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Blister spot is an economic disease of Mutsu (Crispin) apple fruit in Eastern North America and is caused by the bacterium Pseudomonas syringae pv. papulans. It is seldom of economic importance on other apple cultivars although minor outbreaks of blister spot have occurred on Fuji and RedCort. It first occurred in the Okanagan Valley in 1995 on Mutsu.
Blister spot is first observed as small raised blisters that develop on fruit from early to mid July (Fig. 1). The blisters continue to expand during the growing season becoming purplish-black and ranging from 1 - 5 mm in diameter at harvest. A few to more than 100 superficial lesions may develop on each fruit.
Figure 1. Apple infected with blister spot. (BCMA)
Problems with blister spot can be prevented when the orchard is established by avoiding the cultivar Mutsu. Fuji apples are very susceptible to blister spot and should not be grown close to Mutsu which is the only known overwintering host for this disease. Copper fungicides provided effective control of blister spot in Ontario and tribasic copper sulphate (Copper 53W - United Agri-Products) is registered for its control on Mutsu and other sensitive apple cultivars.
Aliette WDG is also registered for blister spot control. Begin application at petal fall (2.5 kg/ha or 1 kg/acre) with subsequent applications at 7 day intervals, for a maximum of 3 applications.