פירוש על ביכורים 2:6

שוה לאילן – that we “go” (i.e. determine it) following the formation of fruits or leaves. For we count [the] three [years towards] “Orlah”/being uncircumcised fruit (i.e., fruit-trees of the first three years) from the time when its leaves/fruit form, and similarly for that which is in its fourth year (which was carried to Jerusalem to be eaten there). And similarly for the seventh year produce, for its fruit/leaves formed in the sixth year and were completed in the seventh year, it is permissible [to eat from them]. But nevertheless, it is exempt from tithing, for concerning [the laws of] tithing, we follow after harvesting like the laws of vegetation, and since it is harvested in the seventh year, it is exempt from tithing.

Introduction 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, don't 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 one way to a vegetable. is similar to a tree in respect of orlah, fourth year plantings, and [the law of] the seventh year; 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).

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. Rabbi Eliezer says: it is similar to a tree in all ways. 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.