Advice

Bacterial canker (Pseudomonas syringae pv. morsprunorum and P. syringae pv. syringe)

Plants affected

Most common on cherries and plums, but may also affect apricots, peaches and ornamental Prunus species including cherry laurel (P. laurocerasus)

Symptoms

Brown spots may develop on leaves in the spring, sometimes surrounded by a yellow halo. The spotted areas fall out giving a distinctive 'shot hole' appearance. Cankers enlarge rapidly on the stems and branches in spring. Stems and branches with cankers exude gum - particularly on cherries. Leaves on cankered stems are yellow, then rapidly shrivel and die.

Cause

The bacteria Pseudomonas syringae pv. morsprunorum and the closely related P. syringae pv. syringae cause this disease. Infections occur through leaf scars and wounds. These give rise to small cankers in which the bacteria survive the winter.

Control

Pruning should be carried out between June and August during dry weather, as infection of the branches occurs in autumn and winter. Between these months there is also little risk of infection, via the pruning wounds, by the damaging fungal disease silverleaf. If pruning out canker-affected branches, wipe the pruning tool with disinfectant between each cut.

Spray the foliage, branches and trunk thoroughly with Bordeaux mixture or copper oxychloride (Murphy Traditional Copper Fungicide) in late summer and autumn to protect leaf scars and bark wounds from infection. Three applications of the fungicide are necessary, at monthly intervals from about mid-August to mid-October.