Vampire pumpkins and watermelons are a folk legend from the Balkans, in southeastern Europe, described by ethnologist Tatomir Vukanović. The story is associated with the Roma people of the region, from whom much of traditional vampire folklore, among other unusual legends, originated.The validity (or lack of same) of this legend is discussed at this Wikipedia talk archive page.
The belief in vampire fruit is similar to the belief that any inanimate object left outside during the night of a full moon will become a vampire. According to tradition, watermelons or any kind of pumpkin kept more than ten days or after Christmas will become a vampire, rolling around on the ground and growling to pester the living. People have little fear of the vampire pumpkins and melons because of the creatures' lack of teeth. One of the main indications that a pumpkin or melon is about to undergo a vampiric transformation (or has just completed one) is said to be the appearance of a drop of blood on its skin.
Photo credit ("I bought the melon at the Raleigh, North Carolina farmers' market and set it on the counter in an air-conditioned kitchen for four days, achieving the results photographed here. Upon observing the phenomenon, I wouldn't doubt that less scientific people supposed the watermelon had some relation to vampires.)
Via Biomedical Ephemera, or: A Frog for your Boils.