P&PDL Picture of the Week for
June 5, 2006

Fire blight

Gail Ruhl, Sr. Plant Disease Diagnostician, Dept of Botany & Plant Pathology, Purdue University

Fire blight is a serious bacterial disease that is most damaging in years when above normal spring temperatures are coupled with frequent rains during the blossoming period. The bacterial pathogen, Erwinia amylovora infects about 75 different plant species, all in the Rosaceae family. Trees most severely affected in Indiana include many varieties of apple and pear, susceptible crabapples, mountain ash, hawthorn, cotoneaster, and pyracantha. Ornamental pear ‘Aristocrat’ has exhibited epidemic blossom blight this year.

Fire blight may appear as a blossom blight, shoot blight, or branch and trunk canker. Infected blossoms wilt and turn brown to black. The bacteria may move from the blossoms, down the pedicel, and into the fruit spur. Leaves then wilt, die, and turn brown to black, usually remaining attached to the tree for the summer. Shoot blight is easily recognized by the rapid dieback of shoots. Often the tip of the shoot bends over to resemble a shepherd’s crook. If the infection continues down a shoot or flower spur into a larger branch, then a canker may form. The canker is often sunken, with darkly colored bark. The pathogen overwinters in cankers on the trunk and branches and in the spring droplets of sticky, amber-colored bacterial ooze form from these cankers. Insects and splashing rain spread the bacteria from the droplets to blossoms and twigs. Warm weather and rain during the flowering period are optimal for infection to occur.

We do not recommend pesticide (streptomycin and copper) spraying at this time since there are no pesticides that will cure or prevent further spread of blight. Secondly, be careful not to over fertilize or attempt any other cultural practices that will stimulate plant growth since succulent growth is more susceptible to infection by the fire blight bacteria.

Extensive pruning of fire blighted trees at this time will generally only result in spreading the disease and stimulate the growth of even more new, susceptible tissue. If only a “limited” number of blossom clusters and
branches are affected then prune out affected areas, cutting 10-12 inches below any symptoms of disease. Sterilize cutting tools between each cut by dipping them in a freshly made solution of 1 part liquid bleach added to 9 parts of water, however, be sure to rinse tools thoroughly with water before putting them away to prevent corrosion from the bleach water. Pruning shears may also be dipped in a solution of 70% denatured alcohol or 5% Lysol. No doubt some stem/limb death will occur over the summer. In late winter, prune out all dead tissue (dormant pruning does not require sterilization of cutting tools). Dormant copper sprays may be used following label directions.

The following control measures are provided as information for next season:

  • Streptomycin is a bactericide that must be applied every 4 days during the blossoming period. It is labeled for use by the homeowner on apple, pear, and crabapple, starting when 10 to 20 percent of the blossoms are open. Streptomycin is available in homeowner-usable formulations.
  • The preferred cultural management practices for homeowners are proper pruning, balanced fertilization, and planting resistant varieties.

For more information on Fire Blight - BP-30

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Fire blight

Fire blight

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service