Apples, like many other tree fruits, have a number of diseases that infect both leaves and fruit. The picture to the right shows some of the different diseases - the apple on the right has scab, the apple in the middle has sooty blotch/fly speck (blotches and specks on the top of the apple), and the apple on the left has a summer rot (bitter rot). In order to control these diseases, you should understand the life cycles.
Scab is primarily a spring disease. The fungus overwinters on infected leaves on the orchard floor. In the spring, spores are released from the leaves and spread by wind to infect newly emerged apple leaves. As spring progresses more spores will be produced by the newly infected leaves which will infect more leaves and fruit. A tree with a serious scab infection can be defoliated and the fruit will be ruined.
The easiest way to control scab is to plant disease resistant trees like Pristine, Enterprise, Goldrush and Liberty, to name a few. If you have trees that are susceptible to the fungus then the tree must be sprayed on a regular basis when the new leaves emerge in the spring, particularly during a wet spring. Removing or destroying leaves on the ground during the winter will also reduce the disease inoculum.
Summer rots develop much later in the season when temperatures start to rise. There are a number of different summer rots (black rot, white rot, bitter rot) and certain varieties may be more susceptible then others. The only way to control summer rots is to spray a fungicide on a regular basis (every 10 days to 2 weeks depending on rainfall).
Sooty blotch and fly speck are also diseases that build up in hot weather. Sooty blotch is just like the name says and forms sooty patches all over the fruit. Fly speck forms little, black specks on the fruit. Both of these diseases are superficial on the fruit and the damage is only cosmetic. Most fungicide sprays that control summer rots will also control sooty blotch and fly speck.