Frost rings are marks on fruit ususally apples or pears. These marks are formed when freezing temperatures damage the skin of the fruit. The temperatures do not last long enough to kill the flower or fruitlet. Frost rings are most often found when a killing frost has occured in the spring, where many flowers were killed but also many fruits survived. So frost rings are less likely when a severe freeze has killed almost all of the fruit, but common when a frost has killed some of the fruit. Often much of the fruit on a tree, in an orchard or even in a region are affected.

Frost rings can take many forms. They may be complete rings around the fruit but often they are only partial rings or patchs of russeted tissue. Here are two picture that I have of frost rings in apples. The one to the left is of a young fruit about a month after the frost. It is easy to see that the tough callus tissue of the frost ring is constricting the growth of the fruit. The picture below is of a different apple at harvest time. Unless you grow fruit it is unlikely that you will see these surface blemishes on the fruit since marked fruit are removed and used for processing when apples are packed for shipment to grocery stores.