Common crabapple diseases...

Many of the newer 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.

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 that have varying degrees of susceptibility to disease, could still possess enough merit in the form of flowering characteristics, growth pattern, and shape. Understanding their limitations, these plants are perfectly acceptable in many landscape situations.

There are four diseases that pose a serious threat to crab apples: apple scab, a fungus characterized by black sooty spots or leaves and corky spots on fruits. Fire blight is a bacterium that turns twigs and branches black, eventually killing the tree. Cedar-apple rust is a fungus that creates rusty, corky spots on leaves and, if serious, can defoliate the tree. Less of a threat is powdery mildew is a white fungus that attacks leaves, flowers, and fruits during warm, humid weather and is unsightly.