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How to Manage Pests

UC Pest Management Guidelines

Apple blossoms infected with bacterial blast.


Bacterial Blossom Blast

Pathogen: Pseudomonas syringae

(Reviewed 8/06, updated 3/09)

In this Guideline:


Fruit buds infected in early bloom stop growing, turn brown and papery, and may drop off. Later infections affect flower petals and stems, and fruit cluster bases turn brown or black. Infection usually does not move beyond the base of the fruit cluster; sometimes, however, fruit spurs are killed. Flower symptoms may closely resemble fire blight, but bacterial ooze is never present with blast as it is with fire blight.


Pseudomonas syringae is a widespread bacterium that infects many plant species and occasionally causes bacterial blossom blast on apples. Frost or freezing temperatures and free moisture are required for Pseudomonas infection to occur. Warm, dry weather halts its development.


Bacterial blossom blast is difficult to control. When possible, use overhead sprinklers to prevent freezing. Monitor temperatures wherever freezing is most likely to occur in the orchard and turn on sprinklers when the temperature drops below 34°F (1.1°C). Turning on sprinklers before the initial drop in temperature helps avoid the damage that results from evaporative cooling. Uniform coverage of all tree parts to produce a combination of water and ice is essential. Finally, keep sprinklers going until temperatures are well above 32°F (0°C) and the danger of frost is past.

Organically Acceptable Methods
The use of overhead sprinklers to prevent freezing and sprays of approved fixed copper products or streptomycin sulfate are organically acceptable methods.

Treatment Decisions
As an alternative to using overhead sprinklers, treat weekly with copper materials or antibiotics from green tip through bloom if blossom blast has been a problem and a freeze is likely. Apply chemicals before a freeze; treatment after symptoms appear is ineffective.

Common name   R.E.I.+ P.H.I.+
(trade name) Amount to Use (hours) (days)

Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy. When choosing a pesticide, also consider information relating to environmental quality. Not all registered pesticides are listed. Always read label of product being used.
  (Agri-Mycin 17) 4.8 oz/100 gal 12 50
  COMMENTS: Streptomycin-resistant strains of blossom blast bacteria are present in some areas. Do not exceed 48 oz/acre/application.
B. FIXED COPPER# Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Application of copper-containing materials beginning at or about green tip may cause fruit russetting on some cultivars, including Granny Smith. Copper-resistant strains of blossom blast bacteria are present in some areas. Not all copper compounds are approved for use in organic production; be sure to check individual products.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
# Acceptable for use on organically grown produce.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1,4,9,11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Apple
UC ANR Publication 3432
W. D. Gubler, Plant Pathology, UC Davis
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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