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Diagnostic Tip: Fire Blight or Canker?

September 2, 2002

We have seen both diseases in abundance in 2002. Details on fire blight can be found in issue no. 2 of this newsletter; and details on cankers are in issues 2, 11, and 13. Sometimes these diseases look alike, and diagnosis can be confusing. Both problems kill the cambium on woody plant material. Both may initially cause wilting and browning of foliage because water flow is restricted with infection. Fire blight tends to happen more quickly; but if you do not keep a daily vigil, you may miss this fact.

Fire blight causes young shoots to wilt and quickly turn brown. The newest growth is most susceptible. Often the tip of the shoot curls over in a "shepherd's crook." This crooking is not always present. Fire blight is caused by a bacterium. Keep in mind that no fruiting bodies form with bacterial diseases. The fire blight bacterium may enter stem tips, a flower bud, or side shoot. The browning it causes is continuous from the tip back to where tissue turns green again. Green tissue is not interspersed with the dead tissue. Besides, the sapwood under the bark is dead as far back as leaves die. Peel back the bark, and you see brown wood underneath.

Cankers are caused by fungal pathogens and therefore often have fruiting bodies embedded in the dead tissue. Canker fungi often cause localized dead wood; this is the area we refer to as the canker. Usually the dead area is surrounded by live, green wood. If you proceed down the stem, you find more dead spots--not a continuous death of wood as with fire blight.

Consider the weather pattern when trying to distin-guish between fire blight and cankers. Fire blight occurs most frequently in spring and fall during wet periods. Infection occurs on the most succulent tissues on rapidly growing plants. We see summer infection as well but not in periods of drought. Cankers occur all season long and infect plants under stress.

Always check reference books to determine whether a plant has been reported to host fire blight. Most plants can get canker diseases. When in doubt, send a sample to the Plant Clinic or consult with your local Extension office.

Author: Nancy Pataky


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