A hole which was caused by a thorn while the Esrog was still on the tree is kosher even if a piece is missing, if the entire area of the hole is covered by scar tissue. This applies even if there is still a piece visibly missing from the Esrog being that the missing area is fully covered by the scab. If however the scab is not fully covering over the hole, or there is no scab at all, and one can tell that there is a piece missing, the Esrog is invalid.
 The reason: As since the hole occurred while on the tree and the scab grew over the hole and covered it, it is considered like a normal growth over the Esrog and hence it is Kosher. [ibid]