Diaper.
That plastic over the foam has holes in it...
The fungus/mold/mildew spores get in it during the manufacturing process. The seat cover leaks anytime the bike gets wet, even from washing. The inside if my stock cover was solid black mildew, the seat foam had pink, green, black, orange, and yellow mushrooms and shelf fungi growing on it. Cool, disgusting, quite a stench, but very cool. 8)
The seat rebuild took roughly 3 square feet of 3/8" foam, by the time I got everything pieced together. Pan shape makes for some odd cutting. I think the rough piece of hide was 18"X24". Again, some funky cutting involved. I wish I'd done a better job on mine, it has two highly annoying (visible) stitch lines on the back of the seat. Oh, use stainles steel sheetmetal screws with flat washers to secure the cover, 1/4-3/8 inchers, I'd post the metric size, but don't remember the conversion formula, at any rate, the screws should be as long as one layer of foam is thick. Takes a lot of screws and washers, but, staple gun staples don't hold up as well.
-WD