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. 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.