One bacterial disease common in the San Francisco Bay Area
is fire blight of the Rosaceae family, which attacks fruit trees, including apple,
cherry, apricot, and pear. Fire blight is caused by the Erwinia
amylovora bacteria. Its common name reflects that an infected tree
looks scorched and blackened as though by fire. Note that bacteria are typically
host-specific, so that each tree species has its own E. amylovora pathogen variation
(pathovar).
Fire blight bacteria are spread to flower buds and nectaries in early spring, in humid warm conditions, either through splashing rain or pollinators. The bacteria invade the tissue, causing the blossoms to wilt and collapse. A brown sticky exudate is produced from the damaged tissue and spreads to young shoots. The shoots become infected, blackened, and deformed in a characteristic “shepherd’s crook” shape. The bacteria overwinter in cankers that form in the bark and wood. In the spring, when the temperature and moisture levels are favorable, the cankers ooze with bacteria, and the life cycle begins again. The exudate is splashed onto nearby blossoms, or attracts pollinators, which pick up the bacteria and carry it to new blossoms.
The best treatment is prevention, which includes planting resistant fruit trees and keeping trees healthy and vigorous to ward off infection. You can spray the tree in the spring with copper-based applications, designed to provide a barrier that prevents bacteria from reaching the blossoms, but timing is critical. The IPM has some guidelines and references for determining the best time to spray, typically based on temperature and humidity. Several sprayings may be needed, since blossoms do not all open at once, and conditions favorable to the bacteria may persist over several months. Once infected, pruning infected limbs or branches 8-12 inches below visible injury can help stop the spread of infection. Some experts advocate sterilizing pruning equipment between cuts; others disagree.
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| This small stand of Pyrus calleryana ‘Aristocrat’ (Aristocrat Pear) street trees in Dublin, California looks to be infected with fireblight. |
Fire blight bacteria are spread to flower buds and nectaries in early spring, in humid warm conditions, either through splashing rain or pollinators. The bacteria invade the tissue, causing the blossoms to wilt and collapse. A brown sticky exudate is produced from the damaged tissue and spreads to young shoots. The shoots become infected, blackened, and deformed in a characteristic “shepherd’s crook” shape. The bacteria overwinter in cankers that form in the bark and wood. In the spring, when the temperature and moisture levels are favorable, the cankers ooze with bacteria, and the life cycle begins again. The exudate is splashed onto nearby blossoms, or attracts pollinators, which pick up the bacteria and carry it to new blossoms.
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The best treatment is prevention, which includes planting resistant fruit trees and keeping trees healthy and vigorous to ward off infection. You can spray the tree in the spring with copper-based applications, designed to provide a barrier that prevents bacteria from reaching the blossoms, but timing is critical. The IPM has some guidelines and references for determining the best time to spray, typically based on temperature and humidity. Several sprayings may be needed, since blossoms do not all open at once, and conditions favorable to the bacteria may persist over several months. Once infected, pruning infected limbs or branches 8-12 inches below visible injury can help stop the spread of infection. Some experts advocate sterilizing pruning equipment between cuts; others disagree.
To learn more about fire blight, visit the IPM website: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7414.html.
You can also search the site for your specific crop: http://www.ipm.ucdavis.edu/PMG/crops-agriculture.html
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