Fire blight is a bacterial disease mostly affecting apples, pears, Asian pears, and close relatives like quinces and hawthorns. It gets its name because it causes young fruits, shoots, and branch tips to appear blackened and shriveled, as though scorched by fire. Dead leaves remain on the tree and shoot tips curl downward.
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Fire Blight Pathology 

Fire blight is mostly contracted through spring blossoms of fruit trees. Honeybees can be a carrier of the disease after they’ve pollinated an infected blossom. Once a tree is infected, it carries the disease indefinitely unless removed through pruning. An infection can be spread from the blossoms and branch tips to the rest of the tree and appears as a black discoloration in the bark.

Weather conditions greatly affect the rate of the disease spread. The disease is most seriously spread during wet springs, especially if it is warm and rainy during bloom-time or pre-boom.

Fire Blight Treatment and Prevention 

An otherwise healthy tree showing signs of Fire Blight can be cared for an managed through pruning of damaged areas. Prune off diseased wood as soon as you notice it to prevent it from spreading. Pruning cuts should be made into healthy wood, at least 6 to 12 inches below where you see any sign of infection.

It is very important to sterilize your tools between each pruning cut (good general practice anytime you’re pruning during active growing season) to avoid spreading the disease. Use rubbing alcohol or a 10% bleach 90% water solution to sterilize. This can be done by dunking your pruner or saw blade or by using a spray bottle to coat the blade in between each cut. Prevention of Fire Blight starts with selecting disease resistant varieties of your trees (see link below). A copper sulfate spray in early spring before infection also can reduce your chances of contracting the disease. Unfortunately, there is no ultimate cure for Fire Blight, and the best way to avoid it is to carefully monitor your trees for early signs and take preventative measures.

MORE INFO 

Here is a good primer on Fire Blight including lists of resistant varieties of apples, pears, and Asian pears:
Sources:

This edition of POP TIPS prepared with assistance from 2014 POP intern Megan Bazin. 
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