Bikkurim, Chapter Two, Mishnah Six
The rabbis seem to have caught comparison fever. Having finished comparing various agricultural gifts, they thought, Why stop now? Our mishnah compares the laws governing etrogim to those governing vegetables and fruits. If you’re sick of talking about produce, dont worry, in the next mishnah we’ll compare different types of blood, and after that we’ll move on to the animal kingdom.
An etrog is similar in three ways to [the fruit of an ordinary] tree, and in three ways to a vegetable.
1) It is similar to a tree in respect of orlah, fourth year plantings, and [the law of] the seventh year;
2) And it is similar to a vegetable in one thing: that its tithing goes according to the time it is harvested, the words of Rabban Gamaliel.
a) Rabbi Eliezer says: it is similar to a tree in all ways.
Section one: When it comes to determining what year of its growth or of a sabbatical cycle the fruit of an etrog tree is in, we go after the time that the fruit begins to bud on the tree and not after the time that it is picked from the tree. If it buds during the third year, then it is prohibited because it is orlah. If it buds during the fourth year, it must be brought up to Jerusalem and eaten there. If it buds during the seventh year, it has the status of sabbatical year produce, which means that it must be treated with a certain amount of sanctity (see my Introduction to Sheviit).
Section two: When it comes to determining what tithe year an etrog is in, meaning determining whether it is in its first, second, fourth or fifth year of a seven year sabbatical cycle, in which case second tithe is given, or whether it is in its third or sixth year in which case poor tithe is given, Rabban Gamaliel holds that we follow the time when the etrog is harvested, as is the case with vegetables. Thus, if the etrog buds during the second year of the sabbatical cycle, but is picked in the third year, the tithe removed is maaser ani (poor tithe) and not maaser sheni (second tithe).
Rabbi Eliezer holds that in all ways an etrog is treated like the fruit of a tree and thus if it buds in the second year, maaser sheni is given even if it is picked during the third year.