May apple

A member of the Barberry family and not a true apple. May apples are oblong in shape and about the size of a large olive; they turn yellow when they are ripe. The flesh is lightly sweet to acidic. If eaten when green (and unripe), May apples can be poisonous. They are most common in the eastern United States, where they are often homegrown, but they are rarely found for sale in the commercial market. May apples are great for preserving.