After 24 hours, a fully grown vegepygmy bursts from the body of any creature slain by russet mold, provided the creature was Small or larger. For each size category larger than Small, the body produces one additional vegepygmy.
A patch of russet mold is unharmed by all effects save for acid, alcohol, or remove disease (or a similar magical effect, such as heal), all of which can kill a single patch of russet mold on contact. Sunlight doesn't kill the mold, but does render it dormant and harmless as long as the sunlight persists.
So this disease appears to act differently then every other one in the core book. Let me see if I break it down correctly.
Round 1: DC 15 Fortitude save to be perfectly fine, or infected
Round 2+: A new Fortitude save to prevent 2 Con damage, but does not cure the disease.
It seems that Remove Disease would work on it, but the wording also implies this is how you kill patches of it. Not the mold that's infecting a victim.
Is there not a set duration for Russet Mold? Will a certain amount of successful saves not stave it off?
It appears that, as is, if someone is infected they would die in a day without Divine help.
The fungus causes Constitution damage, but it is not a disease (immunity to disease does nothing to protect from the mold, per the description above).
As for stopping it, the text is very clear.
"A new Fortitude save can be attempted each round to halt the growth. Although immunity to disease won't protect against russet mold spores, the growth can be halted by remove disease and similar effects."
One Fortitude save stops the mold from growing (stops the effect, in other words), as does a remove disease.
Right, but it's still there. So you have to make a new save the next round and so on. It just doesn't deal the damage when you make the saves. At least the way I read it... Maybe "halt" is just a weird word to use here cause it comes across as being "a temporary stop."