Rif Succah 17a {Succah 31a continues; 34b; 35a; 34a}

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17a

{Succah 31a continues}
If he cannot find an etrog {a citron}, he should not bring instead a lemon or a pomegranate.

And we learn also in the Mishna {in Menachot 27a}: The four species in the lulav-set are prerequisites for one another.

And Rav Chana bar Abba said: They only stated this in the case where he did not have, but if he does have, they do not prevent him {from fulfilling}. And even though he takes each one by itself, he fulfills his obligation, for it is established for us that a lulav-set does not require binding.

{Succah 34b}
Mishna:
A stolen etrog, or a dry one, is invalid.
Of an idolatrous tree or of an idolatrous city is invalid.
Of orlah {the first three years' growth} is invalid.
Of impure terumah is invalid. Of pure {terumah} he should not take {up to fulfill his mitzvah}, and if he took, it is valid.
And of demai {which one is unsure whether it was tithed}: Bet Shammai invalidate and Bet Hillel validate.
Of maaser sheni in Yerushalayim, he should not take, and if he took, it is valid.

{Succah 35a}
Gemara:
The Sages learnt {in a brayta}: {Vayikra 23:40}:

מ וּלְקַחְתֶּם לָכֶם בַּיּוֹם הָרִאשׁוֹן, פְּרִי עֵץ הָדָר כַּפֹּת תְּמָרִים, וַעֲנַף עֵץ-עָבֹת, וְעַרְבֵי-נָחַל; וּשְׂמַחְתֶּם, לִפְנֵי יְהוָה אֱלֹהֵיכֶם--שִׁבְעַת יָמִים.

40 And ye shall take you on the first day the fruit of goodly trees, branches of palm-trees, and boughs of thick trees, and willows of the brook, and ye shall rejoice before the LORD your God seven days.

that the taste of the wood and the fruit is the same. Thus we say that this is the etrog {citron}.
And say it is peppers? It is impossible. What shall he do? If he takes one pepper {grain}, its taking will not be noticeable. If he takes two or three -- one פְּרִי the All-Merciful said, and not two or three fruits. Thus, it is impossible.

{Succah 34b}
Mishna:
If a stain spread over its majority, if it has lost its crown, or the fine rind has been peeled off, or if it is split, or perforated, if ever so little thereof is wanting, it is invalid.

If a stain spread over its minority, if it has lost its stalk, or if that be perforated but it is not missing anything, it is valid.

A dark-colored etrog is invalid. A leek-green one, Rabbi Meir validates it and Rabbi Yehuda invalidates.