Does an etrog need ‘ma’aser’ before use in ‘arbah minim’
Yes, an esrog that grew in E. Yisroel needs have terumos u’maasros taken from it before it can be used. The reason is because the esrog has to be permitted to eat in order for it to be kosher to use for the mitzvah, and until the terumos u’maasros are taken it is considered tevel. On the other hand you may not take a part of the esrog off for terumos u’maasros because then it will be a “choser”. Therefore you will need to find some other esrogim that need terumos u’maasros taken from them in order to make your esrog permitted to use for the mitzvah. Most esrog plantations take terumos u’maasros off from posul esrogim before they are released for sale, so this will usually not be an issue, ( most esrog boxes with good hashgachos say on them that terumos u’maasros were taken). However it is something to remember for those people that go to the orchard to pick their own esrogim.
O:CH 649-5 M:B ibid 45.