The pilot stated that during the landing roll, a gust of wind impacted the airplane, which turned to the left and departed the left side of the 50-feet-wide, turf runway. After the pilot regained control, he attempted to avoid a ditch by imputing right rudder. The left main landing gear impacted the ditch resulting in substantial damage to the left wing spar. The pilot reported there were no preexisting mechanical anomalies with the airplane.