There are 35 species of crabapple trees available with over 700 cultivated varieties. Blossoms appear pink, red or white before the foliage develops in the spring. The crabapple tree produces tiny tart apples that are used in jellies and preserves. The fruit is a source of food for songbirds during the winter. Many varieties of crabapples are subject to fire blight, scab, mildew and infestation with Japanese beetles but selective breeding is creating stronger cultivars.
Fire blight is caused from bacteria (Erwinia amylovora). The disease strikes the blossoms of the crabapple. Eventually the bacteria moves form the flowers to the twigs and branches. The flowers will appear to have a burnt color on them and then the twigs and branches shrivel and turn pitch black. The fruit of the crabapple will also be infected. Branches will begin to have large lesions that ooze a reddish substance. This substance contains bacteria and will easily spread to other crabapple trees in the area. There is no treatment for fire blight. The infected branches need to be removed from the tree and the wounds sealed with a tree wound sealant. If the fire blight continues to spread on the crabapple then the tree will ultimately die. Bactericides have been used on fire blight with very limited success.
Scab (Venturia inaequalis) is a fungal infection that begins to afflict crabapple trees in the spring. Infected trees will exhibit leaves that have purple spots that appear to have a feathery appearance. The spots become approximately half an inch in size. Infected leaves turn yellow and begin to fall from the tree. Many trees will become almost devoid of foliage by midsummer. Scab does not kill crabapple trees but it does render them unsightly. All infected leaves should be raked up and disposed of to prevent spring spread. Prune the crabapple to offer good airflow and help prevent the fungus. Fungicide sprays for scab are often effective at preventing infection.
Crabapple mildew (Sphcerotheca mali and magnus) can reduce the crabapple fruit production. The fungus can also kill flower buds, which limits crabapple formation. The mildew commonly infects young green shoots and appears as yellow spots on the underside of the leaves. The surface of the leave can exhibit small grey hairy growth patches of mildew. Eventually the infected leaves will curl, dry and fall from the tree. The disease can be controlled by spraying fungicide for crabapple trees infected with mildew.
According to the University of Kentucky, the Japanese beetle was first introduced into the United States in 1916. The beetles are a metallic green shade with copper wings. They enjoy feasting on the crabapple leaves. Japanese beetles lay their eggs in the soil beneath the trees and the eggs produce grubs. Grubs can be controlled with an application of soil insecticide. Adult beetles can be controlled with pyrethroid products such as cyfluthrin bifenthrin, esfenvalerate, permethrin,deltamethrin and lambda cyhalothrin. Follow the directions on the label for insecticide application.
Many crabapple cultivars offer superior disease resistance. According to the Department Of Agriculture and the University of Massachusetts Ames White, R.M.J. 102., golden gem, molten lava, coral cascade and Mt. Arbor special crabapples are resistant to fire blight, mildew and scab.