I bought myself a prepackaged Esrog yesterday and found it to have a really large pimple like bump towards its lower half. Other than that it is a beautiful, clean, and yellow expert which is large and properly shaped. Is my Esrog still kosher?
Yes, your Esrog remains Kosher even though it may not be the most beautiful due to this pimple like bump.
Explanation: There are various aspects which can invalidate the Kashrus of an Esrog, including the growth of pimples known as a Chazazis. However, according to most opinions, including the opinion of the Alter Rebbe, a Chazazis is only able to invalidate an Esrog if it is made up of two individual pimples or blisters which are next to each other. However, a single pimple is not considered a Chazazis and does not invalidate the Esrog. Furthermore, even according to the opinions which consider even a single pimple to have the same status as an invalidating Chazazis, it only invalidates if it is apparent at first sight, and even then if it is found in the lower half of the Esrog, below the Chotem, it does not invalidate if there is only one of them found there. Accordingly, it is clear from the picture of your Esrog that you sent me that your Esrog is 100% Kosher according to all opinions
Sources: See Admur 648:19-24; Michaber 648:9 and 13; Mishneh Sukkah 34b; See regarding definition of Chazazis and that it must contain two blisters next to each other: Admur 648:19 in parentheses and 648:22; M”A 648:16; Mabit 3:49 Other Opinions: Some Poskim rule that even one blister is considered a Chazazis. [See M”B 649:49] see regarding necessity to be apparent upon first sight: Admur 648:22; M”A 648:16; Mabit 3:49; M”B 648:46; Piskeiy Teshuvos 648:14; See regarding that below the Chotem a single Chazazis remains valid: Admur 648:19; Michaber 648:9; Mishneh Sukkah 34b and Sukkah 35b; M”A 648:12; Rosh