The wooded hills of the Upper Tiber Valley, now swathed in gray mist and gold foliage, have a lot to hide.

Truffles, one of the world's most expensive delicacies, grow wild on the buried roots of some oak, willow, hazelnut and poplar trees here. Finding and unearthing the rare fungus is a ritual that rewards the diligent hunter and his essential companion, the dog with the well-trained nose.

This season, the forest is yielding its delicious prize but withholding a dark secret: Who is poisoning the truffle dogs and turning the autumn hunt into a slaughter?

Since the hunt started Sept. 28, at least 40 dogs have died after eating meat morsels laced with strychnine or weedkiller and dropped in the woods, according to dog owners, veterinarians and forestry service workers in this corner of Umbria, 110 miles north of Rome.

No one has been accused of the killings, but everyone in these woods, which cover 1,000 square miles and are the public property of eight hill towns, believes that the dogs are victims of an undeclared war among some of the valley's 1,833 licensed truffle hunters.

"The more truffles you can find, the richer you're going to get, so if your dog is better than my dog at finding truffles, then let's get rid of your dog," said Malcolm Holliday, president of the Anglo-Italian Society for the Protection of Animals. "This is exactly what's behind it -- this mentality."

Malicious poisoning in the pursuit of truffles is not new in rural Italy, but Holliday and other veterinarians here say there have never been so many dogs killed in one season.

Dog poisonings are not officially recorded, but no one in authority can recall more than 20 in an entire year in this valley, and many of those deaths were attributed to poison laid down by pheasant hunters each spring to kill foxes that compete for their prey.

What appears to be happening now is a cruel twist of the law of supply and demand: An unusually dry summer stretched into October, shriveling Italy's normal crop of white truffles by about half and doubling their price, to as much as $70 per ounce on the wholesale market. This stiffened competition to find them, raising the peril for the dogs.

"When the price goes this high, war breaks out," said Luigi Bigi, a veterinarian who has seen much of the carnage. "The dogs lose."

Italy is the only major producer of white truffles, which are hunted in Umbria, Tuscany and Piedmont until mid-December. The white truffle, or Tuber magnatum pico, is even rarer and more pungent than the precious black truffle, Tuber melanosporum, which is harvested in the spring in France, Spain and the same parts of Italy.

The truffle's versatility and value to the regional economy are on display this time each year under a sprawling white tent in the center of this ancient and prosperous town. The Truffle Fair features more than 30 varieties of truffle-seasoned oils, cheeses, sauces and dried pasta -- as well as whole truffles the size of golf balls.

Truffle hunters, mostly farmers and retired people with the time to search, earn as much as 70 percent of the wholesale price for the musky nuggets.

"Depending on how much time you invest, how much you know and how good your dog is, you can make as much as 10 million lire {about $6,000} in a season," said Fabrizio Gragnoli, a garbage collector who stretches his income by truffle hunting on weekends with Camillo.

Camillo -- half pointer, half setter -- was one of the first to taste poison this fall. As he sniffed the woods in early October, his legs wobbled and then folded under him. Gragnoli rushed him to Bigi, who pumped Camillo's stomach and saved his life.

Most poisoned dogs die in agony before they reach Bigi, whose office has become a trading post for information about the slaughter.

Some of the dogs ingested poison placed near their homes -- suggesting that the killer knew exactly which animals he wanted to eliminate. Others were hunting game and ate poisoned bait apparently meant for truffle dogs.

Some truffle hunters have reacted by muzzling their dogs, only to realize that this makes them sniff less effectively.

The Upper Tiber communities have started training 50 "Guardian Angels" to help roughly the same number of police officers patrol the woods around the clock. These unarmed volunteers, mostly truffle hunters themselves, will have power to make arrests, but most hunters are skeptical that one guard per 10 square miles is enough to stop the poisonings.

Most hunters, in fact, are loners. Regional truffle production statistics are vague, for instance, because the hunters are loath to share information that might interest a curious neighbor or a probing tax inspector. Their secrecy may compound the poisoning problem, other townspeople say, by discouraging wider collective action to stop the killing.

"There's too much fanaticism in this business today," lamented Lazzaro Frattini, 63, a legendary truffle hunter. "When prices were low, it wasn't like this. Truffle hunters would at least stop and say hello to each other. Now there are too many interests, too much jealousy and not enough pity for these dogs."