The selection and subsequent increase of resistant forms of the pathogens occurred similarly to that described previously for arthropods. Resistance of V. inaequalis to the benzimidazole fungicides became widespread in the mid and late 1970s throughout the Southeast. Consequently, the benzimidazole fungicides are no longer recommended for apple scab control in most southeastern states. Resistance of V. inaequalis to dodine has been reported from several northern apple producing states but has not been reported from the Southeast. Resistance of the fire blight bacterium, E. amylovora , to streptomycin has been reported from California, Michigan, and several other states, but not in the Southeast.
Several strategies are available for minimizing the likelihood of resistance development.
To avoid the development of dodine resistance, limit dodine use to two or three applications a year. In areas of the United States where dodine resistance is a problem, dodine was often used ten to twelve times through the growing season. Historically in the Southeast, dodine has been used only two to four times, early in the season. A similar strategy should be used to avoid resistance to streptomycin. Applications of streptomycin should be made only when needed during periods favorable for infections.