# It is important to buy an etrog that has a reliable hechsher (certification that it is not grafted) in order to be sure that is not grafted. <ref>The Rama (Responsa 226) writes that an etrog that is the product of an etrog tree grafted with a lemon tree is invalid because it no longer qualifies as an etrog. The Levush (649:4) writes likewise but for a different reason. He asserts that a grafted etrog is invalid since it was created in violation of kilayim (grafting trees of different species) even if a non-Jew did it. The Shevut Yaakov (1:36) disagrees with the Levush’s reasoning, since we hold that non-Jews are not obligated in the laws of kilayim. The Shevut Yaakov quotes a story in which the Shach permitted making a bracha upon a grafted etrog, but he adds that the Shach later retracted. In sum, almost all poskim consider a grafted etrog to be invalid, including the Magen Avraham (648:23), Taz (648:3), and Chazon Ovadia (p. 223). Rabbi Zvi Sobolofsky (“The Laws of the Daled Minim,” min. 3) advised getting an etrog with a reliable hechsher in order to make sure that it is not grafted. </ref>
# An etrog must be at least the size of an egg. <ref> See Mishna (Sukkah 34b), Rambam (7:8), and Shulchan Aruch (648:22). </ref>
# If the pitom falls off or the stem on the bottom of the etrog is completely removed, the etrog is invalid. <ref> The Mishna (34b) establishes that if the pitom fell off an etrog, the etrog is invalid, but if the oketz fell off, it is valid. The Gemara (35b) explains that the pitom is the rod-like protrusion (“buchanto”). Rashi (s.v. Tani) cites a dispute between his teachers about how to explain what the pitom and oketz are. Rabbeinu Yaakov explained pitom as the protrusion on top of the etrog and the oketz as the thick stem on the bottom. Rabbeinu Yitzchak, however, explained that oketz refers to the outer part of the thick stem on the bottom, while pitom refers to the inner part of that stem, meaning that the etrog would be invalid only if the stem on the bottom is totally removed, even the part that penetrates the etrog. Rashi sides with Rabbeinu Yaakov.
* The Rabbeinu Chananeil (36a) explains that the Gemara was not explaining the Mishna at all but rather adding another potential disqualification. According to his view, the pitom is on the top of the etrog and “buchanto” is on bottom. He adds that the oketz is the outer part of the stem on bottom, and if that is removed, the etrog remains valid. The Rif (17b) and Rambam (8:7) agree. The Rosh (3:16) writes that the minhag was to follow Rabbeinu Chananeil. Shulchan Aruch (648:7-8) concurs. Mishna Brurah (648:31) writes that if the bud on top of the pitom falls off, one should avoid using the etrog unless it is the nicest one available.