Freezing Tolerance of Citrus Hybrids USDA 17-11 and USDA 119 Compared to `Hamlin' Orange and Satsuma Mandarin

Citrus hybrids USDA 17-11 [Citrus grandis L. × (C. paradisi Macf. `Duncan' × Poncirus trifoliata (L.) Raf. `Gotha Road')] and 119 [(C. paradisi Macf. `Duncan' × P. trifoliata (L.) Raf. `Gotha Road') × C. sinensis (L.) Osb. `Succory'], `Hamlin' orange [C. sinensis (L.) Osb.], and satsuma mandarin (C. unshiu Marc.) were planted March 1993 and 1994. Trees were irrigated and fertilized in an identical manner. In 1993, electrolyte leakage readings were taken monthly using 17-11, 119, and satsuma leaf discs. Leaf killing point (LKP) LT50 averaged from –8 to – 9C by mid-November for all selections. In 1994, leaf discs from 17-11, 119, and `Hamlin' orange were sampled weekly to determine LKP. USDA 119 had the lowest LKP and acclimated the fastest during the fall. By the end of November, there was no significant difference in LKP (–6.5C) between USDA 119 and 17-11, although both selections were significantly more freeze-tolerant than `Hamlin' orange (LKP–40C), which showed no significant decrease in LKP until the 6 weeks after the hybrid selections began acclimating. Citrus hybrids 17-11 and 119 can survive in freeze-susceptible areas that are marginal for other commercial citrus.