The number of truffles being found is also falling. We only discover two in Raymond's field that morning. There are not many, either, when we visit the local truffle market, held in Lalbenque every Tuesday between December to March, the harvesting season. A dozen farmers stand behind the traditional truffle bench on which their tiny baskets lie. The baskets hold just three or four truffles each.
A pompous ceremony opens this, the first market of the year. Members of l'Ordre du Diamant Noir (the Guild of the Black Diamond), wearing black robes and large hats and bearing silver medals on their chests, proclaim the mysteries of their prized mushroom. It is not until the whistle is blown and the market declared open that people start to talk. "Thirty years ago, we sold them by the tonne," says farmer Henri Bourgnou, "but who wants to grow truffles today? They need too much care and too much time for too unpredictable a result."
Tuber melanosporum, this subterranean fungus, will only appear 10 years after an oak or hazel tree has been planted, when the roots are sufficiently developed. Good climatic conditions are essential to its growth: it needs regular watering and a mild winter. In 1998, the summer was too dry and the winter came too early. On top of this, wild boars destroyed entire crops by digging up the fields. As a result, the region's first crop of the year is disast-rous. Fewer than 15kg are sold on the market this Tuesday. The farmers' only consolation is the price these precious few fetch: FFr2,200 (pounds 240) per kilogram, or pounds 25 for a single truffle the size of a plum.
"It's madness," says Pierre-Jean Pebeyre, whose family has been selling truffles for more than 100 years. They arrive at the Maison Pebeyre factory from all over France, Spain and Italy, and are very carefully chosen. "We select them on their colour - the blacker they are, the riper - and on their consistency. They have to be hard but not too hard, like a firm rubber ball. Then we scratch the skin a bit to see the inside. A good truffle is black with white veining, and with good contrast between the black and the white." Maison Pebeyre sells the truffles on fresh for around FFr4,000 (pounds 440) a kilogram, or makes them into preserves for luxury shops including Lafayette Gourmet in Paris, where 50g (two cherry-sized truffles) cost FFr428 (pounds 47) - which works out at an astonishing pounds 915 per kilogram. It is the price that confines this little mud-loving mushroom to the ghetto of super-luxurious foods.
At the start of this century, France produced around 1,000 tonnes of truffles a year, a figure which has now dropped to just 50 tonnes. The Maison Pebeyre dominates the market (it sells 20 tonnes annually), but Pierre-Jean knows that its future depends on the revival of truffle growing. To encourage farmers, the government has promised FFr5m (pounds 0.55m) over the next five years - a sum that will be matched by the regional governments. "But if a tree gives no truffles, there are no subsidies from the state or from Europe, as there are for a poor wheat crop," says Pierre-Jean.
The modern misconception that truffles are difficult to cook with is based on their cost and rarity. Nineteenth-century cookbooks show that truffles were as common then as garlic or potatoes, and meals would often include several whole truffles. As an astounded Ken Hom once remarked: "They couldn't get rid of the bloody things!"
Antony Worrall Thompson chose Italian white truffles as his luxury to take to his desert island. And Ken Hom finds French black truffles "so wonderful and so unique" that he has moved to Quercy "for love" of them. "I use them in my spring rolls," he says, "because with truffles, you really just want to steam or warm them. You don't need a lot to experience their flavour. Cook them with eggs and you will understand the glories of truffles ... "
Simplicity is the key word with this subterranean mushroom. The truffle has a delicate but surprisingly intense flavour that will penetrate a whole dish. Its earthy, slightly pungent taste fills the mouth. It isn't like a mushroom, nor anything else I've ever tasted before. To savour it, just put a thin slice of truffle, topped with coarse salt, in buttered bread and heat it in a hot oven for two minutes - it makes a wonderful snack. At Le Rendez-Vous in Cahors, chef David Blanco prepares a potato pancake (galette) with duck comfit and truffle for us. It is simple and delicious, the delicately cooked slices of truffle giving a crunchy touch to the meal and flooding it with their strong aroma.
The future of the truffle may soon depend on the scientific research that began 20 years ago to improve yields. But how, where and even why truffles grow at all remains mysterious. Unlike strawberries, baby truffles (mycelium) cannot simply be sown under a tree and mature truffles picked six months later. So far, the only successful scientific intervention has been treating tree roots before they are planted to accelerate the growth of the fungus. But the older, traditional farmers look down on those innovations. They are convinced that truffles and their development should remain a mystery, as an intrinsic part of their appeal.
The National Institute for Agronomic Research has even developed a synthetic truffle aroma now, that can fool the truffling dogs; the awful fear, these traditional farmers say, is that one day it will replace the aromatic fungus of which they are so proud.