Recently, I have been getting a few phone calls on a disease called Fire Blight. Fire Blight (Erwinia amylovora) is a devastating bacterial disease that affects many plants, but especially apples and pears. Being a bacterium, a wounded area is needed for infection. Unlike fungal pathogens, bacteria cannot penetrate the host on their own. Fire blight occurs early in the year during warm and humid weather. Young, succulent growth is very susceptible to this disease. Cultural practices such as proper pruning, correct fertilization, and site selection can help to minimize fire blight. Pruning during the dormant season is the preferred time; however, if an infection is severe in the spring, then measures need to be taken. If steps are not taken to remove some of the blighted areas of the tree, then fast-spreading infections can seriously harm the tree and rootstock. The Catch-22 about pruning during the growing season is that it may spread the disease even more! If the infection is light and isolated, it is appropriate to prune out the infected branches. But, if the infection is widespread, cutting may not help much. Heavy pruning done during the growing season will generate new, succulent growth for the blight to attack. Young trees are especially at risk, therefore scouting for the disease early in the season and subsequent removal of infected areas are necessary. If the bacteria gets into the rootstock it can kill the tree, especially young trees up to 7 years old. Pruning of fire blight infected shoots should never be done during wet conditions, including heavy dews. Any free-flowing water can wash the bacteria from an infected site to a fresh pruning wound. Sanitation of pruning equipment is very important to control fire blight. Pruning equipment should be sterilized in a 10% bleach solution for one minute between cuts. Using a pair of pruners will help speed up the process. Pruning during the dormant season doesn’t require any sterilization. Also, do not leave winter prunings on the ground too long because they can be a source of inoculum once the weather warms up. When cutting during the growing season, be careful not to brush pruned branches against other parts of the tree if possible, as this can spread the disease as well. Using a copper spray product in the dormant season helps control fire blight. During the season, antibiotics can be used, but these should not be used without caution as resistance can occur.
The following links have lists of apples, apple rootstocks, and pears and their susceptibilities to fire blight:
http://www.caf.wvu.edu/Kearneysville/tables/fbrootsus.html — apple rootstocks