Fire Blight on Crabapples
Bacterial disease affecting plants in the Rose family
By: Sandy Feather ©2008
Penn State Cooperative Extension

Q. My crabapple tree bloomed beautifully this spring, but some of the blooms seemed to die. Now I'm noticing that some of the branches look like they are dying at the tips. Can you tell me what might be wrong, and what I can do to help my crabapple tree?

A. Judging by your description and the branch sample you sent, your crabapple (Malus spp.) is suffering from a common disease known as fire blight. This bacterial disease, caused by the bacterium Erwinia amylovora, affects many plants in the rose family, including apple, cotoneaster, crabapple, hawthorns, pears, pyracantha and serviceberries. Fire blight enters susceptible plants through wounds and through natural openings, such as the nectaries of flowers. Bees and other pollinators commonly spread the disease as they work the flowers. The development of fire blight is favored by warm, wet spring weather.
  
Initial symptoms include the sudden wilting and shriveling of individual flowers or entire flower clusters, as happened with your tree. Flowers typically turn brown, then black, and remain attached to the tree after they die. As the infection spreads from flower clusters down the twig, leaves begin to die. They also turn brown-black and remain on the tree.
  

Crabapple creating a fantastic flower show

Susceptible plants often escape the initial blossom infection during cool spring weather because temperatures are not warm enough for the disease to develop. In that case, the first symptom you notice may be shoot blight. Succulent new growth wilts and soon becomes withered and brown. It looks as though the tree has been singed by fire; hence the name fire blight. The tips of affected shoots curl over into a characteristic shepherd's crook shape. Scattered shoots on the tree may be infected simultaneously, giving the entire tree a scorched appearance. On close examination you should be able to see small, slightly sunken reddish-brown or black cankers where the scorching stops or where the affected shoot attaches to a larger branch. During warm, wet spring weather, you may notice liquid oozing from older cankers that has an unpleasant odor.
  
As this ooze drips onto lower parts of the tree, the bacteria can gain entry to the trunk through natural openings in the bark known as lenticels. (Lenticels lined up in horizontal patterns give cherry bark its characteristic appearance). The disease can also enter through pruning wounds, storm damage, or through damage from lawn mowers or weed whackers. Infections low on the tree, on the root collar or trunk are often fatal.
  

Removing infected branches is the primary control for fire blight. This should only be done during dry weather when the tree is dormant. Older recommendations called for pruning infected branches whenever they appeared, but recent research indicates that pruning during the growing season results in more rapid spread of the disease, even when pruning tools were properly sterilized between cuts.
  
Remove infected branches at least 4 inches below the base of a canker. Disinfect pruners with a 70 percent alcohol solution between cuts to reduce spread of the disease. Avoid pruning or working around infected plants when the weather is warm and wet. Send infected prunings out with the trash or burn them rather than composting them.
  
Avoid excessive fertilization that promotes a lot of succulent growth, which is most susceptible to infection. Also, remove root suckers and water sprouts, as these are very succulent and create a perfect point of entry for the bacteria. Root suckers are upright shoots that grow from the base of the tree. Water sprouts are upright shoots that grow straight up from branches, often as a result of old pruning cuts.
  
The best way to treat fire blight is to minimize the chance of your tree getting it by growing resistant cultivars of crabapples. These include 'Adams,' 'Callaway,' 'David,' 'Dolgo,' 'Harvest Gold,' 'Indian Summer,' 'Jewelberry,' 'Liset,' 'Profusion,' 'Red Baron,' 'Selkirk' and 'Sentinel.' Although resistance is not immunity, it helps to start with a plant that is not highly susceptible.

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