What is Fireblight?
Fireblight is a destructive bacterial pathogen that affects a large range of plants residing in the Rosaceae family. Fire blight attacks blossoms, leaves, shoots, branches, fruits, and roots. The bark at the base of blighted twigs becomes water soaked, then dark, sunken, and dry; cracks may develop at the edge of the sunken area. Young twigs and branches die from the terminal end and appear burned or deep-rust colored. Branches may be bent, resembling what is commonly referred to as a “shepherd's crook.” Dead leaves and fruit remain on the branches.
Initially, the disease often enters the tree through natural openings, especially flowers and wounds in the spring. Once established in the tree, fireblight quickly invades through the current season's growth into older growth. Fireblight can be spread from diseased to healthy plants by rain, wind, and pruning tools. Fireblight is identified by three main symptoms: shepherd’s crook, blackened leaves, and bacterial canker. Fireblight cankers on stems appear as slightly sunken, dark discolored areas with a narrow callus ridge. The inner bark of the infected stem turns from green to brown. The visual symptoms may slightly vary depending on weather conditions and plant variety. During warm and moist conditions, infected tissue may exude bacterial ooze in droplets.
How to remove Fireblight
During spring and summer, prune out infected branches 8 inches below the damaged tissue. Avoid pruning when the plants are wet. Dip pruning tools in 70 percent isopropyl alcohol (rubbing alcohol) or 10 percent bleach solution (1 part bleach to 9 parts water solution) between each cut. Wash and oil shears when you are finished. These practices avoid spreading the pathogen.
Avoid heavy nitrogen fertilization, especially in summer, when succulent growth is most susceptible to fireblight infection. Avoid splashing water. Chemical control is not always effective and needs to be applied preventively. Therefore, in years when warm, humid, wet weather coincides with flowering and leaf emergence, spray plants with a fungicide containing basic copper sulfate (Kocide) or an antibiotic (Agrimycin) to reduce infection. Applications of Agrimycin need to begin at the start of blooming and continue every 3-4 days during the bloom period until flowers are gone. Application of Kocide should begin at bloom and continue every 7 days during bloom. Re-application following rain may be needed.