Mould and fungus are the banes of my life. Living in South West England's damp climate in an old house I am constantly battling the spread of tiny filaments across my lenses. So often I pick out a lens, hold it up to the light and there is a new fan of fungus making its way across the surface of one or more lens elements. If it wasn't for the fact that I disassemble and clean the lenses myself, I'd have given up years ago rather than keep on paying a fortune to have lenses cleaned. I've tried sealed Tupperware containers with bags of silica gel, but that seems to make no difference. Whether lenses are stored in the light or dark doesn't seem to make any difference, but I do think that storage in lens or camera cases, whether leather or plastic, does seem to make matters worse.
One observation I'd make is that Pentax/Takumar lenses seem worse than most. The few lenses I have that are Vivitar, Tamron, etc. and a couple of lenses on non-Pentax cameras seem far less prone to the problem.
Steve