There are two main reasons to develop a spray routine for your apple tree: disease and insects. Some of the sprays you use are preventative, while others are meant for immediate control.
Types of Sprays
Fungicide sprays control the many warm-weather diseases of the apple tree. Horticultural oil controls insects such as scale. Apple maggot is controlled with lime-sulfur. Insecticide sprays will be needed for other insects.
Spray the apple tree with horticultural oil while dormant, then again when the leaves are 1/2 inch and again right before the tree blooms, when the small buds begin to turn pink. Apple maggot control begins before the tree produces foliage in the spring with a lime-sulfur spray. To control the many other insects that attack apple trees, use an insecticide when the blossoms start to drop and then three more times, at two-week intervals. To control summer diseases, spray the fungicide in early June and then again every 14 days through the middle of August.
Carefully calibrate your sprayer in accordance with the product's manufacturer instructions. Make sure the entire tree is covered with the spray material. Many fungicides and insecticides have been listed by the Environmental Protection Agency as possible human carcinogens, so wear protective clothing during application and shower after you are finished.