Symptoms and Signs
Infections caused by Pseudomonas syringae 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 or 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
A preventive spray 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
- Plant windbreaks and use bushy cultivars with relatively few thorns to prevent wind injury.
- Prune out dead or diseased twigs in spring after the rainy period to reduce the spread of the disease.
- Schedule fertilization and pruning during spring or early summer prevents excessive new fall growth, which is particularly susceptible to blast infection.
Organically Acceptable Methods
Use cultural controls and copper and Bordeaux sprays in organically managed citrus groves.
Treatment Decisions
In the Sacramento Valley where blast is an annual problem, spray each year at the onset of cool, wet periods.
| Common name | Amount to use | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | BORDEAUX# (10-10-100) | 10–25 gal/tree | See comments | See comments |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| COMMENTS: Apply from October through November, before the first rain. On mandarin trees, apply after fruit is picked to avoid undesirable residue. Not all copper compounds are approved for use in organic production; be sure to check individual products. For information on making Bordeaux mixture, see UC IPM Pest Note: Bordeaux Mixture, UC ANR Publication 7481. Be sure to follow label directions as well. For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank-mixing products that contain the same a.i. Use restricted entry interval and preharvest interval of the most restrictive label of those products used in tank mix. | ||||
| B. | FIXED COPPER# | Label rates | 24 | 0 |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| COMMENTS: Not all copper compounds are approved for use in organic production; be sure to check individual products. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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. 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. |