X-639 Citrandarin

X-639 Citrandarin is a rootstock originally sourced from South Africa. UF/IFAS describes it as a hybrid of Cleopatra mandarin and Poncirus trifoliata, developed in the 1950s and first available in Florida in 1994. A Citrus Research International profile also places its origin in the early 1950s; the availability date is not the date of the cross.^{⁠[5]⁠[18]}

In Florida guidance, X-639 is described as vigorous and relatively large, with 8–12 feet suggested for in-row spacing. UF/IFAS reports tolerance to Phytophthora and citrus tristeza virus and relatively good performance under moderate soil salinity; a 2021 extension article cautions that vigor can delay early productivity and that no rootstock fully prevents HLB damage. These are qualified profile statements, not numeric ratings.^⁠[5]⁠[8]

A 2022 report calls a recovered mutant rootstock from one unusually vigorous X-639-rooted Valencia tree “XXX-639”; the nearly 50 fruit were on the Valencia scion. A 2025 SWFREC presentation describes a putative X-639 mutant labeled “STR-4-1” and a similar Valencia-on-X-639 observation, but does not name XXX-639. The proposed correspondence is an inference, and neither account is a replicated performance trial.^{⁠[7]⁠[19]}