Answer: As the name implies, fireblight causes shoot tips to blacken and wilt -- almost as though they've been burned. It is caused by Erwinia amylovora, a bacterium that overwinters in cankers on infected wood. Therefore, a fungicide -- which targets only fungal diseases -- won't control it. Fire blight is spread by insects, pruning tools, and splashing water. Bacteria enter healthy blossoms under the right conditions. A temperature of 65F or higher, plus a trace of rain or high humidity are required for infection. Because exacting environmental conditions are necessary for infection to occur, fire blight may be very serious some years and appear hardly at all in others. It affects apple trees as well as pears.
Pruning is a good way to keep the disease from spreading. Continue to remove infected stems and branches, cutting back several inches into healthy wood. Be sure to disinfect your pruners between cuts so you won't spread the problem as you're trying to remove it. (Use rubbing alcohol or a 10 percent bleach solution to disinfect your pruners. When you've finished pruning disinfect again, then wipe them dry and apply a little mineral oil to keep the pruners from rusting.)
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