Source: John Hartman, University of Kentucky, Bugwood.org
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 to apply streptomycin at ½ pound per 100 gallon dilute rate 10 to 14 days 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. Refer to the streptomycin label for specific use recommendations and limitations.