Swingle Citrumelo

Swingle citrumelo is a grapefruit × Poncirus trifoliata hybrid produced by Walter T. Swingle in Florida in 1907. UCR places USDA rootstock trials in Florida and Texas about four decades later, naming Bill Cooper, Ed Olson, and Heinz Wutscher among the personnel, and credits Wutscher with much of the later development that made Swingle widely used. UF/IFAS documents its subsequent widespread use in Florida.^{⁠[1]⁠[14]⁠[15]}

Rootstock performance depends on soil and graft combination. UCR reports decline on some Florida soils and damage where Phytophthora and Diaprepes occur together; a regional survey associated tree performance with soil series and landscape position. Australian and Florida accounts list specific scion incompatibilities, including Eureka and Meyer lemons, Pera, Murcott, and Roble.^{⁠[1]⁠[14]⁠[15]⁠[20]}

Grower reports describe Swingle fruit with rough or bumpy rind and flavors ranging from lemon-grapefruit to orange-like, sometimes with trifoliate notes. One grower says it can substitute for lemon in pie but cautions against relying on its zest. Individual measurements are not cultivar-wide estimates: one North Carolina sample weighed 140 g and read 11 °Bx.^{⁠[12]⁠[13]⁠[16]⁠[17]⁠[18]}

Cold reports describe different outcomes rather than a single hardiness limit. One grower reported a 6°F low with a strand of lights near the tree base and a few dead branches; another reported survival at 8°F without damage but gave no event date, duration, site, or protection details.^{⁠[16]⁠[17]}

A study using diploid and tetraploid Swingle 4475 SRA 928 with Mexican- and Persian-lime scions found fewer HLB symptoms in field lime scions on tetraploid Swingle, with stronger reported tolerance for the triploid Persian-lime combination. This result applies to the tested material and grafts, not all Swingle trees or a claim of immunity.^⁠[21]