Scabs are a fungal disease that cause fruits, leaves, and roots to obtain hardened tissue. Sometimes the tissue will crack. This is a major problem for fruits such as apple, crabapple, peach, and nectarines. It can infect other plants through the leaves, but usually sticks to fruit trees. The disease is prevalent in rainy season.
Rain tends to release the spores into the air.
And once it lands on a susceptible plant, the water on the plant aids the growth of the disease on the new host. Good control is generally easy if you plant resistant varieties. Another idea is to spray every few weeks with vegetable oil or another horticultural oil to prevent infection in the first place. Spray on your plants. If you have a huge orchard, you'll need to adjust for use in the machinery you usually use to spray your trees.