Unfortunately some crabapple and apple trees are susceptible to a fungal disease called apple scab. Symptoms of apple scab usually start on the undersides of leaves. Spots, at first, are small, irregular lesions that are light brown to olive green in color. As infection continues, lesions become more circular and velvety olive green to black in color. Leaves may curl and scorch at the margins.
Leaves usually turn yellow and fall off in mid-summer.
If the fruit stems become infected, fruits may drop early. The apple fruits may develop scabby lesions. The temperature affects the severity of infections. In order for infection to occur in cool weather, the plants must remain wet relatively longer than in warm weather. With existing crabapple trees you can do nothing and let the tree defoliate each summer.