Crabapple trees can grow as tall as 50 feet, depending on the variety. They produce clusters of pink, white or red flowers in the spring, followed by a bitter 2-inch in diameter fruit. The trees--which come in weeping, columnar, spreading, vase shaped or pyramid shaped varieties--are also susceptible to a variety of diseases
Apple scab is a fungus that attacks both apple and crabapple trees. It lives in the diseased leaves and--if they are not picked up and disposed of--the fungus will survive the winter and strike again when the spring rains come. Apple scab attacks both the leaves and the fruit and can destroy the whole crop. It appears on the leaves as soft olive green spots that grow into thick scabs. Fruit is vulnerable when it is young and shows the same symptom.
Cedar-apple rust is a fungus that attacks apple, flowering crabapple, hawthorn, juniper and cedar trees. It can cause the leaves to fall off early and the fruit to become distorted and also fall off. Orange/yellow spots, less than a ½ inch in diameter, will appear on the top surface of the leaves in late spring or early summer. The spots will become orange as they grow. Small black structures called pycnia will form in the middle of the spot. During wet weather, an orange-colored ooze the consistency of gelatin can be seen coming from the pycnia. This is followed by an orange cup-shaped structure called aecium forming on the bottom of the leaf right under the lesion on the top.
Fire blight attacks many types of apple, including crabapple--and rose bushes and mountain ash, hawthorn, cotoneaster and pear trees as well. It is a bacterial infection that starts in the blossoms or flowers and spreads from there to the twigs and finally to the branches. The flowers will turn brown and wilt, and the twigs will become blackened and shriveled up. In serious cases, cankers will appear on the branches. Repeated infections can be fatal.
Wetwood is a bacterial infection that affects the central core and/or the bark of many trees. In addition to apple and crabapple, it can attack elm, cottonwood, aspen, willow, ash, fir, maple, birch, hickory, beech, mulberry, oak, sycamore, poplar, cherry, plum and linden. The central core of the tree will turn yellow/brown and will be wetter than the surrounding wood. It will also be under pressure from internal gas and that, combined with the moisture, will cause a bad smelling, slimy ooze to seep from where the branches join the trunk, leaving a gray or white crust on the trunk.
The ooze is toxic and can kill the cells of the tree as well as anything it falls on.