Diseases Many new flowering crabapples are disease resistant or tolerant. Disease resistance involves genetic resistance to infection by disease causing organisms. Disease tolerance implies the plant may be affected by certain diseases but are of little health significance to the plant. Unfortunately, few crabapples possess all desirable characteristics of exquisite flowers, fruit, foliage, growth habit, and disease resistance. This does not mean that other cultivars should not be used.
Many crabapples are slightly susceptible to certain diseases and yet have great merit.
By accepting and understanding their limitations, these plants are perfectly acceptable in many landscape situations. This fungal disease first affects emerging leaves in the spring, during moist conditions, and then moves to the fruit. Scab causes dark, leathery spots with a corky appearance on the fruit. On mature leaves, the infections appear as black, velvety spots that are slightly raised. As the disease develops, leaves turn yellow and drop prematurely.