As many as 50 Mediterranean fruit flies were found yesterday in the northern end of the richest agricultural valley of California, and a new quarantine was clamped down in the state as California continued battling to save its agriculture from a multibillion-dollar disaster.

State and federal officials announced the new quarantine, covering more than 200 square miles of the 1,500 square miles of Stanislaus County, after an emergency meeting late yesterday. The county is apparently the point where the destructive pest is making the trip from coastal areas to the fertile San Joaquin Valley.

Agriculture officials had hoped to contain the spread of the fruit-and-vegetable-destroying Medfly within the small growing region south of San Francisco where four counties are now under quarantine.

But late last week the pest broke through the barrier of low mountains and entered California's central valley, the state's great agricultural basin that supplies the nation with 97 percent of its apricots, 92 percent of its grapes, 66 percent of its peaches, and large portions of other crops. Altogether California grows about half the fresh fruits and vegetables consumed in America.

There is now no physical barrier to prevent the fly from infesting the entire 400-mile-long valley. If that happened and a statewide crop quarantine were enforced, the economic loss to the state and its agricultural industry could be $15 billion before the crisis was ended, according to state spokesman Annie Zeller.

It was probably careless travelers who brought the new danger to the state, Zeller said. The flies found yesterday, at least two of them confirmed to be fertile, as well as the six found in the previous four days, were found near a highway rest stop.

"We think somebody just got careless and took some infested fruit from Santa Clara County in their car," Zeller said. The travelers apparently got through the State Highway Patrol roadblock with some unnoticed fruit, and might have stopped at the rest stop and thrown away the skin or pit of some fruit that contained fruit fly larvae.

"This is the very thing we've been afraid of," Les Hubbard of the Western Growers' Association told United Press International. "We're scared stiff."

The tiny Mediterranean fruit flies can invade some 200 different fruits and vegetables by boring into them to lay their eggs. When the eggs hatch, the larvae eat the fruit or vegetable from the inside out before growing into adults and seeking new crops to infest.

In Stanislaus County, there is a $742 million farm output annually, chiefly in tomatoes, melons, walnuts, and apricots.

As soon as the flies were found there, near the town of Westley, the eradication team moved in to spray the area with the pesticide malathion. Between midnight and dawn yesterday, six helicopters sprayed 80 square miles of the area.

More spraying was scheduled for Monday and Tuesday behind the front lines in the valley, and officials continued to put out greater numbers of traps in the valley to try to monitor the spread of the fly. William Helms of the U.S. Department of Agriculture said that more than 30,000 traps are already out, making the monitoring effort the largest of its kind in this country in at least 30 years.

The quarantine halts all possibly infested products from being shipped out of the quarantine region before being treated. After treatment the products may go to market. Helms, assistant administrator of the Agriculture Department's research service, said that the whole state is now technically under federal quarantine, and it is actually the area of enforcement which is expanding with new discoveries of flies.

Doyle Wallace, head of the Western Stanislaus farmer's cooperative, said many of the crops could still make it to market even if they were infested, but many farmers would have to absorb the cost of treating them for fly larvae.

The cannery process for tomatoes, which includes cooking, automatically destroys the larvae. Another method of killing them is to put them in an airtight fumigation tank and pump it full of an insecticidal gas. Potential problems have cropped up with that method, however. Federal investigators tested 25 fumigation centers last week, and 22 of them failed to meet federal standards, so they could not be used now to clear fruit out of quarantine. Yet another potential treatment is to store the produce for many days at temperatures below 40 degrees.

Any of the methods costs money and results in some delays and crop losses.

There was some concern and disappointment in Florida as well yesterday, where another fruit fly was found near Tampa. Four flies were found a week ago, and spraying was begun. The state had gone five days without finding any flies until yesterday.