The man accused in the stabbing death of a 49-year-old Gustine grandmother pleaded not guilty to murder charges Friday. Alfonso Guzman, 52, entered the plea in Merced County Superior Court in Los Banos. He appeared on a television monitor from a live video feed at the Merced County Jail. Gustine police said they believe Guzman killed his sister-in-law, Linda Guzman, after an argument at her Sycamore Avenue home Wednesday. Police said Linda Guzman's 6-year-old granddaughter witnessed the stabbing. Alfonso Guzman remains at the county jail without bail. Alfonso Guzman had been living with Linda Guzman and her husband for at least several months, police said. The homicide is the seventh in Merced County this year and the first in Gustine since June 2008.