The book of Leviticus commands that the fall festival of Sukkot, which begins Sunday night, should be celebrated by taking palm, myrtle, and willow branches along with a pri ets hadar—usually translated as the “fruit of goodly trees”—and “rejoicing before the Lord.” By the 1st century CE, most Jews seem to have accepted that the fourth, ambiguous item was a citron (an etrog in Hebrew), which is used in the Sukkot ritual to this day. Rachel Scheinerman presents a brief history of this fruit, which was the first in the citrus family to make the journey from eastern to western Asia:
The etrog is indigenous not to the Land of Israel but to China. Several millennia ago it grew most abundantly in Yunnan, a southwestern Chinese province, where it is still used, to this day, in traditional Chinese medicine. . . . From there, it traveled to northeastern India and westward across the subcontinent, . . . and became a component of traditional Ayurvedic medicines used to treat everything from stomach complaints to hemorrhoids to infertility. One of the Buddhist gods, Jambhala (the etrog was sometimes called jambhila), was often depicted holding an etrog as a symbol of fertility because of its high density of seeds. (One is reminded of the medieval Ashkenazi custom for a pregnant woman to bite off the tip of the etrog at the end of the holiday.)
When Darius I conquered India in 518 BCE, the fruit spread to Persia. Now it was called wādrang, which seems to be the linguistic precursor to the Aramaic word etrog (and also the English word orange). . . . [T]he etrog was regarded in Persia as a prized exotic species and carried from province to province. Throughout their vast empire, [which included the Land of Israel], the Persians built royal outposts bedizened with generously irrigated ornamental gardens called pairidaeza or “paradises.” . . .
One of these paradises was constructed after the Babylonian exile and before the conquests of Alexander the Great—that is, sometime between 538 BCE and 332 BCE—in Ramat Raḥel, which sits on the outskirts of present-day Jerusalem and is today home to a kibbutz. Archaeologists have recently positively identified (based on an analysis of fossilized pollen trapped in the plaster of one of its pools) the presence of eleven native and foreign species in that paradise, including the etrog. . . .
Once Leviticus 23:40 was understood by Jews as a description . . . of three bound boughs, known collectively as a lulav, and an etrog, this combination became an important Jewish symbol—alongside other symbols of Temple ritual like the menorah, shofar, firepan, and ark—found on synagogue walls, coins, and tombstones. In fact, the presence of a lulav-and-etrog icon signals to archaeologists that an ancient synagogue is Jewish rather than Samaritan, [since] the Samaritans [interpret the verse differently] and therefore have not incorporated this symbol.
Read more on Jewish Review of Books: https://jewishreviewofbooks.com/articles/3467/the-peculiar-history-of-the-etrog/