Proc. Fla. State Hort. Soc. 99:33-35. 1986. RANGPUR LIME X TROYER CITRANGE, A HYBRID CITRUS ROOTSTOCK FOR CLOSELY SPACED TREES W. S. Castle A combination of diseases, repeated freezes, and other University of Florida, IFAS factors has reemphasized the importance of rootstocks in Citrus Research and Education Center Florida. Moreover, the effects of these factors illustrate the 700 Experiment Station Road inherent weaknesses in virtually all citrus rootstocks and Lake Alfred, FL 33850 the need to continually search for new, improved ones. Another recent trend related to rootstocks has been C. O. YOUTSEY the shift toward more closely spaced trees, particularly FDACS, Division of Plant Industry within the row (7,8). Rootstocks well-suited for dense plan Citrus Budwood Registration Bureau tings have not been available although such stocks are 3027 Lake Alfred Road being evaluated and one appears particularly promising Winter Haven, FL 33881 (1, 3, 4, 8). It is a hybrid of Rangpur lime and Troyer D. J. Hutchison citrange (RxT) and has been under study in Florida for 18 United States Department of Agriculture yr. During this period, trees on RxT have demonstrated Agricultural Research Service sufficient commercial potential to justify our presentation 2120 Camden Road in this report of their performance and a description of Orlando, FL 32803 RxT and its characteristics. Additional index words. Blight, tristeza, tree size control. History Dr. J. R. Furr, formerly a plant breeder with the U.S. Abstract. A hybrid of Rangpur lime (Citrus limonia Osb.) and Department of Agriculture (USDA) at Indio, California, Troyer citrange [ C. sinensis (L) Osb. x Poncirus trifoliata (L.) made pollinations between Rangpur lime and Troyer cit Raf.] (RxT), produced in the U.S. Department of Agriculture range in 1953. He observed from subsequent evaluations citrus breeding program at Indio, California, has been tested that the resulting hybrid plants were, in general, cold in Florida since 1968. The rootstock fruit are pear-shaped and hardy, semi-dwarfing as a rootstock, salt tolerant, and pro contain 10 to 20 polyembryonic seed/fruit. Seedlings are ex duced mostly nucellar seedlings. One selection (C54-64-8), cellent nursery plants and appear to be compatible with most brought by the USDA to their foundation farm near Lees- scions. Trees grown on RxT are precocious, substandard in burg, has been the seed source for virtually all experimen size, yield efficiently, and are suitable for close planting. tal work in Florida including the studies reported here. 'Valencia' sweet orange and 'Marsh' grapefruit ( C. paradisi Bud propagations of this selection are maintained by the Macf.) trees on RxT, planted in 1968 and maintained at 12 Division of Plant Industry (DPI), Citrus Budwood Regis ft in height in a 10 ft x 15 ft setting, consistently yielded in tration Bureau in Winter Haven, Florida. the top 25% among trees on 18 rootstocks. Fruit size has been Field trials with RxT began in 1968 with several exper moderate while juice quality has been above average. iments planted by Dr. Richard L. Phillips who was then Younger plantings of 'Bearss' lemon ( C. limon Burm. f.), 'Mur- employed at the Citrus Research and Education Center cott' (a possible C. reticulata Blanco hybrid), 'Minneola' (CREC), Lake Alfred. Dr. Phillips' research efforts were tangelo (C. paradisi x C. reticulata), and several sweet focused on dwarfing rootstocks and in a search for new orange varieties on RxT have performed similarly and appear plant material to evaluate, he obtained seeds of RxT from promising for densities approaching 250 trees/acre. Trees on Dr. Furr. RxT have withstood recent freezes despite their smaller size. Blight has affected some trees and they are susceptible to Nursery Characteristics exocortis viroid; however, trees are tolerant to citrus tristeza virus and no field loss has been experienced from Four seedlings of RxT were planted in 1968 at the phytophthora foot rot. CREC farm near Davenport and are representative of other seed trees in Florida. They began to bear in 1973. Many rootstocks have been explored commercially in These seedling trees are now about 15 ft tall and each the Florida citrus industry. Among these, rough lemon (C. produces 100 to 200 lb. of medium to small-sized, pear- jambhiri Lush.) and sour orange (C. aurantium L.) have been shaped fruit annually. The fruit are yellow in color and dominant but this trend is changing. Trees on rough each contains about 10 to 20 seeds. The trees have been lemon have been greatly affected by blight. Carrizo cit unprotected from cold weather but have continued to range replaced rough lemon but it is also susceptible to fruit, receiving only minor damage from numerous blight and as a result has declined in popularity. freezes. In some years, they have been partially deciduous Sour orange has long been the standard for cold toler probably because of the trifoliate orange parentage in ance and high quality fruit in Florida. Current epidemics Troyer citrange. of tristeza in several areas of Florida seriously threaten the In the nursery, stands of RxT seedlings are uniform, future of sour orange because of its susceptibility to this vigorous, upright with virtually no branching, and disease- free. The leaves are usually unifoliate but sometimes virus disease. bifoliate or trifoliate leaves appear. No problems have been Florida Agricultural Experiment Station Journal Series No. 7792. experienced during the budding, forcing, or growing pro- Proc. Fla. State Hort. Soc. 99: 1986. 