Blister spot is a disease of apple fruit caused by the bacterium, Pseudomonas syringae pv. papulans (Rose) Dhanvantari. Although over 20 cultivars of apple have been reported as susceptible to the bacterium, the disease is usually of economic importance only on Mutsu in New York State. If uncontrolled, the disease generally affects 5 to 60 per cent of the fruit in an orchard. The bacterium does not cause extensive decay of the fruit, but makes them unsuitable for fresh market use.