Citrumelo
Citrumelo is also called Swingle citrumelo trifoliate hybrid, because it is cold hardy and is a hybrid between a 'Duncan' grapefruit and a trifoliate orange, developed by Walter Tennyson Swingle.
Citrumelo is widely employed as a citrus rootstock, being resistant to the severe citrus [tristeza virus] and to Citrus phytophthora [root rot|phytophthora root rot] as well as to blight, cold, and citrus nematode. The trees can survive temperatures into the teens ; one specimen in North Carolina reportedly survived temperatures below zero Fahrenheit.
The fruit are yellow, and up to 4 inches across in size. Their taste is described as "like a cross between a lemon and a grapefruit", or "if sprinkled with sugar like an ordinary grapefruit, harvested perhaps a bit too early".