On July 28, 2013, about 1544 central daylight time, a Robinson R44, N9681R, descended during a turn and impacted terrain during an aerial application near Capron, Illinois. The helicopter sustained substantial damage. The commercial pilot sustained serious injuries. The helicopter was registered to and operated by Back Nine Aerial LLC under 14 Code of Federal Regulations Part 137 as an aerial application flight that was not operating on a flight plan. Visual meteorological conditions prevailed for the flight that originated from East Troy Municipal Airport (57C), East Troy, Wisconsin. The pilot stated that he departed from 57C at 0640 to perform a series of aerial applications of fungicide on fields near Harvard, Illinois. About 1445, while flying downwind about 150 feet above a field, the helicopter began to descend. The pilot then turned the helicopter into the wind and increased pitch to arrest the descent, which resulted in a low main rotor RPM. There was not sufficient altitude to recover the main rotor RPM, and the helicopter settled into a wooded area contacting trees. The helicopter came to rest in an upright position. According to a Federal Aviation Administration inspector, there was a 3-4 inch tree limb that was broken and exhibited features consistent with being cut by the main rotor blade. Usable fuel was present aboard the helicopter, and there were no mechanical anomalies with the helicopter that would have precluded normal operation.