Nothing says ugly on your fruit like apple scab. The good news is that the apple is still edible. It is caused by a fungus, Venturia inaequalis. The fungus overwinters in dead leaves and apples on the ground, and then infects the leaves and fruit buds of the tree in the spring. Infection of the fruit can cause rough spots (hence the name) or even be so extensive that it causes the fruit to split during growth. A really bad infestation can even defoliate your tree and weaken or kill it. Infection is dependent on temperature and leaf wetness.
You will first notice apple scab as mottled spots on the leaves of the plants. These will later turn brown. The leaves also may get brown around the edges, or turn completely brown and fall off the tree. Later, as the little apples start to enlarge, you will see the spots on them as well. Obviously, when you see these symptoms, the infection has already taken place and the best you can do is spray to prevent secondary infections. Prevention is the best way to deal with this disease.
Start with sanitation around your apple tree in the fall. Make sure you remove all the fallen leaves. Most homeowners do not compost hot enough to destroy the spores, so it is best if you burn the leaves or remove them entirely. If mowing chops up the leaves, you can add some compost accelerator to make sure the leaves completely decompose. During the dormant season make sure your trees are well pruned to promote an open structure allowing light to penetrate through all the branches and to allow good air flow. If you spray for apple scab, timing is everything. Remember that any fungicide will be toxic to all fungus. Some fungus is beneficial to your tree. For instance, fungus in the soil holds nutrients, helps the tree roots to absorb water, and resists pathogenic fungus. So the least number of applications of spray the better - even sulfur. When the flower buds on your tree are turning pink, spray the tree with a sulfur spray. Then repeat the spray prior to any extended rain (more than 6 hours of continuous leaf wetness). During this period through petal fall is the most vulnerable time for the tree to get scab.
Sulfur spray: Mix 8 tbs. liquid sulfur, 1 tsp. NuFilm-17, 2 tbs. liquid seaweed in a gallon of water.
Other organic fungicides
There are some other fungicides acceptable to organic standards. Some of them claim to work for apple scab. One of these is a phosphorous acid preparation called Exel LG. Another is Serenade, which is a Bacillus subtilis preparation. I have not found many references in the literature on how effective these are. I might try them next spring and report how they worked.
Some years are just too wet for complete control of scab. 2010 was one of them for my area west of the Cascades. We had warm periods followed by the wettest spring in recent history. Which brings me to to my last thought about this disease - plant resistant cultivars when possible. Beyond the very resistant list, there are trees that are at least more resistant than others. I was recently walking through the Home Orchard Society apple orchard and noticed that some apples have some scab, but not completely covered with scab. You probably already have a tree in your yard, but if you are planning on planting one, you would benefit by looking at cultivars that are resistant to the diseases in your area. For the western northwest, scab is the one to be concerned about. Farther south, and east, fireblight is another resistance to look for.