The Torah mentions the that one should take the “fruit of a beautiful tree” during Sukkot, and according to Oral Tradition, this refers to a citron, or Etrog. One issue that has arisen is that of grafted Etorgim; a reglar citron tree is delicate, requires much care and only bears fruit after several years. As such, some farmers grafted the Etorg tree with other fruit, such as lemons, and the resulting hybrid produced more fruit in less time. This grafted citron is known as Etrog Murkav and is Halachically invalid for use in the Arba Minim (Four Species).
Citrons originating from Morocco have a longstanding tradition as being bona fide Etrogim, and this has been upheld by Rabbi Machluf Abuhatzira (Yafe Sha’a § 54) and Rabbi Baruch Toledano. Rabbi Haim David Hazan (Yishre Lev) of Izmir, basing himself on several North African rabbis, testified on the validity of Moroccan Etrogim, as did Rabbi Yaakov Ettlinger (Aruch LaNer, Tosefet Bikkurim, § 645). Furthermore, the Brisker Rav and Rav Elyashiv were careful to seek out Moroccan Etrogim for use on Sukkot. Since Moroccan Etrogim are known to be seedless, Rabbi Israel Harpness wrote that this may be a disqualifying property, however his opinion was rejected by numerous rabbis.
Furthermore, citrons which have what is known in horticulture as a stigma, or Pitam in Hebrew, are known to be preferable for the Mitzvah of Arba Minim, and Moroccan Etrogim possess them. As well, since Israeli Etrogim this year (5776) may have a questionable status with regards to the laws of Shevi’it, there is a further advantage to using Moroccan Etrogim.
Summary: Moroccan Etrogim have a strong and authentic tradition of being Halachically valid, and are known to be especially beautiful.