How to Manage Pests
UC Pest Management Guidelines
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
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.
The use of overhead sprinklers to prevent freezing and sprays of
approved fixed copper products or streptomycin sulfate are organically
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.
||Amount to Use
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.
||4.8 oz/100 gal
Streptomycin-resistant strains of blossom blast bacteria are present in some areas. Do not exceed 48 oz/acre/application.
OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
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.
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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