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Ministry of Agriculture, Food and Rural Affairs

Blister Spot

Blister spots are often detected near the calyx end of fruit growing on the outside of the tree canopy facing the sun Blister spot lesions eventually become purplish black with a tan centre expanding no more than 4-5 mm in diameter Midvein necrosis of leaves on tender shoots infected with the blister spot bacteria Crusty brown lesions on the midvein and petiole of the lower surface of blister spot infected leaf
Click to enlarge.

Beginner

Scientific Name
Psuedomonas syringae

Identification

  • Fruit symptoms first appear as small water-soaked raised blisters on the fruit surface often near the calyx end.
  • Lesion matures to a brown blister with a purplish black margin expanding to no more than 4-5 mm in diameter and that penetrates no more than 1-2 mm into the flesh.
  • The lesions do not develop into fruit decay, but several to more than 100 blister spots can occur on a single fruit.
  • Leaf symptoms appear as a midvein necrosis on leaves of tender shoots.
  • Crusty brown lesions occur on the midvein of the lower leaf surface, causing affected leaves to become curled and puckered.
  • Occasionally, other fungi enter the blister spot lesion resulting in decayed areas on the fruit.

Often Confused With
Apple scab on fruit
Fireblight on the leaves

Period of Activity
Blister spot can be observed in orchards from petal fall through late June.
Young Mutsu fruit are most susceptible beginning two weeks after petal fall lasting for about six weeks (late July). However fruit symptoms often do not appear until mid to late July after which time the bacteria can no longer infect the fruit.

Scouting Notes
Regular scouting is the best way to assess disease levels. When scouting susceptible varieties such as Mutsu and Shizuka note infected leaves and fruit when scouting other pests. Fruit lesions often occur on fruit growing towards the outside of the tree canopy facing the sun.

Thresholds
None established. Apply fungicides preventatively to susceptible varieties, Mutsu and Shizuka.

Advanced

Scientific Name
Psuedomonas syringae

Identification           
Initially symptoms appear as small water-soaked raised blisters associated with the lenticels (fruit pores) on the fruit surface. The first spots are often detected near the calyx end of fruit growing towards the outside of the tree facing the sun. Symptoms often do not appear until mid to late July after which time the bacteria can no longer infect the fruit. The lesion eventually appears as a brown blister with a purplish black margin expanding to no more than 4-5 mm in diameter on the fruit surface. Lesions rarely penetrate the flesh to more than 1-2 mm deep. The lesions do not develop into fruit decay, but several to more than 100 blister spots can occur on a single fruit reducing the fresh market quality significantly.

Leaf symptoms appear as a midvein necrosis of infected leaves on tender shoots. Crusty brown lesions occur on the midvein of the lower leaf surface, causing affected leaves to become curled and puckered.

The disease causes superficial blemishes to the fruit, making them unsuitable for fresh market. Occasionally, other fungi enter the blister spot lesion resulting in decayed areas on the fruit.

Often Confused With
Apple scab on fruit
Fireblight on the leaves

Biology
The bacteria overwinter in apple buds, leaf scars and diseased fruit left on the orchard floor from the previous season. Although infected apple buds appear healthy, the bacteria multiply during the spring and are rain splashed to leaves and other plant surfaces throughout the orchard. The bacteria survive and multiply on leaf surfaces including weeds in the orchard, without causing any disease symptoms. Warm, humid or wet conditions during the spring and early summer favour a build up of bacterial populations and subsequent infections of leaf veins and fruit. During late spring or early summer, a brief shower is all that is required to distribute the bacteria onto the developing fruit where they infect through the fruit pores or lenticels. Young Mutsu fruit are most susceptible beginning two weeks after petal fall lasting for about six weeks (late July). After this point the lenticels on fruit are no longer susceptible to infection by the bacterium.

Period of Activity
Blister spot can be observed in orchards from petal fall through late June. The bacteria overwinter in apple buds, leaf scars and diseased fruit left on the orchard floor from the previous season. Warm, humid or wet conditions during the spring and early summer favour a build up of bacterial populations and subsequent infections of leaf veins and fruit. Symptoms often do not appear until mid to late July after which time the bacteria can no longer infect the fruit. Young Mutsu fruit are most susceptible beginning two weeks after petal fall lasting for about six weeks (late July).

Scouting Notes
When scouting susceptible varieties such as Mutsu and Shizuka note infected leaves and fruit when scouting other pests. Fruit symptoms often occur on fruit growing towards the outside of the tree facing the sun.

Thresholds
None established. Apply fungicides preventatively to susceptible varieties.

Management Notes

  • When planting a block of Mutsu trees obtain disease-free nursery stock and avoid planting near older Mutsu blocks where blister spot is present.
  • The cultivar Shizuka has similar appearance and quality of fruit as Mutsu but is much less susceptible to blister spot and is an alternative cultivar to plant in orchards with a history of blister spot.
  • Avoid the use of overhead irrigation during the period of fruit susceptibility.
  • A newly planted “clean” block of Mutsu trees often remains disease-free for a significant period of time. Eventually the blister spot bacteria spreads into the orchard, initially causing low levels of infections but if left unmanaged, 80% or more of the fruit can become infected.
  • The level of damage may decline in some years due to weather conditions unfavourable for bacteria spread and disease development.
  •  Once in the orchard, blister spot cannot be eradicated, but proper management minimizes fruit damage each year. Recent research shows certain products with bactericidal properties are effective at reducing the level of infection.
  • For more information on products registered for blister spot control, consult OMAFRA Publication 360, Guide to Fruit Production - Chapter 3 Apples (PDF) or Apple Calendar only (PDF):