Many ornamental crabapple trees perform well without any insecticide or fungicide treatment.
However, some otherwise desirable crabapple varieties are very susceptible to leaf disease, especially scab. When conditions are favorable, scab often virtually defoliates them by mid to late summer. Although affected trees sometimes bloom beautifully for years despite such defoliation, leaf drop compromises their aesthetic value and may weaken the trees in the long run.
Disease resistance should be a prime consideration when selecting an ornamental crabapple tree.