Effects of Fire Blight Spray on Pear Trees

Types

• Fire blight sprays are available at most garden supply centers under various trade names. Sprays generally contain streptomycin sulfate, which is a General Use Pesticide (GUP) registered for use on fruits. Fixed copper sprays and Bordeaux sprays (a mixture of copper sulfate and hydrated lime) are also used to control fire blight.

Russetting

• Sprays containing copper oxychoride may cause significant russetting -- the appearance of small brown spots on the surface of the fruit -- when applied to some varieties of pear, such as Anjou and Bartlett. Russetting makes the pear less attractive in appearance, but does not affect the eating quality of the fruit. Streptomycin sprays may also cause russetting when applied in large quantities.

Resistance Buildup

• Frequent application of streptomycin-based fire blight sprays may eventually result in streptomycin-resistant strains of Erwinia amylovora. The bacterium genetically alters itself to become resistant to the chemical. Patterns of this resistance have been discovered in areas where fire blight is commonly controlled with streptomycin. Alternate applications of other sprays can help to delay a buildup of resistance.

Dormant Application

• Applying copper sprays to pear trees during the end of their dormant period and during the green tip bud stage has the effect of reducing the amount of the infecting agent that is produced in overwintering fire blight cankers. How effective this practice is depends on a number of factors, such as weather conditions during the green tip bud stage and the presence of bees and insects, which may spread the infecting agent. Despite these variables, copper spray application is likely to result in less blight during bloom.