The Agricultural Research Service, U.S. Department of Agriculture hereby releases to nurserymen and growers the US-1516 citrus rootstock. This rootstock selection originated from a 1975 cross of African Pummelo (Citrus grandis) x Flying Dragon Trifoliate Orange (Poncirustrifoliata) made at the USDA Date and Citrus Station at Indio, California by Dr. Herb Barrett of the USDA, ARS, USHRL, Florida. Hybrid seed from the cross was planted at the A.H. Whitmore Foundation Farm, Groveland, Florida in 1976 and grown to fruiting. Field testing of US-1516 was planned and conducted by Dr. Kim Bowman, in collaboration with or support from industry partners, including Florida Citrus Research Foundation, Florida Citrus Production Research Advisory Council, and Florida Citrus Research and Development Foundation. Dr. Greg McCollum (also of USDA, ARS, USHRL) collaborated in the evaluation of fruit quality
from field trials. The major positive attributes of this new rootstock are induction of superior tree health, superior fruit productivity, and good fruit quality on sweet orange trees grown on the Florida ridge and infected with Candidatus Liberibacter asiaticus (Las), the causal agent of huanglongbing (HLB). This rootstock is being released for commercial use in Florida because of
the urgent need for new citrus rootstocks that have improved tolerance to HLB.
Field testing of the US-1516 rootstock has been primarily at one location and with ‘Valencia’ sweet orange scion. The budded trees were transplanted into the trial in June 2008 at a commercial field site in Polk County owned by Wheeler Citrus, at a spacing of 4.4 m x 7.6 m. The experiment included 21 ‘Valencia’ trees on each of the 17 rootstocks, planted in a randomized complete block design, in ten adjacent rows about 200 m long. In addition to US-1516rootstock, the trial contained most of the common citrus rootstocks used in Florida, including Carrizo (Citrus sinensis L. Osbeck x Poncirus trifoliata L. Raf.), Kuharske (C. sinensis x P. trifoliata), Swingle (C. paradisi Macf. x P. trifoliata), Cleopatra (C. reticulata L. Blanco), Kinkoji (C. obovoidea Takahashi), US-812 (C. reticulata ‘Sunki’ x P. trifoliata ‘Benecke’), US-897 (C. reticulata ‘Cleopatra’ x P. trifoliata ‘Flying Dragon’), and US-942 (c. reticulata ‘Sunki’ x P. trifoliata ‘Flying Dragon’). Border trees with the same scion were planted on each end of the rows and in the two adjacent rows. Soil was Candler sand, with good natural drainage, and a gentle slope. Irrigation in the block was by under-tree microjets.
Of the 357 trees originally planted in the trial, 52 trees were removed because of very poor health (primarily symptoms ofHLB). All of the 305 trees that remained in the trial in July 2015 from the original planting tested positive by PCR for Candidatus Liberibacter asiaticus (Las), and 304 trees showed visible leaf symptoms of HLB. Compared to other rootstocks in the trial, US-1516
rootstock had the lowest number of trees removed (zero) because of poor health, and the best average tree health rating. Fruit production of ‘Valencia’ on US-1516 in this trial was measured in April of each of four seasons from 2012-2015 when the trees were 4-7 years of age. Cumulative fruit yield for trees on US-1516 rootstock, at 211 kg/tree, was second highest in the trial, following trees on US-942 at 227 kg/tree. In the last season of harvest (2015), fruit yield for trees on US-1516 was the highest of any rootstock, at 65 kg/tree. ‘Valencia’ fruit quality for trees on US-1516 was good, with large fruit size (212 g/fruit), intermediate total soluble solids (TSS = 9.41 percent), high TSS/acid ratio (12.5), and high fruit juice color number (38.6) at harvest time in April. The size of trees on US-1516 rootstock at 7 years of age was relatively large, with a scion trunk cross sectional area of 86 cm2 and a canopy volume of 6.28 m3, which was similar to trees on Carrizo, Kuharske, and US-942.
For field testing, US-1516 was propagated by seed, and seedlings were observed to be highly uniform. Source plant material for US-1516 has been provided to the Florida Bureau of Citrus Budwood Registration clean budwood program (3027 Lake Alfred Road – Highway 17, Winter Haven, Florida 33881) and will be distributed, following USDA release, according to Florida Department of Agriculture and Consumer Services regulations. Small quantities of seed and plant tissue for research, as well as additional information on US-1516 may be obtained from Kim D. Bowman, USDA, ARS, USHRL, 2001 South Rock Road, Ft. Pierce, Florida 34945 (firstname.lastname@example.org). Genetic material of this release will be deposited in the National Plant Germplasm System where it will be available for research purposes, including development and commercialization of new cultivars. Appropriate recognition should be made if this germplasm contributes to the development of a new breeding line or cultivar.