Sukkah, Chapter 3, Mishnah 6. This mishnah deals with the disqualifications of the esrog: it first lists the things that disqualify an esrog, and afterward those very same things in a measure or manner in which they do not disqualify it.
The disqualifications listed in the mishnah:
"V'chein chazazis al rubo, nitlah pitmaso, niklaf, nisdak, nikav v'chaser kol shehu - pasul" - So too, if a chazazis covers most of it, if its pitam was removed, if it was peeled, split, or perforated and is missing any amount at all, it is invalid:
Chazazis al rubo - a chazazis is a type of scab that forms on the esrog, and when it covers most of the esrog, it is invalid.
Nitlah pitmaso - the pitam is the point at the top of the esrog where the flower once was.
Niklaf, nisdak, nikav v'chaser kol shehu - even the smallest missing amount renders it invalid.
Explaining "nisdak": Some explain that this refers to a split running lengthwise along both sides, while others explain that even if it split on one side only, if the split reaches the middle section, it is invalid.
Explaining "nikav v'chaser": The commentators disagree as to whether this is a single case or two separate cases:
Some explain that it is a single statement: it was perforated and material was lost from it at the same time.
Others explain that these are two separate laws: nikav - a hole that passes from one side to the other, even if nothing at all is missing from it. For example, if a needle was pushed through it all the way, creating a hole that passes from side to side without any of the flesh of the esrog being removed, it is invalid, because the hole goes all the way through. Chaser - even if only a small amount is missing, and even though the hole does not pass from side to side, as long as something is missing, it is invalid.
Explaining "niklaf": The Gemara distinguishes between an esrog that was entirely peeled and one that was only partially peeled. Rashi explains that if it was partially peeled it is invalid, because it looks spotted; but if it was entirely peeled, as long as it is only the very thin outer layer and the esrog remains green in appearance, with nothing missing from the body of the esrog itself, it is valid, since it does not look spotted and its outer greenness remains in place.
The valid cases listed in the mishnah:
"Alsah chazazis al mi'uto, nital uktzo, nikav v'lo chaser - kasher" - If a chazazis covers only a minority of it, if its stem was removed, or if it was perforated but nothing is missing, it is valid:
Chazazis al mi'uto - when the chazazis covers only a minority of the esrog.
Nital uktzo - the uketz at the bottom of the esrog, that small piece of the stem which remains after it is cut.
Nikav v'lo chaser - a hole that does not cause anything to be missing from the body of the esrog.
Regarding the law of a chazazis on a minority of the esrog, three conditions must be added:
It is valid specifically when the chazazis is in one place, and that place is a minority of the esrog and not the majority.
If it is in two or three places, even though the total amount is small, it is invalid.
And if it is on the upper part of the esrog, even in a single spot and even in a small spot, this is a problem.
The color of the esrog:
"Esrog haKushi pasul" - a black esrog is invalid. However, the Maharsham explained that this applies only in a place where it is not the nature of the esrog to be dark or black, but in a place where this is its normal appearance, it is valid.
"V'hayarok k'karti, Rabbi Meir machshir v'Rabbi Yehudah posel" - Regarding an esrog whose color is green like a leek, a very deep green, the Tannaim disagree: Rabbi Meir declares it valid, and Rabbi Yehudah declares it invalid, because it looks like a fruit that has not ripened.
In summary: In this mishnah we learned the disqualifications of the esrog - a chazazis covering most of it, a removed pitam, peeled, split, and perforated with any amount missing - along with the explanations of their details; and in contrast, the valid cases - a chazazis on a minority of it, a removed stem, and a hole with nothing missing - along with their conditions. We also examined the law of the esrog's color: the black esrog, and the dispute between Rabbi Meir and Rabbi Yehudah regarding one that is green like a leek.