NARRATOR：

Now listen to part of a lecture on the topic you just read about.

FEMALE PROFESSOR：

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

Precautions like cleaning shoes and using bike scrubbers might help a little but they will not make a huge impact.

The problem is that P. ramorum spores do not spread just through human assistance.

Studies have detected the spores in water streams following rainfall.

It appears that the spores are frequently picked up by rainwater and washed into water streams, which then carry them long distances.

This type of water-assisted spread is much harder to control.

Second, the fungicides referred to in the reading provide effective protection against P. ramorum only when they’re injected directly into the tree trunks.

But their effect lasts only for a few months, so the injections have to be repeated.

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.

Can you imagine forest managers having to inject every single oak in a large forest with a dose of fungicide every few months?

That sounds impractical and also too expensive.

Third, as the reading explains, clear-cutting destroys the vegetation around the sick oak trees, because some of that vegetation may be infected.

But keep in mind that 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 a dangerous disease.

However, the vegetation in many forests of the western United States includes tree species that are very rare and cannot grow back easily.

So, clear-cutting in these forests would mean destroying many healthy rare trees.

This would represent a greater ecological damage than the P. ramorum damage we’re trying to prevent.