While maneuvering at low level over a field during an aerial application flight, the pilot took his hand off the collective to adjust the sun visor because the sun was in his eyes. The pilot reported that he had used his leg to block the collective from moving as he adjusted the visor. The helicopter hit mild turbulence and bumped his leg away from the collective. The collective slipped down, and the helicopter impacted the ground and rolled over on its side. The helicopter sustained substantial damage to the tailboom and cockpit. The pilot stated that there were no mechanical failures or malfunctions with the helicopter's flight control system or engine prior to the accident.