The 10-acre brush fire was caused by a person mowing dry grass in the heat of the day, said Cal Fire Capt. Jane Schmitz.
The tractor did not have a spark arrestor, and that led to the start of the blaze, Schmitz said in a news release.
Wildland mowing should also be done before 10 a.m., when plants retain more moisture than in the hotter afternoon hours.
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The fire was contained by 4:57 p.m.
Nearly 60 firefighters are battling a blaze this afternoon near Nacimiento Lake.
The blaze was reported at 3:40 p.m. and by 4 p.m. had burned between five and 10 acres, according to a Cal Fire spokesman.
The fire is burning near the intersection of Gateway Drive and Nacimiento Lake Drive. Two structures are being threatened by flames.
Temperatures at the scene are well over 100 degrees.