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 or blast of flowers and young shoots. The sour sap phase of decline may not show gum and cankers, but the inner bark can be brown, fermented, and sour smelling. Flecks and pockets of bacterial invasion in bark occur outside canker margins. Frequently, infected trees sucker from near ground level; cankers do not extend belowground.
Comments on the Disease
Pseudomonas syringae 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. Young trees, 2 to 8 years old, are most affected. The disease rarely occurs in first year of planting and is uncommon in nurseries.
Management
Planting trees that are budded or grafted about 32 inches above the root crown can help suppress bacterial canker infections. 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. Trees on Lovell peach rootstock are more resistant than others; those on plum rootstocks are most susceptible. Delayed pruning may help.
In light sandy soils and some heavy soils, successful control has been achieved with preplant fumigation for nematodes. Application of copper during dormancy has not been shown to protect against bacterial canker.
Chemical Control
Nematodes stress trees, which predisposes them to bacterial canker. Preplant fumigation for nematode control reduces the severity of bacterial canker in newly planted orchards. The benefits of preplant soil fumigation for control of bacterial canker usually lasts only a few years; in some areas only limited improvements in disease control occur following soil fumigation. For additional information, see the nematode section. Bactericide applications have no reliable effect on bacterial canker and their use is not recommended.
| 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. | ||||
| PREPLANT | ||||
| A. | METHYL BROMIDE* | 300–600 lb | 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. | ||||
| ‡ | 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. |