The conditions you describe sound like apple scab. This fungal disease is present every year, but some years it is more severe than others. Its development is accelerated by wet weather. The primary infection period is early spring, but if wet weather continues through the summer, additional secondary infections can occur. Defoliation can be severe on susceptible varieties of crabapple. You can spray susceptible trees with a fungicide to prevent the disease. It will take at least two or three sprays at weekly intervals beginning with bud break in spring. Rather than setting yourself up for yearly spraying, however, consider replacing your susceptible crabapple. Numerous scab-resistant varieties are available at local garden centers.