Among the most beautiful and cold-hardy flowering trees, crabapples traditionally have been plagued and killed by fire blight. Caused by an invading bacterium, symptoms include browning of flowers, twigs and foliage. Although infected branches can be removed to control the blight, selecting crabapple varieties with natural resistance is the best preventative solution.
What is a Crabapple?
Crabapples are botanically classified alongside regular apples in the genus, or species group, of Malus. Genetically, both crabapple and apple trees are similar since they both have ornamental flowers and produce fruits. Generally speaking, if the fruits of the tree are smaller than 1/2 inch in diameter, the tree is regarded as a crabapple.
These trees, or shorter tree-like shrubs, are derived from complex hybrid crossings of wild apples species from the cold-temperature regions of North America, Asia and Europe. There are hundreds of crabapple cultivated varieties, or cultivars, extant today.
Crabapple trees, like any other plant, can be attacked by insect pests and disease pathogens during their lifetime. However, all apples historically have been susceptible to several common threats in varying degrees: powdery mildew, rust, apple scab, and fire blight.
Environment can play a role in a crabapple tree's susceptibility to pest or disease. If a tree is stressed or weakened by drought or a physical wound from a wind-broken branch, it can be at increased risk of invasion by a pathogen or boring insect, for example. Fungal diseases may be more prevalent in overly wet, cool climates.
Fire blight is caused by a bacterium called Erwinia amylovora. It is present in many garden landscapes, but becomes problematic with crabapples only when the bacterium infiltrates plant tissues. Symptoms include the immediate browning of flowers and drying of branch twigs. Soon thereafter, foliage browns and more parts of the tree branch die. On the twigs and branches you will see small, rounded, reddish-brown depressions, called cankers. In severe cases, cankers or branch woods have an oozing, bad-smelling gel.
Contracting Fire Blight
The fire blight bacterium enters and attacks the crabapple through pores or wounds. Pollinating bees can spread the bacterium among trees and flowers, where it may enter the tree through the nectaries of the flowers. The sugary sap oozing on infected trees can also attract the insects, putting them in direct contact with the fire blight and enabling them to carry it to other flowering trees.
Flowers are by no means the only way for the bacterium to enter the crabapple tree. Cracks or wounds in the bark of branches, trunk or surface roots can permit entry, too.
A healthy, well-groomed crabapple tree is much less likely to succumb to fire blight, but it is at constant risk and exposure to the bacterium. Plant breeders recognize the dilemma with crabapple susceptibility to fire blight and have worked for decades creating hybrids and selections that tend to shun fire blight, having diminished effects of exposure to the bacterium.
Often a crabapple variety will have a plant label that mentions its disease tolerances, using terms such as "slight tolerance," "good tolerance" or extreme tolerance" to a specific threat, such as fire blight. Conversely, a label or print literature may forewarn with the phrase "highly susceptible to," which immediately tells you the plant will have difficulty with the pest or pathogen if encountered in your garden.
To avoid issues with fire blight, including losing an entire tree because of it, only plant and grow crabapple varieties regarded as "highly resistant" or having "excellent tolerance" to fire blight. Such varieties include, but are not limited to: Adams, Adirondack, Callaway, Centennial, David, Dolgo, Harvest Gold, Profusion and Tina.
Consult your cooperative extension office for local recommendations of crabapple cultivars that resist fire blight and perform well in the climate and soils of your region.