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Dr. Jeffery Iles, Professor and Chair of the Department of Horticulture at Iowa State University in Ames, Iowa, states "there are many crabapples that possess all, or at least many, desirable characteristics we associate with crabapples. While it may be true there are many crabapples that have disease problems, there are many more that do not."
Diseases
Apple Scab
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 leaves, scab infections first appear in May or early June as olive-green or oil-soaked spots. On mature leaves, the infections appear as black, velvety spots that are slightly raised. As the disease develops, leaves turn yellow and drop prematurely. If the tree is heavily infected, defoliation can occur by early summer.
Control can be achieved one of two ways. Remove the trees that are highly susceptible and select other less susceptible disease-resistant crabapples. Alternatively, apply fungicides as leaves begin to emerge, at two weeks and again four weeks after the first application.
Frog-eye Leaf Spot
Symptoms of this fungal disease are typically small, dark brown spots (dead leaf tissue) outlined by a thick, dark purple circle. Frog-eye leaf spot is found commonly on many flowering crabapples and its effect, from heavy defoliation to no impact, depends upon susceptibility to this fungus.
The best course of action is to select crabapples, which are resistant or tolerant to this disease.
Fireblight
This devastating disease is caused by a bacterium, Erwinia amylovora. Symptoms appear as death of new terminal shoots in late spring or early summer. These shoots appear to be scorched by fire. The leaves remain attached to the blighted shoot which develops a characteristic curvature at the tip, commonly called a "shepherd's crook."
Fireblight often progresses down through the shoot and forms a canker in the older tissue. Cankers are typically sunken areas that are dark brown to purplish in color. An orange or amber gum may ooze from these infected parts. As the bark dies, the area becomes slightly depressed.
Control of fireblight can be easily achieved if these guidelines are followed. First and foremost, select plants that are genetically resistant to fireblight. If that is not an option then sanitation, removal, and disposing of blighted branches and shoots are the best alternatives.
Insects/Pests
Flowering crabapples are relatively undamaged by most insects. Although they are frequented by various types of caterpillars, leafhoppers, leaf-rollers, leafminers, and Japanese beetles, these pests rarely cause significant damage to the tree. The web forming caterpillars (i.e. eastern tent caterpillar, fall webworm) are easily pruned out or removed with a gloved hand.
Japanese beetles and other pests may be controlled with insecticides. Control may be warranted in young trees if one-third to one-half of the foliage is affected.
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