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第1段
1 .<-NARRATOR:->Now listen to part of a lecture on the topic you just read about.
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第2段
1 .<-FEMALE PROFESSOR:->Unfortunately, the methods described in the passage have serious limitations that make them ineffective or impractical in many situations.
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第3段
1 .Precautions like cleaning shoes and using bike scrubbers might help a little but they will not make a huge impact.
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2 .The problem is that P. ramorum spores do not spread just through human assistance.
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3 .Studies have detected the spores in water streams following rainfall.
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4 .It appears that the spores are frequently picked up by rainwater and washed into water streams, which then carry them long distances.
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5 .This type of water-assisted spread is much harder to control.
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第4段
1 .Second, the fungicides referred to in the reading provide effective protection against P. ramorum only when they’re injected directly into the tree trunks.
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2 .But their effect lasts only for a few months, so the injections have to be repeated.
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3 .Now, this type of treatment might work if you want to protect a few oak trees in a city park, but it’s just not very practical in protecting whole forests that contain thousands and thousands of oak trees.
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4 .Can you imagine forest managers having to inject every single oak in a large forest with a dose of fungicide every few months?
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5 .That sounds impractical and also too expensive.
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第5段
1 .Third, as the reading explains, clear-cutting destroys the vegetation around the sick oak trees, because some of that vegetation may be infected.
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2 .But keep in mind that some of that surrounding vegetation is in fact healthy, and it gets destroyed in the process anyway.
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3 .Now, in some forests it makes sense to sacrifice some healthy vegetation in order to stop the spread of a dangerous disease.
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4 .However, the vegetation in many forests of the western United States includes tree species that are very rare and cannot grow back easily.
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5 .So, clear-cutting in these forests would mean destroying many healthy rare trees.
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6 .This would represent a greater ecological damage than the P. ramorum damage we’re trying to prevent.
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