Dormant oil sprays are for certain insects only, and do not control scab. Resistant varieties are the best way to manage scab, especially for ornamental crabapples. Some commonly grown crabs, such as Almey, Hopa, Eleyi and Radiant are very susceptible to scab. There are many resistant cultivars available, so be sure to ask for them when selecting crabapples. Consider replacing extremely susceptible trees with resistant cultivars.
Some apples also have resistance.