33 cesses or with scion compatibility. Commercial greenhouse from less than 1 to 100 acres. The scion varieties are 'Ham- and field nurserymen have propagated trees for coopera lin', 'Pineapple', and 'Valencia' sweet orange, 'Marsh' and tive experiments and compare it favorably in its nursery 'Redblush' grapefruit, 'Bearss' lemon, 'Murcott' mandarin, characteristics to Carrizo citrange. and 'Minneola' tangelo. The DPI began to include RxT in some of its plantings in 1976 which along with the field Pathological Characteristics experiments are the basis for the following horticultural Seedlings of RxT were rated susceptible to Phytophthora information. parasitica Dast. in the USDA (Orlando) screening program Soil adaptability. Fruiting trees on RxT have been estab as compared with Carrizo citrange which is the test stan lished on soils, such as Astatula, Candler, Myakka, Pineda, dard and is considered tolerant. Under field conditions, and Immokalee fine sands, that are commonly used for virtually no loss has occurred in cooperative experiments. planting citrus in Florida, even though these soils differ In controlled tests, also performed by USDA (Orlando) markedly in their physical and chemical characteristics. personnel, RxT was rated tolerant to citrus tristeza virus. Tree growth and vigor. Trees on RxT grow at a slower In field trials with exocortis infected scion material, rate than those on most other commercial rootstocks. In 'Bearss' lemon on RxT was unaffected by a mild form (2) the 1968 plantings, this moderate vigor has resulted in while 5-yr-old trees at the DPI farm had a moderate reac substandard-sized trees about 50 to 75% of the height of tion. No trees have been inoculated with severe isolates; those on stocks such as rough lemon or Carrizo citrange (3, 4). In younger field trials planted over a wider range however, exocortis viroid should be avoided because both of soils, the growth of trees on RxT has varied apparently Rangpur lime and Troyer citrange are susceptible. The in response to soil type. Trees on RxT planted in deep reaction of trees on RxT to xyloporosis is unknown. sandy soils (Astatula or Candler fine sand) have achieved Rangpur lime x Troyer citrange was rated susceptible their moderate size more slowly than ones growing on to the burrowing nematode, Radopholus citrophilus Huettel, other soils. For example, in 2 field trials, RxT-rooted trees in controlled tests, but has not been examined for citrus growing in Basinger, Pomona, or Immokalee fine sand are nematode, Tylenchulus semipenetrans Cobb, tolerance. In the 50 to 75% as tall as those on C. volkameriana Pasq. after 6 field, trees of 'Pineapple' orange on RxT appear to be and 8 yr, respectively. No measurements of RxT root sys growing well in a citrus nematode-infested site; however, tem development have been taken. other trees on susceptible rootstocks are also growing satis Yield arid fruit and juice quality. Trees on RxT are preco factorily. cious and have produced as many as 30 or 40 fruit/tree Trees on RxT are affected by blight, but the incidence within 2 yr after field planting in Florida. Such heavy, early has been low in CREC field trials planted before 1980. In bearing can be detrimental and has resulted in slow growth a recent survey of the DPI foundation planting, fewer trees and broken branches on young trees. This precociousness on RxT had blight than those on other stocks such as may account for some of the relative variation in tree rough lemon and Volkamer lemon (9); however, these ob growth rates observed at different sites. servations are based on a small number of trees that are Yield data for Florida trees on RxT in several experi only 10-yr-old. Data from established, larger-scale com ments have been reported (1, 2, 3, 4). The behavior de mercial plantings are required to determine the relative scribed below is based on the older field trials. Unpub susceptibility of RxT to blight. lished data from younger plantings support these general observations. Horticultural Characteristics Production per tree is often lower than for trees on Nearly 20 field trials with trees on RxT have been plan other stocks because trees on RxT are smaller, but they are ted by CREC and USDA personnel beginning in 1968. efficient and bear well considering their smaller canopy Most of these are grower-cooperative experiments planted volume (Table 1).