During wet spring weather, about the time of vegetative bud-break, spores are produced in reproductive structures in the fallen leaves and fruit. These spores are forcibly discharged and young, wet leaves become infected. The leaf spots develop from 8 to18 days later, depending on the temperature. Secondary infections occur throughout the growing season whenever environmental conditions favor infection. These later infections often result in severe defoliation, which can weaken trees especially if crabapples are defoliated repeatedly.
Defoliation can also reduce the abundance of flowers the following spring.
If you are purchasing new trees, the best management strategy is to plant resistant cultivars. Many cultivars with good horticultural traits and disease resistance are available from local nurseries and should be planted in Delaware landscapes. Adams, David, Dolgo, Donald Wyman, Jewelberry, Ormiston Roy, Red Jade, Professor Sprenger, Sugar Tyme, White Cascade, Malus sargentii , and Malus seiboldi var. Rake and destroy or compost fallen leaves and fruit. This is an important preventative measure since the fungus overwinters in the infected leaves.