Apples love disease. And bugs. Or at least disease and bugs love apples. Pesticides and insecticides were not used widely in North American apple growing until the late 19th century. In part, it was because many of the pests that troubled orchards elsewhere had not yet made the trip to the New World.
Public perceptions of how fruit should look also discouraged pesticide use: some pest damage was accepted as natural and unavoidable. Most people saw no problem with bumpy, discolored, and pock-marked apples. Still-life paintings of fruit from before the 19th century clearly show fruit with insect damage.
Pesticides entered the market once growers began producing more fruit for market and fresh eating rather than for home consumption and cider. Instead of imperfect fruit, the marketplace demanded perfect, shiny, blemish-free fruit. This has meant that pesticide use on apples has remained higher than on most other crops.