The commercial pilot was making a passenger flight under Title 14, CFR Part 135. He said he was landing at an off airport site, to drop off hunters. During the landing roll he said the airplane started to drift off the runway. He said he applied rudder and brake to correct for the drift. The pilot said the airplane was skidding in the mud, and the nose wheel encountered a hole. The nose wheel separated from the airplane, and the left wing struck the ground, damaging the spar. The pilot reported there were no mechanical problems with the airplane prior to the accident.