MSU Extension Integrated Pest Management

Blister spot

Disease

Blister spot

Pseudomonas syringae pv. Papulans (Rose 1917) Dhanvantari 1977

Distribution: Economically damaging only on the variety Mutsu (Crispin).

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Lesions begin as small, darkened, water-soaked areas, generally around lenticels and typically on the lower half of the apple. Small raised blisters form shortly thereafter, becoming purplish black as they expand (A). The infections are shallow, not extending more than 1–4 mm into the fruit flesh. The epidermal layer covering the blister dies and will often flake off the surface (B). The lesions are generally circular and rarely become larger than 4–5 mm in diameter. Leaves of Mutsu may also experience a mid-vein necrosis (C).

• Crops Affected: apples

  Management

  Blister spot is managed primarily through the timely application of bactericides during the three weeks after petal fall.