They are known as "black diamonds" and fetch astronomical prices at Christmas and New Year. But France's seasonal black truffle frenzy has turned violent after a farmer shot dead a trespasser he feared was trying to steal his prized tubers.

Laurent Rimbaud, 32, a truffle grower from the Drôme region of southern France, appeared before prosecutors today after turning his hunting rifle on a man he spotted loitering among the oak trees of his truffle patch.

Rimbaud, head of the union of young farmers in the Drome, told police he was patrolling his truffle fields in the village of Grignan when he was frightened by an intruder who he thought was armed. He shot the man twice in the thigh and head. The man, 43, a father of two from a neighbouring village, died shortly afterwards. Local press reports said he was found not with a gun, but a knife for cutting out truffles. The state prosecutor said the man was known to police for theft.

The killing has already become a cause célèbre within the close-knit truffle community of southern France. Around 250 supporters, mainly farmers and truffle growers, immediately staged a march in favour of Rimbaud, a part-time volunteer fireman and the well-connected son of local farmers and wine producers. Truffle producers on the march said they lived in fear of snafflers digging up their fields at night and their livelihoods were under threat.

The mature black truffles that are native to southern France are in high demand at Christmas and sell for between €800 and €1,000 a kilo after being sniffed out by truffle dogs. The Tricastin area where Rimbaud farms is the biggest area of black-truffle production in Europe and its truffles carry a special appellation.

French chefs recommend the truffle is cooked with eggs or foie gras or turned into ice cream.

Growers have complained of feeling under threat from cheaper black truffles from China. From mid-November to mid-March, and particularly in mid-January when the pungent truffle flavour is said to peak, farmers say trespassers scour their land at night looking for truffles.

"Our truffle fields are like open-air safes [full of money] and when times are hard certain thieves help themselves," Joel Barthélémy, deputy head of the Tricastin truffle growers' union, told La Provence newspaper. "But I've told my fellow producers never to patrol their fields with a gun. The temptation is too high."