Gemara

2b (Mishnah (R. Gamliel)): An Esrog is like a tree in three ways.

3a: An Esrog has the nature of vegetables. Just like vegetables grow from all water (we irrigate them), and we take Ma'aser according to Lekitah (when they are picked), also Esrogim.

(Mishnah - R. Eliezer): An Esrog is like a tree in all ways.

Rosh Hashanah 15a (Rabah): If Chanatah (budding) of an Esrog was in Shishis (the sixth year), and it entered Shemitah, it is exempt from Bi'ur (eradication when it is no longer available for Chayos in the fields). If Chanatah was in Shemitah and it entered the seventh year, it is exempt from Ma'aser, but Bi'ur applies to it.

Question (Abaye): Granted, you are stringent in the Seifa (you are unsure whether it has Kedushas Shemitah, so you obligate making it Hefker, therefore it is exempt from Ma'aser). However, in the Reisha you exempt from Bi'ur. You must be sure that we follow Chanatah. If so, Ma'aser should apply to it!

Answer (Rabah): (Even if it is not Hefker,) everyone handles it (when they enter the field to take other fruits), therefore it is exempt from Ma'aser (like Hefker).

(Rav Hamnuna): If Chanatah was in Shishis and it entered Shemitah, it is like produce of Shishis in every way. If Chanatah was in Shemitah and it entered Sheminis, it is like Sheminis in every way.

(Beraisa - R. Yosi): Avtulmus testified that Ma'aser of an Esrog depends on Lekitah (but Shemitah depends on Chanatah);

In Usha, Chachamim voted and concluded that Ma'aser and Shemitah of an Esrog depend on Lekitah.

15b (R. Yochanan and Reish Lakish): If Chanatah was in Shishis and it entered Shemitah, it is like Shishis in every way.

Ravin: R. Yochanan says so even if it was only a k'Zayis in Shishis, and it grew greatly in Shemitah.

Sukah 39a (Mishnah): One who buys a Lulav from his friend in Shemitah, the seller gives the Esrog for a gift, because one may not buy it in Shemitah.

39b: The Tana holds that the status of an Esrog depends on Lekitah. Some Tana'im holds that it depends on Chanatah.

Rishonim

Rambam (Hilchos Ma'aser Sheni 1:5): The Esrog is the only tree fruit that is like a Yerek (vegetable). For Ma'aser and Shevi'is, we follow Lekitah. If it was picked in the third year, after Tu bi'Shvat, we separate from it Ma'aser Oni, even though the fruit was finished in the second year (in which we separate Ma'aser Sheni). The same applies if it was picked in the fourth year before Tu bi'Shvat. If it was picked (that year) after Tu bi'Shvat, we separate Ma'aser Sheni.

Rambam (6): Even though we follow Lekitah, if a sixth year Esrog entered Shemitah, even if it was only a k'Zayis and it became heavy, Ma'aser must be taken from it.

Rebuttal (Ra'avad): This is wrong. R. Yochanan and Reish Lakish said that if Chanatah was in Shishis and it entered Shemitah, it is like Shishis in every way. The Rambam understood that one is liable for Tevel, and it is exempt from Bi'ur. If so, we follow Chanatah for Shemitah. However, Ma'aser Sheni or Ma'aser Oni depend on when it was picked, like R. Yosi.

Defense (Ri Korkus and Radvaz): The Ra'avad assumes that R. Yochanan and Reish Lakish hold like R. Yosi. It is difficult to say that they argue with the conclusion in Usha. Rather, they explain that in Usha they enacted to be stringent to be concerned for Lekitah in Shemitah, unlike R. Yosi who follows Chanatah to exempt from Shemitah. R. Yochanan and Reish Lakish teach that (even though we treat it like Shemitah,) we are stringent to consider it Tevel. Chachamim gave both stringencies because it is a tree, and it grows on all water (it is irrigated).

Question (Shoham v'Yashfe, brought in Likutim in Frankel Rambam): Since it is Safek Shemitah and the owner is forced to make it Hefker, it should be exempt from Ma'aser!

Rambam (Hilchos Shemitah 4:12): Shemitah of Yerek depends on Lekitah. Even if an Esrog was as big as a bean before Rosh Hashanah, and it became heavy in Shemitah, we take Ma'aser from it like sixth-year fruit.

Kesef Mishneh: Rabah holds that we follow Chanatah for Shemitah, like R. Yosi. The Stam Mishnah in Sukah is like Rabanan of Usha, who follow Lekitah. The Rambam was unsure whom we follow. Also, regarding Ma'aser, he was unsure if we follow Chanatah, like R. Eliezer, or Lekitah, like R. Gamliel. He ruled stringently about both of these.

Radvaz: It is unreasonable that R. Yochanan argues with the conclusion in Usha. Rather, they explain that in Usha they enacted to follow Lekitah for Ma'aser, i.e. to determine which Ma'aser is given (Oni or Sheni). They did not discuss a sixth year Esrog that entered Shemitah. R. Yochanan teaches that we are stringent to obligate tithing it and observing Kedushas Shemitah. Even if most grew in the seventh year, we follow Chanatah and obligate tithing it. Even if most grew in the sixth year, we follow Lekitah and Shemitah applies to it.

R. Yochanan teaches that we are stringent to obligate tithing it and observing Kedushas Shemitah.

Poskim

Shulchan Aruch (YD 331:126): Esrog is unique among fruits. It is like Yerek, and we follow Lekitah, both for Ma'aser and for Shemitah. Nevertheless, if a sixth year Esrog entered Shemitah, even if it was only a k'Zayis when Shemitah came, Ma'aser must be taken from it.

Gra (191): The Rambam and Shulchan Aruch rule like Rabanan of Usha, for the Stam Mishnah in Sukah is like them. The Ra'avad rules like R. Gamliel, because R. Yochanan, Reish Lakish and Rabah all hold like him.

Chazon Ish (Shevi'is 7:13): An Esrog picked between Rosh Hashanah and Sukos of Shemitah has Kedushas Shemitah because we follow Lekitah. Regarding Ma'aser, before Tu bi'Shvat is considered last year's produce.