How to Manage Pests
UC Pest Management Guidelines
Pathogen: Pseudomonas syringae
(Reviewed 10/14, updated 10/14)
In this Guideline:
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. Orange or red flecks and pockets of bacterial invasion under the bark occur outside canker margins. Frequently, trees sucker profusely from near ground level or on the limbs below infected areas; 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 bacterium is commonly found on healthy as well as diseased plants and becomes pathogenic only on susceptible or predisposed trees.
The disease is found almost exclusively in replanted orchards where ring nematodes flourish or in locations where spring frost is a problem. The disease is worse in low, gravelly, sandy spots, soils with shallow claypans (2-3 feet deep), or other soil conditions that lead to weakened growth. Vigorous trees are less susceptible to bacterial canker, while young trees (2–8 years old) are most affected. The disease rarely occurs in the first year of planting unless the ground is not fumigated before planting. It is uncommon in nurseries.
The key to bacterial canker management is control of ring nematodes and maintaining healthy, vigorous trees. Any management practice that improves tree vigor (e.g., lighter, more frequent irrigation with drip or microsprinklers, improved tree nutrition [especially nitrogen], etc.) will help reduce the incidence of this disease.
It is very important to fumigate sandy soils when apricot trees are to be planted following an old apricot, peach, almond, or other Prunus spp. orchard. Rootstocks of plum parentage (e.g. Myrobalan, Marianna 2624) are highly susceptible to bacterial canker. Lovell peach rootstocks are more tolerant than Nemaguard or apricot rootstocks. In soils with high levels of ring nematodes, annual fall treatments with a nematicide are beneficial. There is evidence that pruning during the dormant period may make trees more susceptible than pruning after trees become active in spring or pruning in summer. Copper sprays applied at the beginning and end of leaf fall have been tried with highly variable results &—resistance to copper may be a factor.
In light, sandy soils and in some heavy soils, control has been achieved with preplant fumigation for ring nematodes. Nematodes stress trees, which predisposes them to bacterial canker. The benefits of preplant soil fumigation for control of bacterial canker usually last only a few years.
UC IPM Pest Management Guidelines: Apricot
J. E. Adaskaveg, Plant Pathology, UC Riverside
Acknowledgment for contributions to Diseases:B. L. Teviotdale, Kearney Agricultural Center, Parlier