A U.S. official said today two more Asian nations, Taiwan and South Korea, have effectively banned California fruits and vegetables even though Japan has decided to drop similar prohibitions.

William Helms of the U.S. Agriculture Department's animal and plant health inspection service said the two countries have been refusing California fruits and vegetables because of Northern California's Mediterranean fruit fly infestation. The State Department announced today, however, that Japan had agreed to drop a similar ban on any California produce that was not fumigated or cold treated.

California state officials said Taiwan imported only $5.2 million in fresh fruit in 1980 and South Korea $1.3 million. These figures are minuscule in comparison to Japan's $118.5 million in fruit imports.

Federal officials and California farmers have feared that overseas restrictions could lead to a sharp drop in the international market for California produce if the ban spreads. Only a small fraction of California farmers, who produce half of U.S. fruits and vegetables, are equipped now for extensive fumigation and cold treatment.

A State Department statement today said that Japanese officials agreed, after discussions in Washington and Tokyo, to accept California fruits and vegetables that simply have been certified free of Medflies. Federal officials have already banned all shipments of fruits and vegetables out of a 2,374 square mile quarantine area in northern California. Fruits grown in the rest of the state where the insect has not appeared are certified free of the fly and do not require special treatment.The area under quarantine is primarily suburban and most of the richest agricultural areas of the state are still shipping produce without restrictions.

The State Department said the arrangement, accepted by Japan, "is consonant with procedures governing movement of these same fruits and vegetables in interstate commerce within the United States." Courts have overturned attempts by some southern states to ban, as Japan tried to do, all Medfly-susceptible California fruits and vegetables not fumigated or cold treated.

The state department said a U.S. technical team would leave for Japan today to consult with Japanese officials on the California anti-Medfly campaign and discuss the list of fruits and vegetables that are and are not susceptible to infestation. The Japanese ban would have applied to all California fruits and vegetables, even though melons, lettuce and corn are not Medfly hosts.

California shipped more than $100 million in fresh fruits and about $3.3 million in fresh vegetables to Japan in 1980. That is only a small portion of a $14 billion state agricultural industry, but officials feared the ban could have spread.