Fire Blight: Symptoms, Disease Cycle, and Management for Rosaceous Plants

Pathogen and Hosts

Fire blight is caused by the bacterium Erwinia amylovora, attacking >75 species of trees and shrubs including apple, pear, quince, mountain ash, crabapple, hawthorn, cotoneaster, serviceberry, and pyracantha. The bacterium overwinters in darkened, slightly sunken cankers on infected plants. In spring, bacteria are dispersed by insects, rain, wind, and animals.

Symptoms

• Twigs, branches, and leaders wilt and blacken, especially during flowering.
• Affected twigs/branches may bend into a shepherd's crook.
• Blackened flower parts remain attached.
• Cream-colored liquid may ooze from cankers in wet spring conditions.

Life History

Bacteria carried from infected tissue/oozing liquid to natural openings/wounds by flower-visiting insects, rain, wind, birds, crawling insects. Infection sites: nectaries, flower parts, hydathodes, stomates, small wounds on succulent twigs/branches. Succulent tissues blackened and killed; bacteria move into and girdle branches/trunk. Slightly sunken, darkened canker forms with a distinct dark line at its edge. Plants most susceptible during flowering and new shoot development, but spread can continue later in season.

Favorable Conditions

• Fertilization promoting succulent growth increases susceptibility.
• Moderately high temps (70–81°F / 21–27°C), high relative humidity, and rainfall during flowering = optimum.
• Hail or windblown soil injury opens tissue to infection.

Management

• Grow resistant varieties whenever possible.

Resistant Crabapple Cultivars: Adams, Callaway, David, Dolgo, Harvest Gold, Indian Summer, Jewelberry, Liset, Profusion, Red Baron, Selkirk, Sentinel.

Resistant Pyracantha Cultivars: Mojave, Navaho, Teton, Shawnee.

Resistant Cotoneaster: C. anoenus, C. adpressus, C. canadensis, C. dammeri var. radicans, C. horizontalis, C. microphyllus, C. praecox, C. zabelii.

Resistant Hawthorn (Crataegus): C. arnoldiana, C. coccinea, C. crus-galli, C. douglasii, C. phaenopyrum, C. prunifolia, C. punctata 'Ohio Pioneer', C. viridis 'Winter King'.

Resistant Mountain Ash (Sorbus): S. aucuparia, S. intermedia.

• Do not purchase or plant infected material. Plant only fire blight-free trees/shrubs.
• Remove severely infected plants.
• Once disease begins, implement 3-pronged management:
  1. Dormant season: Closely examine susceptible plants; prune out infected tissues (blackened twigs, branches, flower parts; sunken, darkened cankers). Prune in dry weather, cutting ≥4 inches below canker. Disinfest tools between cuts.
  2. Growing season: Do not prune infected tissues. Pruning during growing season spreads disease even with careful tool cleaning (Blachinsky et al., 2003, Plant Disease 8:1077-1082).
  3. Apply bactericide (where registered) before bud break to protect flowers from initial infection. *Some E. amylovora populations resistant to copper and/or streptomycin.

Fire blighted flowers; Fire blighted twig with discolored canker area under bark.