Pacific Northwest 1998 An Online Guide to Plant Disease Control

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Pear -- Pseudomonas Blossom Blast and Dieback
 
Cause: Pseudomonas syringae pv. syringae, a bacterium. Rain and low temperatures, especially frost-inducing temperatures during bloom, increase incidence of blossom infection. Two common genetic traits increase the bacteria’s ability to cause disease. Most produce a powerful plant toxin, syringomycin, that destroys plant tissues as bacteria multiply in a wound. Bacteria also produce a protein that acts as an ice nucleus, increasing frost wounds that bacteria easily colonize and expand.

Severe on wood--'Old Home' and Asian pear cultivars.

Severe blossom blight--'Packham's Triumph', 'Bartlett', 'Eldorado', 'Anjou', and 'Bosc'.

Less severe blossom blight--'Comice', 'Forelle', red 'Anjou', and red 'Bartlett' cultivars.

Apple is not as susceptible and generally does not need control.

Symptoms: Infection may cause blossom blast, leaf spots, dieback of twigs and spurs, dormant-bud death, and bark cankers. At first, bark cankers are light brown, irregular patches on limbs. Later, outer bark and some underlying tissues may wholly or partly slough away.

The blossom blast may closely resemble that of fire blight but is different in that blast seldom extends more than 1 to 2 inches into the spur and never involves a bacterial exudate. The blossom blast also may resemble that caused by Botrytis. May also cause canker on young trees.

Note that blossoms become increasingly necrotic when inoculated with higher concentrations of bacteria.

Shoot dieback from this bacterial disease.

Whole shoots can dieback from this disease.

Under favorable conditions many shoots can dieback in the spring.

Flowering pear can be a host also as seen on this Pyrus betulifolia.

Pyrus ussuriensis is another host.
 
Cultural control:

  1. Protect from frost using a variety of methods including overhead irrigation or wind machines.
  2. Prune Asian pear cultivars in summer.
  3. Remove and destroy infected tissue.
Chemical control: Resistance to the active ingredients in these products has been detected in the Hood River, OR area.

  1. Champ Formula 2 at 1 to 1.3 gal/A before fall rains and again before spring growth starts. Excessive dosages may cause fruit russet. Champion WP can be used for organic production. 24-hr reentry.
  2. Copper-Count-N at 8 to 12 quarts/A. 12-hr reentry.
  3. Cuprofix Ultra 40 Disperss at 7.5 to 10 lb/A. 12-hr reentry.
  4. Kocide 3000 at 5.25 to 7 lb/A before fall rains and again before spring growth starts. Excessive dosages may russet fruit. 24-hr reentry.
  5. ManKocide at 12 to 16 lb/A before fall rains and again before spring growth starts. Excessive dosages may russet fruit. 24-hr re-entry.
  6. Nu-Cop 50 DF at 12 to 16 lb/A. May cause fruit russet. Nu-Cop WP can be used for organic production.24-hr reentry.
  7. Streptomycin or terramycin applied at early bloom to control fire blight helps control Pseudomonas blossom blast. There are several registered trade names for products that contain streptomycin sulfate including Agricultural Streptomycin, Agri-Mycin 17, Bac-Master, Farmsaver, Firewall and Streptrol.
References:
Whitesides, S.K. and R.A. Spotts. 1991. Induction of pear blossom blast caused by Pseudomonas syringae. Plant Pathology 40:118-127.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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