Causal Agent

Xanthomonas pruni (synonym X. campestri pv. pruni or X. arbicola pv. pruni)

 

Hosts

Almond, Apricot, Nectarine, Peach and Plum.

 

Symptoms

The pathogen causes spotting of the leaves, fruit and new twigs. If the disease has been a problem in past seasons, cankers may be visible on older limbs. Leaf spots appear as angular lesions which are often delimited by veins. It may be easier to see the leaf spots if the leaves are held up to the sunlight. The lesions are usually surrounded by a water-soaked area that is somewhat translucent when held up to light. The center of the lesion usually drops out and leaves a shot-hole appearance. Early symptoms on the fruit are purple-black flecks that may have a water-soaked border. The spots enlarge and the spot will become sunken. Occasionally, the flesh will be cracked if the infection occurs early in the season. The fruit lesions generally make the fruit unmarketable. Two types of cankers appear on the twigs and are categorized as spring or summer cankers. The spring cankers will develop as darkened, broken areas that are near last year's growth. Twigs will occasionally be girdled by spring cankers and the shoot will be blighted. Summer cankers will appear in mid-season and will develop on the middle of the twig on this year's growth.

 

Control

The plants should be monitored from spring through mid-summer. Examine leaves for leaf lesions. They are often more concentrated near the major veins and when held up to the light, it is easier to see the water soaking around small flecks. There is no established threshold for when to begin treating for bacterial spot, but treatments should start when the first lesions are observed on the leaves. If the disease has been a problem in the past, it may be advised to start treating at budbreak. Protective chemicals should be applied at 7-14 day intervals until the weather turns hot and dry. If the weather is predicted to be wet with moderate temperatures, the period between applications should be lower (7 days). If the weather is expected to be dry, then the interval may be extended. Other cultural methods that can be used include raking up and destroying fallen leaves, prune branches to increase air circulation in the canopy and selecting resistant cultivars.