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Blossom blast

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Pear blossom blast Pseudomonas syringae

Overview

  • Blossom blast of pear is a bacterial disease caused by Pseudomonas syringae. Pseudomonas syringae is favored by cool, wet weather and is distributed by splashing rain and insects. The severity of blossom blast can be decreased by a combination of a delayed-dormant copper spray plus two streptomycin sprays applied during bloom, however, it must be applied pre-symptom.
  • Symptoms of blossom blast include blackening of the calyx end of individual fruitlets, entire blossom clusters with cluster leaves remaining unaffected, or complete death of clusters including both the blossoms and the leavees. Severe outbreaks of blossom blast on pears are usually associated with spring frosts because frost injury provides entry sites for infection.
  • The severity of blossom blast can be decreased by a combination of a delayed-dormant copper spray plus two streptomycin sprays applied during bloom, however, it must be applied pre-symptom.
  • From: Rosenberger, D.A. 1996. Pear blossom blast. Scaffolds Fruit Journal (June 3); 4. Blossom Blast.

Symptoms and disease cycle

Blossom blast is a bacterial disease caused by Pseudomonas syringae. Pseudomonas syringae is favored by cool, wet weather (as compared with fire blight bacteria, which are favored by warm, wet weather). Pseudomonas syringae survives and grows on the surface of many different plant species and is distributed by splashing rain and by insects. Severe outbreaks of blossom blast on pears are usually associated with spring frosts because frost injury provides entry sites for infection. In the Columbia County (NY) orchards I observed, many of the developing fruitlets had frost rings, indicating that fruit were damaged by a frost (probably on May 14). However, less than 5% of the flower clusters were affected by blast.

Symptoms of blossom blast include blackening of the calyx end of individual fruitlets, blackening of entire blossom clusters with cluster leaves remaining unaffected, or complete death of clusters including both the blossoms and the leaves. Young leaves near affected clusters may have small nondescript necrotic leaf spots. Unlike fire blight, blossom blast does not spread into larger limbs or cause extensive damage to pear trees. However, heavy infections can seriously reduce fruit set.

In its early stages, when clusters of blossoms are just beginning to turn black, blossom blast can easily be confused with fire blight. However, symptoms of blossom blast are likely to appear earlier in the season than would be expected for fire blighted blossoms. (The expected appearance of blossom blight symptoms associated with fire blight can be determined using the MaryBlyt program.) Fire blight bacteria produce a toxin that is rapidly transported to adjacent leaves and causes blackening of leaf veins, whereas perfectly healthy leaves are often found just below clusters of flowers killed by blossom blast. Clusters affected by fire blight frequently have drops of bacteria ooze on their surface, sometimes visible only with a hand lens, whereas ooze is not visible with blossom blast. Although no fruit grower is ever happy about having diseases in their trees, most pear growers are happy when someone diagnoses dead flower clusters as resulting from blossom blast rather than from fire blight.

From: Rosenberger, D.A. 1996. Pear blossom blast. Scaffolds Fruit Journal (June 3); 4. Blossom Blast.

Chemical control

Nothing can be done to change the course of the disease after symptoms of blossom blast appear, and no further spread of the disease will occur after bloom. When anticipated in advance, the severity of blossom blast can be decreased by a combination of a delayed-dormant copper spray plus two streptomycin sprays applied during bloom (See Plant Dis. Reptr. 61:311-312 [1977] and The GoodFruit Grower, 1 July 1986, pages 22-23). Unfortunately, streptomycin sprays timed to control fire blight infection periods will usually be applied too late during the bloom period to provide complete control of blossom blast. One of the Columbia County orchards affected this year (1996) had been protected with copper plus two streptomycin sprays timed for fire blight.

From: Rosenberger, D.A. 1996. Pear blossom blast. Scaffolds Fruit Journal (June 3); 4. Blossom Blast.

Resistance (Eastern U.S.)

Not applicable

Non-chemical control

Not applicable