Mountain Ash (Sorbus spp.)-Fire Blight

See:

Cause Erwinia amylovora, bacteria that enters the plant through blossoms, vigorously growing shoot tips, young leaves, and wounds. This is the same fire blight that attacks apple, pear, and other closely related plants. It usually goes unnoticed on ornamentals such as cotoneaster, hawthorn, and pyracantha. Rain and pollinating insects easily spread the bacteria. It is a problem mostly east of the Cascade Range when warm temperatures occur during bloom. European mountain ash (S. aucuparia) and the American mountain ash (S. americana) are very susceptible.

Symptoms Blossom clusters may appear blighted. Blossom blight may be inconspicuous and easily go unnoticed. Shoots suddenly wilt, turn black, and die back, presenting a "scorched" appearance. Purplish cankers may develop on the shoots. During wet or warm weather, cankers may ooze brown sticky droplets. Newly infected wood is reddish; older infections are black.

Cultural control

  • Avoid wounding plants.
  • Immediately prune out and destroy infected tissues. Make pruning cuts at least 6 inches below infected tissues. Sterilize tools between cuts. Do not do regular pruning at the same time as blight removal.
  • Use moderate amounts of nitrogen fertilizer to minimize vigorous growth of susceptible shoots.
  • Plant resistant trees such as S. americana 'Decorah'.

Chemical control

  • Dormant season for holdover cankers.
    • Bordeaux 8-8-100. Group M1 fungicide. O
  • Prebloom and bloom period.
    • Badge X2 at 1.5 to 2 lb/A. Group M1 fungicide. 48-hr reentry. O
    • CuPRO 5000 at 1.5 to 5 lb/A but only up to 2 lb/A when new growth is present. Group M1 fungicide. 48-hr reentry.
    • Phyton 27 at 2.5 to 4 oz/10 gal water. Group M1 fungicide. 48-hr reentry.
  • Junction at 1.5 to 3.5 lb/A. May be useful when bacteria are resistant to copper products alone. Group M1 + M3 fungicide. 48-hr reentry.
  • Arbor-OTC is registered for trunk injection, see label for details. Group 41 fungicide (antibiotic). 12-hr reentry.
  • Mycoject Ultra is labeled as a tree injection. Do not inject trees less than 2 inches in diameter. 12-hr rentry.

Note Aliette is registered but not recommended because control is too variable and it is not compatible with copper products.

Reference Van Der Zwet, T., Orolaza-Halbrendt, N., and Zeller, W. 2012. Fire Blight: History, Biology, and Management. APS Press/American Phytopathological Society.