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Fireblight

A common disease of crabapples and other species. Learn about the cause and the multiple control methods.

Fireblight is a common disease of many Rose-family species in North Dakota.  The list of hosts includes apples, crabapples, hawthorn, mountain-ash, cotoneaster, pear and others.  Fireblight is caused by a bacteria that enters the tree via succulent tissue or through fresh wounds.  The recent storms that have been rolling through our region have resulted in perfect conditions for this disease to flourish.

The most common symptoms (see below) are dead branch tips with leaves still attached, curved over at the tip into a ‘shepherd’s crook’.  There is usually a sharp dividing line between living and dead tissue, and in some cases a sunken canker will be found here.  The bacteria can be found even further into the living tissue which is very important when treating the problem.

fireblight, crabapple, Valley City

A heavy infection of fireblight on a crabapple in Valley City, ND.

Dormant-season pruning is the best option for treating fireblight.  Locate the dividing line between living and dead tissue, and go back at least 8-12” into the healthy wood before pruning at a branch connection.  If there are just a few dead branch tips, they can be pruned out during the growing season but it is critical to sterilize the pruning tool between cuts to minimize the potential for spreading the bacteria to new wounds.  Pine Sol® or a 20% bleach solution can be used as sterilizing agents.  Both are corrosive to metal, so rinse and oil pruning tools when finished.  Chemical treatments in the spring can help prevent the disease from establishing, and copper-based fungicides as well as streptomycin are labeled for this purpose.  Follow all label directions and treatment recommendations.

fireblight, canker, margin, crabapple, Valley City

Canker margin on fireblight; living tissue is on the left and dead tissue is on the right.  Pruning should be at least 8-12" into living wood and should occur at a branch connection.

One additional way to prevent fireblight is plant resistant varieties of the host species.  Fireblight ratings of several edible apple varities can be found at: https://www.ag.ndsu.edu/pubs/plantsci/hortcrop/h327.pdf.  More information on the basics of tree pruning is available at:

https://www.ag.ndsu.edu/publications/landing-pages/gardens-lawns-trees/pruning-trees-and-shrubs-h1036

-Joe Zeleznik

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