The proper technique for spraying apple trees begins in the winter, when you prune and spray the trees with dormant oil spray in January or February. In the spring, reapply dormant oil right before the buds open if the infestation of the prior season was heavy. Use a fixed copper spray as the trees bloom in March and April to fight fire blight. Use a fungicide during the spring to fight powdery mildew and apple scab.
Spray apple trees for coddling moth during the summer; this usually occurs in June, but it is appropriate to spray at any point between May and September. If fruit was wormy in the past, apply malathion, permethrin or spinosad two weeks after the petals come off the trees. Repeat that application once a month for the following three months, and repeat the fixed copper application at least two weeks before harvest.
Trees experiencing fire blight have blossoms that first appear gray and waterlogged, but they eventually turn black. During the shoot blight phase, stems turn black and curve to the shape of a candy cane. Bacterial ooze is visible on the stems, and as it travels down the branches, the tree takes on a burned appearance. Fruit produced by these trees shrivel and yield black lesions. Spray fire-blighted trees with streptomycin while they bloom and during periods of relapse; do not harvest the fruit until at least 50 days after application.