On April 27, 1998, at 0715 hours Pacific daylight time, a Schweizer G164D, N31785, was destroyed when it impacted terrain near Robbins, California. The solo commercial pilot sustained minor injuries and no property damage was incurred. The flight had departed Tolle's Flying Service southwest of Robbins on an aerial application flight, and was being operated by Tolle's Flying Service, Inc. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot stated that the rudder post broke on his first application run. This resulted in loss of flight control which caused the airplane to drag a wing tip and cartwheel into the ground. During a postflight inspection of the rudder assembly, it was noted that the rudder post was extensively oxidized (rusted) in the area of the break point. There were also rust deposits on approximately 1/3 of the circumference of the parting surfaces.