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Beginner

This pest affects:

Scientific Name
Xanthomonas arboricola pv. pruni

Identification
Leaves:

• Initially grayish water-soaked spots that develop into angular, brown to black or purple spots along midvein, tips and margins
• Spots dry out and drop, leaving angular and irregular “shot holes” giving the leaves a tattered appearance
• Affected mature leaves with as few as 2-3 lesions may turn yellow and fall off prematurely during July and August

Fruit:

• Small (1-2 mm) circular sunken, tan becoming dark brown, pinpoint spots
• Can develop into larger lesions, forming pits on the surface
• Fruit infected early in the season (up to 4 wks after petal fall) develop deep-pitted blemishes and may exhibit gumming
• Since the infected areas cannot expand with increased fruit size, the spots crack as the fruit matures. Pits or cracks on the fruit surface extend into the flesh and create large brown to black depressed areas on the fruit surface
• Lesions that develop from pit hardening through the pre-harvest period are usually superficial and give the fruit a mottled appearance; however, these shallow lesions may coalesce, resulting in cracking of the skin and subsequent infection by brown rot
• Infected fruit are unsuitable for processing as the spots extend into the flesh and are not easily removed during peeling
• Symptoms are typically more severe on plum than other stone fruit with water-soaked sunken spots, developing into gummy, brown/black necrotic lesions, 1 cm in diameter mid-season

Twigs:

Two distinct types of cankers damage twigs of peach and nectarine. These cankers are different from the large perennial cankers caused by fungi. "Spring cankers" develop on young twigs produced the previous summer. Spring cankers first appear as water-soaked, slightly darkened blisters about the time the first leaves appear. If these cankers encircle the twig, it will die. When conditions are moist, the canker surface has a black, water-soaked appearance. As the season progresses, the tissues over the blister-like lesions rupture and bacteria are released. These bacteria can be spread by windblown or splashing rain and can result in new infections. In time, spring cankers heal and become inactive. "Summer cankers" develop on green twigs of the current season's growth. They usually occur later in the summer after leaf spots are evident. At first, they are water-soaked, dark purplish spots. In time, they enlarge, turn brown to purplish-black and become slightly sunken and round to elliptical with water-soaked margins.

On susceptible plums and apricot cultivars, the bacteria may survive for 2 or 3 years, slowly enlarging and deepening the cankered area. The results are deep cankers deforming the small branches so they have a knotty appearance. Some of these branches may be killed or they may break from the weight of the fruit.

Often Confused With
X-disease – no bacterial oozing; irregular yellow spotting which becomes reddish purple with upward rolling of the leaf at the margins; both healthy and infected symptomatic branches on the same tree

Peach scab – no shot hole symptoms on leaves; lesions on fruit have a velvety, olive appearance

Nitrogen – damage usually most evident along the midrib and entire leaves may turn red. Leaf tissues surrounding shot holes caused by a nitrogen deficiency are more likely to have a reddish color than when bacterial spot is involved.  No fruit symptoms.

Captan spray injury – lesions are circular and occur mostly on young leaves, many of which may be damaged on only one side of the midvein.

Bird pecks – no corky injury to skin

Perennial (Cytospora) canker – black fruiting bodies on the bark surface or under the bark tissue

Period of Activity
Fruit are very susceptible to infection as soon as they are exposed at shuck split through pit hardening but new infections can develop until harvest.

Leaf infections can occur until terminal bud set.

Scouting Notes
On susceptible cultivars, begin monitoring at shuck split and continue weekly through late midseason by examining 50 fruit for small lesions, and 25 of the oldest leaves on each sample tree. Hold the leaves up to the light and look for angular, water-soaked lesions on the leaf blade. These lesions tend to be heaviest along the midvein and at the tip of the leaf, and will be visible within 24 hours of infection, which occurs after a rain or heavy dew. Within three days of initial infection, the lesions will be visible with reflected light and leaves will then begin to show the typical shot hole symptom.

Threshold
No threshold established.

Advanced

This pest affects:

Scientific Name
Xanthomonas arboricola pv. pruni

Identification
Leaves:

• Initially grayish water-soaked spots that develop into angular, brown to black or purple spots along midvein, tips and margins
• Spots dry out and drop, leaving angular and irregular “shot holes” giving the leaves a tattered appearance
• Affected mature leaves with as few as 2-3 lesions may turn yellow and fall off prematurely during July and August

Fruit:

• Small (1-2 mm) circular sunken, tan becoming dark brown, pinpoint spots
• Can develop into larger lesions, forming pits on the surface
• Fruit infected early in the season (up to 4 wks after petal fall) develop deep-pitted blemishes and may exhibit gumming
• Since the infected areas cannot expand with increased fruit size, the spots crack as the fruit matures. Pits or cracks on the fruit surface extend into the flesh and create large brown to black depressed areas on the fruit surface
• Lesions that develop from pit hardening through the pre-harvest period are usually superficial and give the fruit a mottled appearance; however, these shallow lesions may coalesce, resulting in cracking of the skin and subsequent infection by brown rot
• Infected fruit are unsuitable for processing as the spots extend into the flesh and are not easily removed during peeling
• Symptoms are typically more severe on plum than other stone fruit with water-soaked sunken spots, developing into gummy, brown/black necrotic lesions, 1 cm in diameter mid-season

