Secretary of Agriculture John R. Block, terming the situation under control, said yesterday he is optimistic that continued aerial spraying will end California's infestation of Mediterranean fruit flies.

"We have the situation in hand," Block said in an interivew on "Face the Nation" (CBS, WDVM) in which he also sharply criticized the five southern states that are moving to halt the movement of unfumigated California produce into their region.

Although the Medfly is considered a threat to about 200 types of fruits and vegetables, it has not been found in any of California's commercial growing areas. The fly has been spotted only around residential properties.

Block said that the aerial spraying of malathion, a potent insecticide, is "on schedule" and that California, with federal assistance, this week will broaden the area in which traps are set to monitor the flies' movement.

Block said the federal government would go "50-50" with the state on the cost of the aerial spraying program, but he said California must pay for expensive ground spraying and fruit tree stripping operations begun before the aerial spraying. This would leave the state with most of the estimated $53 million bill for the control effort.

Block said he was encourged that traps in a three-county infested area around San Jose were not turning up more fertil Medflies, suggesting that their spread may have been stemmed by the aerial assault.

Despite Block's optimism, problems were cropping up. The first round of spraying of a 150-square-mile infestation zone in the Santa Clara Valley was to have been completed yesterday, but heavy morning fog forced the helicopters to abandon their mission. Weather permitting, the final 30 square miles will be sprayed today.

Another worrisome note was the discovery Saturday of one new larval site in the Sunnyvale area, not far from San Jose. California officials nonetheless expressed hope that the spraying had stabilized the situation.

Block charged that Alabama, Florida, Mississippi, South Carolina and Texas, in their plan to quarantine the California produce that competes with their own, are "taking advantage of the federal government. . . . cIt is our jurisidiction, not theirs." He said the Department of Agriculture will continue its efforts to dissuade them from the ban.

Some California growers, meanwhile, worked hurriedly over the weekend to erect fumigation facilities that would bring their produce into compliance with the southern quarantine ultimatum. "Nobody can remember a crisis like this. Things are in a state of turmoil," said Steve George, an official of George Brothers, a major packing firm.

The agriculture secretary, denying politics figured in his criticism, again twitted Democratic Gov. Edmund G. (Jerry) Brown Jr. for his early refusal to resort to aerial spraying. "I think if he'd been a prominent Republican he might have seen the light earlier and decided to spray," Block said.