Pome Fruit Disease - Blister Spot on Crispin (Mutsu)

The causal bacteria, Pseudomonas syringae, overwinter in the infected buds and multiply on the leaf surface in spring. Rain washes the bacteria onto the fruit where they infect through the lenticels to cause the reddish spot. 

A brief shower is all that is required to distribute the bacteria to new infection sites. The standard recommendation for control of this disease is through timely application of bactericides during the three weeks after petal fall. If the weather pattern of rain continues, two additional sprays should be applied at weekly intervals. Resistance of this bacteria to streptomycin has been documented in commercial orchards in the northeastern United States. An application of fixed copper early in the season (at green-tip stage) is currently being used in many orchards and is effective in reducing disease severity. Phosphite fungicides, such as fosetyl-Al, phosphite salts, and phosphorous acid, have the potential to manage blister spot.