The Rosh writes that if an Etrog does not possess a style, or Pitam, it is Halachically valid as long as it never had Pitam to begin with. There is difficulty with this statement because all Etrogim have a Pitam and three contemporary rabbis clarify the Rosh’s position. Rav Elyashiv clarifies the Rosh by explaining that if as long as the Pitam fell off within a third of the fruit’s growth process. Rabbi Shlomo Zalman Auerbach states that an Etrog without a Pitam is valid as long as the Pitam fell off while the Etrog was still on the tree. Finally, Rabbi Chaim Scheinberg’s approach is that an Etrog is valid if it never developed a Pitam in the first place. Since we cannot be certain as to what the Rosh was referring to, one should only use an Etrog with a Pitam. Moroccan Etrogim are ideal because they have a Pitam and are certainly valid without a doubt.
Summary: Only Etrogim with a Pitam should be used for the Arba Minim. Moroccan Etrogim fulfil this requirement.