According to the pilot flying, the airplane was vectored onto the final approach course and the pilot not flying had configured the airplane to land. The pilot not flying reported that during the approach the airspeed was fluctuating by 20 knots. As the pilot flying initiated a flare to land, the airplane encountered a gust of wind, the airplane rolled left, and full opposite aileron was applied; however, the left wing impacted the runway. The airplane landed, slowed to taxi speed, and exited the runway. Examination of the airplane revealed substantial damage to the left outboard forward wing spar. The wind, approximately 5 minutes prior to the accident, was recorded as 310 degrees at 20 knots, gusting to 33 knots. A peak wind was recorded about the time of the accident as 300 degrees at 33 knots.