Figure 1. Blossom cluster and adjacent shoot with fire blight. (Alan R. Biggs, West Virginia University)
Fire blight is a destructive bacterial disease of apples and pears that kills blossoms, shoots, limbs, and, sometimes, entire trees (Figs. 1, 2). The disease is generally common throughout the United States wherever apples are grown. Outbreaks are typically very erratic, causing severe losses in some orchards in some years and little or no significant damage in others.
Figure 2. Apple shoot with fire blight showing typical "shepherd's crook" symptom associated with wilting. (Alan R. Biggs, West Virginia University)
This erratic occurrence is attributed to differences in the availability of overwintering inoculum, the specific requirements governing infection, variations in specific local weather conditions, and the stage of development of the cultivars available. The destructive potential and sporadic nature of fire blight, along with the fact that epidemics often develop in several different phases, make this disease difficult and costly to control.
Of the apple varieties planted in the eastern region, those that are most susceptible include:
These cultivars are moderately resistant:
All strains of 'Delicious' are highly resistant to fire blight, except when tissues are damaged by frost, hail, or high winds
For more information, see Table of Apple Cultivar Relative Fire Blight Susceptibility.
Additional description of the diversity of fire blight symptoms, the disease cycle, and management recommendations: http://www.caf.wvu.edu/kearneysville/disease_descriptions/omblight.html
Original text prepared by P. W. Steiner, T. van der Zwet, and A. R. Biggs