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. 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. 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.