The pilot in the tailwheel-equipped airplane reported that during landing, the airplane encountered a variable crosswind gust and the left wing lifted. He was able to keep the airplane on the runway and established a landing roll. The ground speed was about 20 miles per hour and the airplane encountered another wind gust. The airplane weathervaned to the left and exited the left side of the runway, rolled into mud and nosed over. The airplane sustained substantial damage to the rudder. The pilot reported that the wind at the time of the accident was variable at 5 knots and gusting to 8 knots. The METAR at the nearest airport was about 12 miles to the west of the accident site, and reported that about the time of the accident, the wind was out from 02 at 6 knots. The pilot landed the airplane on runway 25. The pilot reported that there were no mechanical malfunctions or failures with the airplane that would have precluded normal operation.