Rummel:
"Of course, Zubrin has done just that with straw-man arguments placed for his own convenience."
Zubrin:
"The story goes like this: no Earth organism has ever been exposed to Martian organisms, and therefore we would have no resistance to diseases caused by Martian pathogens. Until we can be assured that Mars is free of harmful diseases, we cannot risk exposing a crew to such a peril, which could easily kill them or, if it didn't, return to Earth with the crew to destroy not only the human race but the entire terrestrial biosphere."

IMO, Zubrin above statement is the straw-man Rummel is referring to.
Who is talking about manned exploration as far as "Martian pathogens"?
Who is worried about some martian disease affecting some crew going to Mars?
Currently, there isn't a crew going. If there is a concern about contamination and a crew going there, it would be (I would imagine) mostly about the crew contaminating Mars.
We probably need to get more infomation about Mars before possibly sending crew- whether there is life on Mars would only be one of the things which might be useful to know before we go.
The main point of Zubrin's statement is to imply that the argument is concerning possible human diseases from Maritian life. This isn't the argument, therefore it's a strawman.
Most people are wise enough to avoid arguing against "straw-man" arguments because it's simply a distraction- they generally simply identify it as a straw-man and leave it alone- I am, of course, "playing with it":)

ME:
"The dutch elm disease comment was a flat out lapse of logic and it doesn't take a degree in biology to see it."

Rummel:
"But readers should be reminded that many Earth organisms, while not coevolved pathogens (for example, ergot fungus, botulism bacteria), are nonetheless dangerous. And Bob should consider his own example (Dutch elm disease) a reminder that organisms, while not human pathogens, can yet cause damage, both environmentally and economically. Still, he insists that Mars life unrelated to Earth organisms couldn't possibly cause harm. How does he know, when we have precisely zero experience with life unrelated to Earth life?"

IMO, Rummel isn't arguing against the strawman part of Zubrin statement but the last part "...but the entire terrestrial biosphere."
In other words what affect could Mars organisms have on Earth organisms.
We don't know anything about Mars life [if it exists]- for instance, there "could" such a things as nanobacteria- " Some scientists believe that life can be very small indeed. Called nanobes, nanobacteria, or nano-organisms, these miniscule structures borrow their name from their unit of measurement, the nanometer." and:
" Folk, for one, believes the nanobes in the Martian meteorite are definitely fossil structures indicative of past life. "
http://spacedaily.com/news/life-01za.html
What is known about these "nanobes"? Practically nothing, it's not even scientifically accepted that they exist.
I'm not saying that Rummel even considers "nanobes" a possiblity, what Rummel is saying is that though Dutch elm disease does not cause human disease, it does affect humans (their elm trees die). It's possible that there is more critical lifeforms (not elm trees) which by being affected by the putative martian life could have more impact on humans living in more urban areas:)