Though poisonous when green, the yellow, egg-shaped May apple can be safely eaten after ripening. This member of the barberry family is about the size of a large cherry. It's lightly sweet and acidic and makes very good preserves. The May apple is found in the East but is rarely available in markets.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.