The pilot had been spreading in the general area of the strip for most of the afternoon. When he commenced operations to the south of the strip an area of sink was experienced on approach to the selected paddock. To allow for this, a reduced weight was uplifted on following flights, and a further six loads were spread successfully. However, as he commenced the turn to line up for the first spreading run with a new load, strong sink and turbulence were encountered. Realising that the aircraft would not clear a line of trees, the pilot continued the turn to the left but the angle of bank suddenly increased and the nose pitched down. The pilot dumped the load and applied right rudder but the aircraft struck the ground heavily. No defect with the aircraft was subsequently discovered which may have contributed to the development of the accident. It was likely that the aircraft was affected by a strong wind gust, which generated a substantial downdraft. The aircraft stalled during the pilot's efforts to avoid the trees, and there was insufficient height available in which a recovery could be effected. It was possible that had the pilot dumped the load before the aircraft stalled, the aircraft may have had sufficient performance to counteract the downdraft.