DO NOT spray bleach on the wood. First of all you would be putting yourself at risk breathing the chlorine spray. Second, the chlorine bleach may not kill the mildew. Third, the chlorine solution may also damage the wood. I read a technical bulletin a couple months ago that addressed this issue. Call a termite/pest control company and have them apply a mildewcide. Be sure the crawlspace is vented adequately, and install a plastic vapor barrier on the ground. If the crawlspace ventilation is lacking, you may have to install a power vent to improve it. Many dehumidifiers are not rated for operation in crawlspaces. Be sure to check the downspouts around the house to make sure the water runs AWAY from the house and foundation.