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TPO 38 阅读 In 1995 a microscopic fungus called phytophthora ramorum, or P. ramorum, was first detected in the forests of the western United States. P. ramorum infects trees and causes particularly serious damage in oak trees: in many infected oaks, leaves wither rapidly, large cracks appear in the bark, and the trees die. A spread of P. ramorum represents a serious threat to the forests in the western states. Several methods of protecting the forests have been proposed. First, stopping P. ramorum spores from spreading would surely be an effective method. Spores are small particles through which all fungi, including P. ramorum , reproduce. Researchers have discovered that

many P. ramorum spores can be found along hiking or biking trails, suggesting humanassisted spread by way of shoes and bicycle tires. A few measures to prevent such humanassisted spread-like encouraging hikers to wash their shoes and installing new bike scrubbers on bicycle trails- would be an effective and low-cost way to stop the spread of P. ramorum. Second, there are a few fungicidal( fungusfighting) chemicals that can be used to protect the oak trees. Some of these chemicals stimulate the oak trees’ natural defenses against the P. ramorum fungus and have been found in small-scale tryouts to significantly reduce the likelihood that the oaks will be infected. A third way to fight P. ramorum is a practice

called clear-cutting. This approach starts with cutting and burning the diseased oaks, but it also involves cutting and burning the seemingly healthy vegetation( bushes and other kinds of trees) surrounding the oaks. This is done because some of the surrounding plants and trees may be infected even though they do not show any symptoms of the disease. Cleaning larger areas of vegetation in places where diseased trees are found is often an efficient measure to stop the spread of infections.

听力 Unfortunately, the methods described in the passage have serious limitations that make them ineffective or impractical on many situations.

Precautions like cleaning shoes and using bike scrubbers might help a little. But they may not make a huge impact. The problem is that P. ramorum spores did not spread just through human assistance. Studies have detected the spores in water streams following rainfall. It appears that spores are frequently picked up by rain water and washed into water streams with them carried along distances. This type of water assistance spread is much harder to control. Second, the fungicide referred to in the reading provide effective protection against P ramorum only when they are injected directly into tree trunks. But the effect last only for few months, so the injection have to be repeated. Now this type of treatment might work if you want to protect few oak trees in the city park. But it just not very practical when protecting

whole forests that contains thousands and thousands of oak trees. Can you imagine forest managers have to inject every single oak in a large forest when dose fungicide every few months. That sound impractical and also too expensive. Third, as the reading explains, clear-cutting destroy the vegetation around the sick oak trees, because some of that vegetation maybe infected. But keep in mind, some of that surrounding vegetation is in fact healthy. And it gets destroyed in the process anyway. Now, in some forests, it makes sense to sacrifice some healthy vegetation in order to stop the spread of dangerous disease; however, the vegetation in many forests of the United State include tree species that very rare and cannot grow back easily. So clear-cutting in these forests would mean destroying many health

rare trees. This would represent the greater ecological damage than the P ramorum damage trying to prevent.


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