BEILIU, China — For the past three decades, Qin Chenghao has been a farmer, tending the trees covering the mountains that rise from the soil of southern China, harvesting their star-shaped fruit. Year after year, the same few traders arrived to buy his crop to sell as seasoning and traditional medicine.

This year, however, Qin's world changed. The star anise dangling from his trees emerged as a source for the key ingredient in Tamiflu, a pharmaceutical known to lessen the severity of avian flu. The output from his 1,300-acre Darong Mountain Star Anise Plantation in Guangxi province is now more than a simple means of spicing up stewed pork. It is a crucial weapon in a global campaign against a pandemic that health experts say could kill tens of millions of people.

The price of Qin's crop has nearly tripled in the past four months, reaching about 80 cents per pound. Stocks have disappeared earlier in the season than anyone can remember, as Chinese pharmaceutical companies snap up what fruits they can to extract the shikimic acid contained within — the substance that is the basis of Tamiflu.

"All of a sudden, our industry has become so important," Qin said, as he reached his chopsticks toward a plate of stir-fried wild greens before halting to answer a call on his cell phone. "Before, it was pretty quiet. Now, I answer my phone all day long. People call from all over the country. It's never been like this before."

With its semi-tropical climate and crowded cities and villages chockablock with pork and poultry farms, southern China is believed to be the source of the H5N1 avian influenza, which has been blamed for the deaths of at least 64 people in Asia since 2003. Now the same area may hold the antidote as well.

Dried star anise — or bajiao, as it is called in Mandarin — is a spice found in many Chinese kitchens, imparting a licorice-like taste to stewed meats. Chinese doctors have long prescribed bajiao to treat colic in babies, as well as headaches, abdominal pain and intestinal distress in adults.

More recently, farmers in northeastern China have mixed bajiao into animal feed because it keeps livestock warm through the winter. Although most star anise is consumed domestically, a small export market also exists. The French, for example, are increasingly using star anise as flavoring for Pernod and other anisette liqueurs. And in the United States, star anise is found in five-spice powders available in grocery stores.

When Tamiflu was invented nearly a decade ago by researchers at Gilead Sciences Inc. in California, they used a different substance, quinic acid from the tropical cinchona tree. But when Switzerland-based Roche Holding AG bought a license to make the drug, it substituted a form of star anise found in southwestern China.

This year, with bird flu hopscotching from Southeast Asia to Turkey to Britain, Roche has embarked on an expansion campaign aimed at increasing the production of Tamiflu tenfold over 2003 levels by the end of next year. Guangxi province — home to 90 percent of China's star anise, which is itself the source of 90 percent of the global supply — has become the heart of a crucial industry.

Health authorities do not recommend using star anise to try to ward off the flu. While production of Tamiflu starts with the acid contained in star anise, it involves multiple chemical steps, some using dangerous explosives, and the resulting drug does not resemble the original material. Moreover, the U.S. Food and Drug Administration warned in 2003 that some star anise teas sold in health-food stores were making people dangerously ill. These were thought to be made from Japanese star anise, which contains a dangerous compound not present in the Chinese spice. The FDA warned that "Japanese star anise in its dried or processed form cannot be distinguished from Chinese star anise through visual examination."

Pharmaceutical companies, who previously purchased little bajiao, are buying half the stocks, say traders. Some of them are processing star anise into shikimic acid for sale to Roche. A freshly sprouted star anise tree takes six years to produce fruit. So supply is likely to remain tight for some time.