Depending on your geographic location in the US, solutions for wet crawl space can range from doing nothing, to encapsulating the whole area. http://www.mrhardware.com/ When putting a fan to vent a crawl space you will want it to turn itself off once the humidity falls below about 60 percent or so.
Most foundation vents come wired with a humidistat but this one ran continuously so I wired in this controller.
If the crawlspace is warm and humid, fungus will grow on any surface that is either porous or slightly dirty. Crawl spaces usually contain concrete or masonry foundation walls and porous materials like wood. These can stay wet, and keep humidity high to support rot, fungus and mildew.
For me in Michigan, moving air with a fan and humidistat under the house will work just fine because I can bring in dry air from outside vents from around the crawl space, not from the house above.
First, do these 2 things to solve most water problems at the foundation and in the crawl space:
1. Place dirt around the foundation to create a slope 6 feet away from the house at 1 inch per foot.
2. Have working gutters and downspouts, or large eave overhangs.