FRESNO, Calif. (AP) _ San Joaquin Valley orange growers have been fending off a heavy freeze that threatens their crops by using hovering helicopters to stir warm air down into colder air near the ground, producers said Saturday.

Temperatures dipped as low as 18 degrees in one Fresno County orchard Friday night, but two growers declined to tally crop losses.

''It's far too early to give an accurate assessment on the extent of the damage,'' Carl Pescosolido of Sequoia and Exeter Orange Cos. said Saturday morning.

But he added, ''it is clear we do have moderate to heavy damage, depending upon the quality of frost protection'' in orange groves. Sequoia and Exeter farms 5,000 acres throughout the lower San Joaquin Valley.

Perry Walker of Riverbend Farms in Sanger said wind machines drew warm air from an inversion layer into the company's groves near the foothills Friday night, averting damage. Cold air near the ground often lies under warmer air which can prevent a freeze if it is stirred and pumped downwards.

''Last night was a good night. I would say the damage was confined to the unprotected orchards in the valley,'' said Walker.

The National Weather Service said Fresno posted a low of 23 degrees on Christmas, tying a record set in 1962. Predictions for cloudy skies in the valley would bring the temperatures up a few degrees, the weather service said.

Oranges begin to suffer serious damage when the mercury dips below 26 degrees for more than an hour, said Pescosolido. The sugar content in oranges also determines when the pulp freezes.

A heavy freeze lasting three to four hours will turn orange pulp to mush and the fruit will be sold below market value for its juice, he said. San Joaquin Valley orchards are leading producers of the state's $363 million orange crop.

''Our assessment is that any citrus farmers without good frost protection would have considerable damage'' on Dec. 24 and Dec. 25, the two coldest nights of the recent cold front, said Pescosolido.

Sequoia and Exter had nine Bell 204 helicopters hovering over orange groves through the valley on Thursday and Friday nights. The helicopters, which lease for $600 per hour per machine, flew as long as eight hours over the chilliest groves Friday, said Pescosolido.

''It's better to do that rather than have no crop at all,'' he said. In some cases the helicopters stirring a warm inversion layer raised the temperature in the groves by five degrees.

Growers also ran water into the orchards, sparked up smudge pots and started wind machines to prevent frost from forming.