DISEASE
Disease resistance should be your first consideration. Many resistant cultivars are available and recommended in order to avoid the most common disease problems. Before making a selection, keep in mind that not all crabapples do well in every location. Disease intensity varies from region to region, and disease strength can vary from year to year. For instance, some crabapples will be more prone to disease susceptibility in areas with greater rainfall than in drier climates. Careful consideration of the following information will be helpful in choosing the right crabapple cultivar. There are four diseases that seriously affect crabapple:

Apple scab is one of the most serious diseases from an aesthetic standpoint, but usually not a serious threat to the health of the tree. It is a fungal disease, which develops in cool, wet springs. On susceptible crabapples, apple scab causes spotting of the leaves, premature defoliation, and unsightly spots on the fruit. There are numerous cultivars that are resistant or very tolerant (still susceptible but with little defoliation) so choose one based on its resistance.
Cedar- apple rust is a less serious leaf-spotting disease common to our native crabapple cultivars. It is usually a problem in areas where native junipers (Juniperus) are planted. Selecting resistant cultivars can control this disease.

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 is a serious bacterial disease of crabapples. Though it is less common then the others, if left untreated it can be fatal to susceptible crabapple cultivars. Select resistant cultivars.  [3]