Symptoms and Signs
Symptoms are most obvious in spring and include limb dieback with rough cankers and amber-colored gum. There may also be leaf spot and blast of young flowers and shoots. The sour sap phase of bacterial canker may not show gum and cankers, but the inner bark is brown, fermented, and sour smelling. Flecks and pockets of bacterial invasion in bark occur outside canker margins. Frequently, trees sucker from near ground level; cankers do not extend below ground.
Comments on the Disease
Pseudomonas syringae is a ubiquitous bacterium that survives in or on plant surfaces, is spread by splashing rain, and is favored by high moisture and low temperatures in spring. The disease is worse in low or sandy spots in the orchard. Vigorous trees are less susceptible to bacterial canker, while young trees, 2 to 8 years old, are most affected. The disease complex rarely occurs in the first year of planting and is highly uncommon in nurseries. It is a frequent problem in replant situations; however, the severity of bacterial canker in an orchard is highly correlated with the presence of ring nematode in the soil.
Management
The key to managing bacterial canker is keeping the trees as tolerant as possible to the disease rather than trying to kill the bacterial pathogen. Problems with bacterial canker can be minimized at planting by carefully selecting the planting site, choosing the least susceptible rootstocks, and following recommended cultural practices regarding pruning and fertilizing. Bacterial canker tends to mostly affect weak trees, so any management practice that improves tree vigor (e.g., lighter, more frequent irrigation, improved tree nutrition, nematode management, etc.) will help to reduce the incidence of this disease. Delayed pruning may help. Lovell peach and Viking rootstocks are usually more tolerant than Nemaguard.
In light, sandy soils and in some heavy soils, control has been achieved with preplant fumigation for ring nematodes. Applications of copper are not effective against bacterial canker.
Organically Acceptable Methods
Cultural controls are acceptable for use in an organically certified crop.
Chemical Control
Bactericide applications have no reliable effect on bacterial canker and their use is not recommended. Preplant fumigation for nematode control reduces the severity of bacterial canker in newly planted orchards. Ring nematodes stress trees, which predispose them to bacterial canker. The benefits of preplant soil fumigation for control of bacterial canker usually last only a few years, and in some areas only limited improvements in disease control occur following soil fumigation.
Following planting, if bacterial canker occurs in an orchard, apply nematicide around all trees in the affected area of the orchard on a yearly basis until the trees are 8 years old.
| Common name | Amount per acre | 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. | ||||
| PREPLANT | ||||
| A. | METHYL BROMIDE* | Label rates | See label | See label |
| COMMENTS: Must be applied under a Critical Use Exemption. Use higher rates for fine-textured soils. Fumigants such as methyl bromide are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone; methyl bromide depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available. | ||||
| B. | 1,3-DICHLOROPROPENE* | |||
| (Telone II) | 27–35 gal | 5 days | NA | |
| COMMENTS: This restricted-use product is applied only by professional fumigation companies.. In California the applications must be applied to soils having a moist surface; this task is difficult to achieve without use of sprinklers unless there is a fortunate rainfall. Do not flood irrigate prepared lands to achieve this surface moisture requirement. Broadcast apply where nematode resistance is unavailable for prevailing nematodes. Fumigants such as 1,3-dichloropropene are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available. | ||||
| ‡ | 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. |
| * | Permit required from county agricultural commissioner for purchase or use. |
|---|---|
| NA | Not applicable. |