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 survives 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 rarely occurs in first year of planting, and is uncommon in nurseries.
Management
The occurrence of bacterial canker is thought to be related to the amount of stress trees are subjected to, including poor nitrogen and/or microelement availability, high ring-nematode populations, previous drought stresses, hardpan, rootstocks that reduce tree vigor, spring freezes, and irrigation methods that wet the tree. If ring-nematode populations are present in an orchard site, preplant fumigation is important. Also important are rootstock selection, proper nitrogen fertilization, and the use of drip or microsprinkler irrigation, which may help with nutrient uptake. Of the rootstocks commonly used for cherries in California, Mahaleb is the most tolerant of bacterial canker, Colt is moderately susceptible, and Mazzard is susceptible. Spring and summer pruning may also help. Fall and dormant season copper sprays have been used by some growers to help manage this disease, but research in California orchards has not shown this practice to be consistent or reliably effective. There is also widespread copper resistance in pathogen populations in commercial orchards.
Treatment Decisions
In light, sandy soils and in some heavy soils, control has been achieved with preplant fumigation for nematodes. Ring nematodes predispose cherry trees to bacterial canker. The benefits of preplant soil fumigation for control of bacterial canker usually last only a few years; in some areas only limited improvements in disease control occur following soil fumigation.
| 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* | 300-600 lb | See label | NA |
| COMMENTS: Use allowed under List of Approved Critical Uses. 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. |
| NA | Not applicable. |