Issue 7, June 6, 2011

Fireblight ... It's Back!

Fire blight is showing up across Illinois. We have been diagnosing it on Callery pear. But, fire blight can affect apples, pears, crabapples, and ornamental pears (yes, Callery pear). You might also see infection on other rosaceous hosts, such as cotoneaster, hawthorn, quince, firethorn, and mountain-ash.

The bacterial pathogen, Erwinia amylovora, that causes fireblight may linger and over winter in cankers. The bacterium will ooze from these cankers in the spring and attract insects. These insects can spread the bacteria to blossoms, fruit, or other plant parts. The heavy rainfall that we had this spring could have also spread the bacteria. Most importantly, don't forget, we also contribute to spread by way of pruning tools.

The fire blight bacterium, infects in warm (>60 degree F), humid conditions. The primary mode of entry into the plant is via flowers, so the critical infection period is during bloom. Therefore, if you are seeing these disease symptoms on your tree, infection has already taken place. Infection can also occur via wounds, especially after wind or hail storms, which have also been frequent in areas of Illinois this year. The bacterium moves systemically in plants to shoot tips. Blighted leaves and blossoms near twig tips appear first. Leaves may wilt and turn brown or black and form the typical shepherd's crooks symptom at stem tips. Stem cankers develop as sunken, cracked areas on stems.

What can you do to manage fire blight? Prune out infected wood in the dormant season, if you can wait. If not, prune in an extended dry period and disinfect pruning tools after every cut. The bacterium may have extended down the stem ahead of the canker. Unfortunately this means wood should be removed 8-10 inches below the edge of the visible canker. Chemical options are limited for home growers because the timing of sprays is so critical. Commercial growers apply copper products in the dormant season and streptomycin at 4-5 day intervals throughout bloom. This is not a disease that you can try to fix with fertilization and watering. You will promote lush growth which is more susceptible to infection by the fire blight bacterium. If planting a new tree that can be a host, try to focus on purchasing a tree with resistance to fire blight. For more reading enjoyment about fire blight, you can go to this link and find the Report on Plant Disease of Fire blight of Apple (Adobe PDF).

As a side note, this sample of ornamental pear (as seen in the picture above) was misdiagnosed last year by an individual that was thought to be a "reliable" arborist. Without going into too much detail, this tree underwent fungicide injections last year. When the disease reappeared this year, the sample was submitted to the U of I Plant Clinic and diagnosed with fire blight. As explained above, fire blight is a bacterial disease and a fungicide application would have no effect on this disease. Please make sure that you call upon a Certified Arborist or expert. You can search for a Certified Arborist in your area on the ISA web site. If an "on-site" diagnosis is made, and pricey chemical recommendations are given, don't be afraid to get a second opinion. As always, you are welcome to send a sample to the U of I Plant Clinic, where you can receive an accurate diagnosis, and unbiased management recommendations. (Stephanie Porter)

Stephanie Porter

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