Why a large fungus? I don't see any reason to have a big and massive fungus. In the environment you describe, mass seems to be at a premium, so a more likely form would be a broad net of sticky strands to collect as much matter as possible - it might be very large, but flimsy. Of course, for your story you might need a massive fungus, but evolution doesn't seem to favor it. Sending away spores with an explosion might be useful on the ground, but in the zero gravity environment of the ring it would be much easier to glide far away from the mother colony (I would expect some self-propelling spores, a bit like oceanic plankton).
The space-travelling plant idea has been done several times, maybe you can invent something stranger? What about some kind of reef-building creatures with a symbiotic ecology, that break loose chunks of the spherical reefs to replicate it? That might produce a similar effect.
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Anders Sandberg Towards Ascension! asa@nada.kth.se http://www.nada.kth.se/~asa/GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y