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Fire Blight or Frost

May 29, 2007

Fire blight is a disease caused by a bacterium, Erwinia amylovora. It appears in the spring as rapid death of stem tips, often with a shepherd’s-crook, scorching of leaves, and a canker at the base of the affected stem area. Common hosts include crabapple, edible apple, pear, cotoneaster, hawthorn, mountain ash, spirea, and pyracantha (firethorn). An image of a pear stem with fire blight follows.

Each spring, the cankers from the previous year’s infection exude an amber, sappy-looking mass of bacteria. Insects and splashing rain move the bacterium to blossoms and branches. Blossoms are the most likely site of infection. Tender new growth may also be infected, as may any stem tissue that has been wounded (such as by insects, hail, or pruning). Fire blight is usually most severe in the spring when above normal temperatures and frequent rains occur simultaneously.

Fire blight causes the entire stem tip to die, with a distinct separation of the affected and healthy tissue. Pear foliage and stems turn black, whereas apple foliage and stems turn brown. Fire blight may continue to spread down the stem and usually ends in a dark, sunken canker at the base of the affected tissue.

Frost damage has been common this spring. Plants began to break buds early in March and April, and succulent growth followed. A sudden temperature drop (in some areas below freezing) killed much of the newly expanding tissue. Damage was sudden, appearing over night. Sometimes only the newest growth was affected, as with the yew in this image.

Many plants exhibited blackened new growth that remained (and may still remain) attached to the plants. More recently, new growth has emerged from the same stems. In most cases, the stem tissue was not killed. Adventitious buds have since developed, and many plants are recovering.

Fire blight and frost symptoms often appear similar and may be difficult to distinguish. First look at the host involved. Fire blight affects only plants in the rosaceae family. Check the University of Illinois fact sheet on fire blight for a list of susceptible plants to help you eliminate fire blight as a possibility. Next, look at other plants in the area. Frost is not selective. It affects any of the deciduous plants in the same type of site exposure. If the spirea in your front yard is frosted, other nearby deciduous plants should also show some leaf, bud, or stem injury. Look specifically at plants that are not hosts of fire blight. Finally, cut into some stem tissue to determine whether only leaves are affected (more typical of frost) or stem tissue is discolored (more typical of fire blight).

If you are still in doubt, send a sample to a plant lab such as the University of Illinois Plant Clinic (http://plantclinic.cropsci.uiuc.edu/). Stem terminals from the affected tips down to an inch into healthy tissue would be ideal. Quick ooze tests with a microscope can confirm the presence of a bacterium. Further culturing is used to confirm the presence of the fire blight organism. As a precaution, always dip shears in disinfectant between cuts when taking these samples.

Author: Nancy Pataky

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