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 below ground.
Comments on the Disease
Pseudomonas syringae survives in or on plant surfaces and is spread by splashing rain. It 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. Properly irrigate and fertilize young trees during the growing season. Trees on Lovell peach rootstock are more resistant than others; and 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 in California.
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. Nematodes stress trees, which predisposes them to bacterial canker. 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 NEMATODES.
| 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/acre | see label | see label |
| COMMENTS: Must be applied under a Critical Use Exemption only. Use higher rates for fine-textured soils. Fumigants such as methyl bromide are a prime source of volatile organic compounds (VOCs), which are a major air quality issue. 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. |