On June 21, 2011, about 1035 Pacific daylight time, a Schweizer Airplane Corp. G-164B, N3627G, departed the runway during landing at Yuba County Airport, Marysville, California. Sutter Butte Dusters, Inc., was operating the airplane under the provisions of Title 14 Code of Federal Regulations (CFR) Part 137. The commercial pilot was not injured, and the airplane sustained substantial damage. The aerial application flight departed from a dirt strip near Gridley, California, about 0930. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that during the takeoff roll, just prior to rotation, the right main landing gear separated from the fuselage. He continued the takeoff and elected to land at Yuba County Airport. During the landing roll, the airplane departed the runway sustaining substantial damage to the lower left wing. Examination of the right main landing gear leg revealed that the outboard segment had separated from the airplane at its root, with the fracture surface in line with the outboard edge of the airframe support structure. The outboard section of the spring leg was sent to the NTSB Office of Research and Engineering, Materials Laboratory Division for examination. Visual examinations of the fracture surface and surrounding material revealed shear lips and granular surface traces initiating from a discolored thumbnail region located slightly forward of the middle point of the lower spring surface. Inspection of the thumbnail region revealed two bands of oxidation containing microscopic fracture features. The inner band was more heavily oxidized, and contained features indicative of fatigue progression. The fatigue initiated on the lower surface and propagated upward into the spring material. The fatigue region measured approximately 0.163 inch wide and had penetrated about 0.072 inch into the gear material. The second outer band displayed lighter colored oxidization features. The overall thumbnail region measured about 0.317 inch wide and had penetrated 0.137 inch into the gear. Outside of the oxidized area the fracture displayed ridges and granular features was consistent with rapid overstress emanating from the thumbnail. Examination of the lower surface of the spring revealed corrosion pitting at the thumbnail origin, and along the line of the fracture. The paint in the area appeared partially intact. Corrosion products were visible on the surface, and pitting was visible under the paint. The paint was also blistered in many areas. The fixed-gear, tailwheel equipped biplane, serial number 729B, was manufactured in 1983. The airplane’s classification was restricted, with a category of agriculture and pest control. A review of the maintenance logbooks revealed a total airframe time of 13,509 hours at the last 100-hour annual inspection, which occurred on June 16, 2011, 10 flight hours prior to the accident.