August 1996
Crabapples are popular landscape trees because of their spectacular flower show in spring, but crabapples can become very unpopular, unsightly trees later in the summer if they lose all their leaves due to a fungal disease called scab. Unfortunately, this is not an uncommon occurrence.
Apple scab, caused by the fungus Venturia inaequalis, affects both apples and crabapples. It is the most common disease of crabapples in Virginia. The disease begins as diffuse, charcoal-gray to brown lesions on leaves. Infected leaves eventually turn yellow and drop off the tree. Severely affected trees can be completely defoliated by midsummer. Some homeowners may think the tree has died after it has lost its leaves; however, while scab weakens the tree, it does not kill the tree outright. Fruits, as well as leaves, are infected by the fungus. The fruit lesions appear as dark, sunken scabs.
Scab can be controlled with fungicides, but sprays must be applied every 10 to 14 days throughout the growing season to achieve effective control, and this is very expensive. A better means of control is to select and plant scab-resistant cultivars. A number of cultivars with high levels of resistance to scab are available. Scab resistance is not the only consideration in selecting crabapple cultivars, however. Other diseases, such as fire blight, cedar-apple rust, and powdery mildew (described below) can affect crabapples. Resistance to these diseases should be considered also.
Fire blight is caused by the bacterium Erwinia amylovora. Although it is not as common on crabapples as scab, it can be serious. Shoots turn brown or black, as if scorched by fire, and leaves cling to the twigs. The tips of affected shoots may be curved, forming what is commonly known as a "shepherd's crook." Sunken, reddish-brown cankers may occur on larger branches or on the trunk. Bacteria overwinter in the cankers and can spread causing new infections the following year. Fire blight symptoms appear in spring. Affected branches should be pruned out well below darkened tissue later in the summer when the bacteria are dormant. Pruning tools should be dipped in a dilute household bleach solution between cuts to prevent spread of the bacteria.
Cedar-apple rust, caused by the fungus Gymnosporangium juniperi-virginianae, appears first as yellow spots on leaves. Later, the spots turn orange and then reddish with tiny black dots in the centers. This disease may cause some defoliation. The pathogen requires two hosts, red cedar and apple or crabapple, to complete its life cycle, so the disease will be more of a problem if red cedars are growing near crabapples. Fungicides can be used to control the disease, but like scab, resistance is a better means of control.
Powdery mildew, caused by the fungus Podosphaera leucotricha, can be a serious problem in crabapples. Leaves and twigs become covered with a white, powdery substance that is actually the mycelium of the fungus. Mycelia are the vegetative parts of the fungus, consisting of branching, threadlike filaments. Early infections may cause curling and twisting of young shoots. Fungicides can be used for control, but resistant cultivars are available.
Of course, a cultivar that has resistance to the major diseases of crabapple is useless unless it has desirable horticultural characteristics. Crabapples come in various sizes and shapes. Some cultivars produce flowers on the current year's growth, others on the previous year's growth. Also, some cultivars bloom every year, whereas others bloom only every other year.
It is possible to select cultivars that are both aesthetically pleasing and have disease resistance. In Virginia, scab resistance is the most important type of resistance. Unfortunately, the literature on disease-resistant cultivars of crabapple is confusing. In some publications, a certain cultivar is listed as highly resistant to scab while in other publications, it is listed as highly susceptible. This apparent contradiction may be due to cultivars responding differently to the scab pathogen depending on the environment in which the resistance trial is conducted.
The cultivars with the most resistance to scab all have white flowers. There are red-flowered cultivars that have moderate resistance to scab.
(This article was originally published in The Plant Protection Newsletter, Volume 9, Number 3, 1990, and reprinted as "Disease Resistant Crabapples," by Mary Ann Hansen, Plant Clinic Manager, Virginia Tech, in The Virginia Gardener Newsletter, Volume 12, Number 2.)