A semi-truck caught fire yesterday afternoon, sparking a small grass fire. CHP investigators say the truck driver was southbound on the 101 just south of Atascadero when a rear tire went flat. He pulled over to the shoulder and noticed smoke. By the time he got a fire extinguisher from the cab, flames had spread to the kindling dry vegetation and the trailer was involved in flames. Southbound traffic was diverted while crews battled the fire. No injuries were reported.