Fungal spores that cause apple scab disease (Venturia inaequalis) erupting through the cuticle of a crabapple leaf. Magnified about 1500xPhoto by Charles Krause
Apple scab (Venuria inaequalis) is a fungal disease of malus specifically, affecting all species and nearly all varieties of apple and crabapple.
Can be managed organically. It is good practice to keep the orchard well pruned to allow for unimpeded airflow. Also important is making sure leaves are decaying quickly on the orchard floor since this is where the overwintering spore load will emanate from.
Generally speaking, scab can be a superficial affliction especially when an orchard is maintained well. It is not a human health issue (some studies show higher antioxidants in scabby fruit). It can lead to rot in storage if the lesions have cracked, as in any damage. This said, some varieties and many in wet years will wreak havoc. This can make the fruit very unsightly and if you are marketing them, unsalable.
There are few apple varieties that are scab immune, but many have tolerable levels of resistance. For commercial growers, it would be wise to choose at least some resistant varieties. It should not perhaps keep you from planting some susceptible types, as there are some wonderful apples out there that should not be neglected because they may get some scab. We have hundreds of varieties in the Walden Heights orchard and my absolute favorites do get scab, but I wouldn’t think of getting rid of them.
Organic growers that sell fruit typically use good orchard maintenance in coordination with an organic fungicide. Often this is sulfur, lime sulfur, or a biological. Prune well and yearly. Do everything you can to get leaves broken down or remove them entirely and compost them. Liming in the fall can help (ashes work too). Add compost to the orchard floor. These measures will all help. For the homesteader you should shoot for a tolerable level of affront to your fruit instead of purity or it will be costly and labor intensive. Commercial growers can do more since they will get a financial return to pay for all the fussing.