The pilot was spraying a bean crop in a valley. During a procedure turn the aircraft began to sink towards trees on the side of the valley. The pilot immediately applied full power and dumped the load, but the sink could not be arrested and the aircraft struck the trees. When the sink was encountered the pilot was committed to continue the turn to avoid terrain. The surface wind in the valley was a light southerly breeze, but there was a strong south-easterly at 2000 feet. The pilot had sprayed the field on previous occasions, and was familiar with the local area. The sink experienced on this occasion was considered to be the result of the strong wind at height combined with the local topography. This type of situation is documented in the Agricultural Pilots Manual. No faults were found with the aircraft that may have contributed to the occurrence. It was evident that when the sink occurred the performance available from the aircraft was insufficient to enable the pilot to avoid collision with the trees.