I N S E C T S

Apple maggot
The apple maggot (Rhagoletis pomonella) is a potentially serious pest of apple as the maggots (larvae) tunnel the fruit, creating characteristic dark tracks. Infested fruits also are prone to spoilage. 
D I S E A S E S

Apple rust (Cedar-apple rust)
Cedar-apple, cedar-hawthorn, and cedar-quince rust are common diseases of apple and flowering crab. These rust fungi spend a portion of their life cycle on rosaceous hosts such as apple, flowering crab, and hawthorn, and another portion on species of Juniperus (which includes eastern red cedar).
Apple scab
Apple scab (Venturia inaequalis) is a common fungal disease affecting apple and crabapple trees. No one will dispute the tedium of raking and disposing of the autumn leaves that litter our lawns. However, should an apple tree happen to grace your yard, this chore may be of extra importance to the health of your tree.
Fire blight
Fire blight is the enemy of rosebushes and apple and pear trees.  It is caused by a bacteria (Erwinia amylovora) which attacks starting generally from the blossoms or flowers and moving up to the twigs and then the branches.  Fire blight gets its name from the burnt appearance of affected blossoms and twigs.
Diseases that affect many trees as well: Powdery Mildew · Wetwood (Slime flux)