Twigs:

Two distinct types of cankers damage twigs of peach and nectarine. These cankers are different from the large perennial cankers caused by fungi. "Spring cankers" develop on young twigs produced the previous summer. Spring cankers first appear as water-soaked, slightly darkened blisters about the time the first leaves appear. If these cankers encircle the twig, it will die. When conditions are moist, the canker surface has a black, water-soaked appearance. As the season progresses, the tissues over the blister-like lesions rupture and bacteria are released. These bacteria can be spread by windblown or splashing rain and can result in new infections. In time, spring cankers heal and become inactive. "Summer cankers" develop on green twigs of the current season's growth. They usually occur later in the summer after leaf spots are evident. At first, they are water-soaked, dark purplish spots. In time, they enlarge, turn brown to purplish-black and become slightly sunken and round to elliptical with water-soaked margins.

On susceptible plums and apricot cultivars, the bacteria may survive for 2 or 3 years, slowly enlarging and deepening the cankered area. The results are deep cankers deforming the small branches so they have a knotty appearance. Some of these branches may be killed or they may break from the weight of the fruit.

Often Confused With
X-disease – no bacterial oozing; irregular yellow spotting which becomes reddish purple with upward rolling of the leaf at the margins; both healthy and infected symptomatic branches on the same tree

Peach scab – no shot hole symptoms on leaves; lesions on fruit have a velvety, olive appearance

Nitrogen – damage usually most evident along the midrib and entire leaves may turn red. Leaf tissues surrounding shot holes caused by a nitrogen deficiency are more likely to have a reddish color than when bacterial spot is involved.  No fruit symptoms.

Captan spray injury – lesions are circular and occur mostly on young leaves, many of which may be damaged on only one side of the midvein.

Bird pecks – no corky injury to skin

Perennial (Cytospora) canker – black fruiting bodies on the bark surface or under the bark tissue

Biology
In Ontario the bacteria overwinters in buds, protected areas on the woody surface of the tree (e.g., cracks in the bark), and in leaf scars that became infected during leaf drop the previous season. Leaf scar infections usually develop into spring cankers.

In late winter as temperatures warm, leaf and flower buds swell, new tissue emerges, and the bacteria begin to multiply. The bacteria are spread from cankers in dripping dew and in splashing and/or wind-blown rain to the newly emerging leaves. These bacteria can also infect through natural openings or wounds. High moisture conditions are very favorable for both leaf and fruit infections.

Peach and nectarine leaves frequently show more severe spotting along the midvein and towards leaf tips because bacteria are suspended in water droplets that run down the leaves and these regions of the leaf dry more slowly.

Severe fruit infections are more common when frequent periods of rainfall or even extended heavy dews and very high humidity occur from late bloom to near pit-hardening. Bacterial spot is more severe in areas where peaches are grown in light, sandy soils than in heavier soils. Wind and wind-blown sand can increase the severity of bacterial spot by creating wounds for the bacteria to infect.

Period of Activity
Fruit are very susceptible to infection as soon as they are exposed at shuck split through pit hardening but new infections can develop until harvest.

Leaf infections can occur until terminal bud set.

Scouting Notes
On susceptible cultivars, begin monitoring at shuck split and continue weekly through late midseason by examining 50 fruit for small lesions, and 25 of the oldest leaves on each sample tree. Hold the leaves up to the light and look for angular, water-soaked lesions on the leaf blade. These lesions tend to be heaviest along the midvein and at the tip of the leaf, and will be visible within 24 hours of infection, which occurs after a rain or heavy dew. Within three days of initial infection, the lesions will be visible with reflected light and leaves will then begin to show the typical shot hole symptom.

Threshold
No threshold established.

Management Notes
Use resistant or tolerant peach varieties when possible.  Babygold 5 and Springcrest are especially susceptible to bacterial spot so these varieties should be avoided. Resistance in plums, nectarines and apricots is not common, although some recent apricot introductions are less susceptible.  Refer to Apricot variety facsheet?

Avoid planting stone fruit in sandy soils. 

Plant nursery stock that is free from cankers.

Maintain adequate fertility to avoid excessive foliar growth or nutrient stress that make trees more susceptible.

Prune to allow thorough spray penetration and more rapid drying of the canopy.

Pruning of visible disease cankers is of little value in controlling the disease.

Avoid overhead irrigation systems that wet the leaves in orchards with a history of bacterial spot.

Test soils for nematode populations before planting.  High populations of ring nematode have also been associated with increased bacterial spot, which may be related to stress caused by this nematode.

There are no products registered for bacterial spot control.  However, some dormant sprays for peach leaf curl may have some activity against the bacterial spot pathogen.

Some information included above excerpted from;

• http://www.ent.uga.edu/peach/peachhbk/bacterial/bacterialspot.pdf
• http://agsci.psu.edu/fphg/stone/diseases/bacterial-spot