Laurent Rambaud, 37, had grown increasingly frustrated with a spate of thefts in the southeastern region of Drome, and on the night of December 20, 2010, headed out into his fields armed with a shotgun.
He came across 43-year-old Ernest Pardo, who was known as a truffle hunter and had previous indictments for theft, who was walking with his dog.
Taking him for a thief and believing him to be armed, Rambaud fired two shots on Pardo, killing him almost immediately.
The charge against Rambaud has been reduced from murder to manslaughter after the court decided the killing was not premeditated, but the 37-year-old still faces 30 years in prison if convicted on Friday.
Rambaud, who was president of the young farmers association in the region and a volunteer fireman, had repeatedly complained about thefts.
Truffles are a delicacy in France, known as "black diamonds" and worth around €1,000 ($1,100) per kilo during the holiday season.
"This is a man who felt in danger that night. He was scared," said his lawyer Alain Fort in opening remarks, who added local farmers had been confronted by armed thieves.
The trial continues, with judges due to give their verdict on Friday.
In 2014 The Local reported how French truffle growers are up in arms over cheaper imports from China which they say are being doctored by unscrupulous chefs and passed off fraudulently as the hugely prized local variety.
In comparison to the price of truffles in France, the pale tuber indicum variety from China and the Himalayan foothills fetches only €30 a kilo.
French trufficulteurs, as the cultivators are known, allege that some eateries spray scents and chemical additives on the Asian variety and pass them off as Perigord truffles to an unsuspecting clientele.