In selecting a crabapple variety, disease resistance should be the first consideration. There are four principle diseases of plants in the rose family:

Apple scab is one of the most serious diseases from an aesthetic standpoint. Caused by the fungus Venturia inaequalis, which develops in cool, wet springs, scab is usually not life-threatening.  Dolgo crab is one among numerous cultivars that are resistant or very tolerant (still susceptible but with little defoliation). Apple scab causes spotting of the leaves, premature defoliation, and unsightly spots on the fruit. Apple scab affects members of the rose family, including nearly all cultivars of apple, crabapple, hawthorn, mountain-ash, cotoneaster, firethorn, and common pear. [4]

Cedar-apple rust is a leaf-spotting disease common to many plants in the rose family when planted near plants in the Family Cupressaceae (Cypress, Arborvitae, Juniper), which serve as host for the fungus Gymnosporangium juniperi-virginianae.  Woody galls form on branches and spring shoots; gelatinous, orange spore-producing bodies emerge from the growths. Orange spots may also appear on the foliage. [5]

Powdery mildew is a fungal disease that can cause considerable damage to leaves and fruit of susceptible cultivars without threatening the health of the tree. Poor air circulation, close association with susceptible apple cultivars, and wet, humid weather conditions will greatly influence disease incidence and severity.

Fire blight,  caused by the bacterium Erwinia amylovora, is a common and frequently destructive bacterial disease of pome fruit trees and other rose family plants. Pear and quince are particularly susceptible. Apple, crabapple, and Pyracantha species are also frequently damaged. Fire blight occasionally attacks hawthorn, Spiraea, Cotoneaster, Juneberry and serviceberry, and mountain ash. Fire blight infections can destroy limbs and even entire shrubs or trees. [3]