The sport pilot was performing touch and go landings on runway 17R. During the fourth landing a wind gust turned the airplane to the left. The pilot attempted to correct the track of the airplane with the application of right rudder, however, the airplane did not respond. The pilot added power to go around and the airplane clipped an airport sign. The airplane landed on the grass adjacent to the runway. An examination of the airplane revealed substantial damage to the firewall. The pilot stated that there were no mechanical anomalies with the airplane prior to the accident. He stated further that the winds were variable and low level windshear had been reported at the airport.