the diseased material you remove, and dispose of it promptly. When the job is done, disinfect your tools thoroughly one last time. Rinse them to remove all traces of disinfectant (it can corrode metal), then dry them before putting them away.