How to Manage Pests
UC Pest Management Guidelines
Bacterial Blossom Blast
Pathogen: Pseudomonas syringae
(Reviewed 8/06, updated 3/09, pesticides updated 10/15)
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.
COMMENTS ON THE DISEASE
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 are organically acceptable methods.
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.
UC IPM Pest Management Guidelines: Apple
W. D. Gubler, Plant
Pathology, UC Davis
Acknowledgment for contributions to Diseases:B. L. Teviotdale, Kearney Agricultural Center, Parlier