Commentary for Sukkah 3:6
עָלְתָה חֲזָזִית עַל רֻבּוֹ, נִטְּלָה פִטְמָתוֹ, נִקְלַף, נִסְדַּק, נִקַּב וְחָסַר כָּל שֶׁהוּא, פָּסוּל. עָלְתָה חֲזָזִית עַל מִעוּטוֹ, נִטַּל עֻקְצוֹ, נִקַּב וְלֹא חָסַר כָּל שֶׁהוּא, כָּשֵׁר. אֶתְרוֹג הַכּוּשִׁי, פָּסוּל. וְהַיָרוֹק כְּכַרְתִי, רַבִּי מֵאִיר מַכְשִׁיר, וְרַבִּי יְהוּדָה פּוֹסֵל:
If most of it (the ethrog) were covered with lichen (it is pasul) [It is in one place that "most of it" is required (to render it "pasul"); but in two or three places, even (if lichen covered) the lesser part of it, it is pasul. And on its pitma (its upper protuberance, as in "the pitma of a pomegranate"), even any amount (so covered) renders it pasul, it being more conspicuous there than in other places.] If its pitma were removed, if it were peeled, if it were split, if it were punctured and something of it were missing, it is pasul. [("if it were peeled":) Only if most of it were peeled; but if part of it, it is kasher. Others say the opposite, viz.: If part of it were peeled, it is pasul; for then it looks "spotted." But if all of it were peeled, it is kasher. "peeled" here does not mean that the skin was removed so that the white could be seen; for in that instance it would be chasser ("lacking") and pasul; but that a thin layer was removed and its appearance is green as before. ("punctured":) If it were punctured through and through, even a very slight puncture, it is pasul. And if it did not go through and nothing were lost, as when he stuck a thick peg into it — if the hole were as wide as an issar (a Roman coin), it is pasul; if less than that, it is kasher. And this is the intent of "if it were punctured but lacked nothing" below, i.e., not punctured through and through, and not as wide as an issar.] If its lesser part were covered with lichen, if its uketz ("tail," peduncle) were removed, or if it were punctured but lacked nothing, it is kasher. An Ethiopian ethrog [one that grows here (in Eretz Yisrael), but is black] is pasul. [But such an ethrog growing in Ethiopia is the norm and is kasher. A leek-green ethrog — R. Meir rules it kasher; R. Yehudah rules it pasul. [All of these are pasul only on the first festival, but on the second and the intervening days, everything is kasher.]
Bartenura on Mishnah Sukkah
English Explanation of Mishnah Sukkah
If a rash spread out on a lesser part of it, if its stem was missing, or if it is perforated but no part of it is missing, it is valid.
An etrog [which is black] as an Ethiopian is invalid.
An etrog which is green as a leek: Rabbi Meir declares it valid And Rabbi Judah declares it invalid.
The Torah says calls an etrog “the beautiful fruit of the tree” or “the fruit of the beautiful tree.” Due to the mention of the word “beautiful (hadar)" the rabbis were more demanding regarding the physical perfection of the etrog than of the other three species. Indeed, to this day people are very concerned that they should have a beautiful “mehudar” etrog. An extreme example of this is the man who paid one thousand dollars for an etrog in the movie “Ushpizin,” a movie which I recommend.
Sections one and two: If there is a rash, which might refer to some discoloration, or things like warts (not just bumps, which are considered desirable in an etrog) on a majority of the etrog, than it is invalid. The pitom is the funny looking mushroom which sticks out of the ends of some etrogim (Google pitom and etrog and you can find some nice pictures). I should note that not all etrogim have a pitom. An etrog without a pitom is valid, indeed in some ways it is preferable because it is less likely to become invalid. The etrog is invalid only if the pitom was there and was then removed. However, if its stem, the part of it which attached it to the tree is removed, it is still valid. The stem is basically not part of the etrog.
Likewise it is invalid if it is peeled, split or any part of it is missing.
Section three: A black etrog is invalid.
Section four: There is a debate over the green etrog, Rabbi Meir declaring it valid and Rabbi Judah invalidating it. In Israel one sees many green etrogim, which never fails to surprise my family (as does the pitom-less etrog).