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Citrus

Bacterial Blast

Pathogen: Pseudomonas syringae

(Reviewed 7/03, updated 7/03)

In this Guideline:

Symptoms
Comments on the disease
Management

SYMPTOMS
Infections usually start as black lesions in the leaf petiole and progress into the leaf axil. Once the petiole is girdled, leaves wither, curl, and eventually drop. Entire twigs may die back. The damage is most severe on the south side of the tree, which is exposed to the prevailing winter winds. Diseased areas are covered with a reddish brown scab. Infections result in small black spots on the fruit.

COMMENTS ON THE DISEASE
Bacterial blast, also known as citrus blast and black pit, is restricted mainly to citrus growing areas in the Sacramento Valley where wet, cool, and windy conditions during winter and spring favor development and spread of the blast bacterium. Leaves and twigs of oranges and grapefruit and the fruit of lemon are most susceptible to infection. The bacterium infects small injuries caused by thorn punctures, wind abrasions, or insect feeding.

MANAGEMENT
Preventative treatment against bacterial blast alone is generally not economical, but sprays against brown rot or Septoria may provide some protection against bacterial blast. Certain cultural practices can reduce the incidence of bacterial blast.

Cultural Control
Planting windbreaks and using bushy cultivars with relatively few thorns help prevent wind injury; pruning out dead or diseased twigs in spring after the rainy period reduces the spread of the disease; and scheduling fertilization and pruning during spring or early summer prevents excessive new fall growth, which is particularly susceptible to blast infection.

Organically Acceptable Methods
Cultural controls and bordeaux sprays are acceptable for use in organically managed citrus groves.

Treatment Decisions
In the Sacramento Valley where blast is an annual problem, apply yearly treatments.

Common name

(trade name) Amount to Use

A. BORDEAUX# (10-10-100) 10-25 gal/tree

COMMENTS: Apply from Oct-Nov, before the first rain. On mandarin trees, apply after fruit is picked to avoid undesirable residue.

# Acceptable for use on organically grown produce.

PUBLICATION

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Diseases
H. D. Ohr, Plant Pathology, UC Riverside
J. A. Menge, Plant Pathology, UC Riverside

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