Swingle Citrumelo-A Promising Rootstock Hybrid

HUTCHISON: SWINGLE CITRUMELO 89

SWINGLE CITRUMELO-A PROMISING ROOTSTOCK HYBRID

D.J.Hutchison

Agricultural Research Service, USD A Orlando

Abstract 'Swingle' citrumelo (CPB-4475) was hybridized by Walter S. Swingle at Eustis, Fla., in 1907, from Citrus paradisi Macf. "Duncan' grapefruit X Poncirus trifoliata (L.) Raf. It is tolerant of tristeza virus and Phytophthora parasitica (root rot) and moderately tolerant of salt. In experimental field plantings, it has proved to bea satisfactory rootstock for grapefruit and sweet orange. Seedlings are uniform and vigorous, with an extensive root system. 'Swingle' citrumelo is asexually reproduced by seeds that are 85 to 95% polyembryonic (nucellar).

On April 1, 1974, the Agricultural Research Service, U.S. Department of Agriculture, released to citrus nurserymen and growers the 'Swingle' citrumelo citrus rootstock, formerly tested as Cit rumelo CPB 4475. This rootstock selection was hy bridized by Walter S. Swingle at Eustis, Fla., in 1907, from Citrus paradisi Macf. 'Duncan' grape fruit X Poncirus trifoliata (L.) Raf. This paper presents the available information on the perform Fig. 1. Upper row, half fruit and seeds of 'Swingle' cit rumelo. Lower row, fruit of Poncirus trifoliata for compari ance of this hybrid as a potential commercial root- son, and whole fruit of 'Swingle' citrumelo. stock. In this technical description, all measurements sweet oranges (C. sinensis (L.) Osbeck); 'Temple* are based on averages: fruitcolor light yellow; (a hybrid of unknown parentage); and red grape medium sized, diameter 63 mm, height 75 mm; fruit. The budwood used in all instances appeared weight 124 g; fruit shape pyriform, stem end col to be from an old-line source and was contaminated lared and slightly furrowed; fruit pubescence with severe strains of exocortis and xyloporosis short and light; external oil glands numerous and viruses. As a result, all except two of these plant slightly raised; rind 2 mm thick, albedo 6 to 8 ings were abandoned because of the ravages of mm thick; axis closed, segments 8 to 11; vesicles these viruses. 'Swingle' citrumelo showed bud- moderately small and juicy; texture moderately union compatibilities with all the scions. fine; flavor highly acid and acrid; seeds 20 to 24 Of the two remaining experiments, one grafted per fruit; polyembryony, 85 to 95%. The tree is with 'Valencia' sweet orange was reported by thorny, vigorous, hardy; medium tall; foliage Gardner et al. (10) in 1967. At the end of 19 dense and evergreen; leaves trifoliolate, elliptic, years, 'Swingle' citrumelo was the most dwarfing medium dark green; center leaflet, length 68 mm, stock in the experiment and was very productive width 29 mm; side leaflet, length 39 mm, width per unit of tree size. The other experiment grafted 18 mm; petiole, length 22 mm, width 3.5 mm. with red grapefruit has now been planted for 26 Fruits, seeds, and leaves are shown in Figs. 1 and years. During the past 8 years, fruit production 2. and tree size on 'Swingle' rootstock have been Extensive field trial plantings in Florida, be visually observed to equal approximately those of gun in the mid-1940's, included 'Swingle' citrumelo sour orange (C. aurantium L.) rootstock in the as one of the experimental rootstocks under con same experiment. The 'Swingle' citrumelo produces sideration. The scion varieties included 'Pineapple', a bench-type rootstock undergrowth. 'Pope Summer', 'Dream Navel', and 'Valencia' In a field trial planted in Florida in 1965 with 90 FLORIDA STATE HORTICULTURAL SOCIETY, 1974

16, 20) in Texas have characterized 'Swingle* cit rumelo as follows: low sensitivity to exocortis and xyloporosis viruses; poor tolerance of iron chloro sis; moderate tolerance of salt, boron, and cold; and good tolerance of midrib necrosis and yellow- vein chlorosis. In California, Carpenter and Furr (1) reported that 'Swingle* citrumelo was outstanding among the citrumelos in its tolerance of root rot (Phyto- phthora parasitica Dast.). This work has been confirmed in Florida by Grimm and Hutchison (unpublished data). The Agricultural Research Service, U.S. De partment of Agriculture, in cooperation with the Instituto Agronomico, Campinas, and the Limeira Citrus Experiment Station in Braizil, in 1946, be gan a series of studies in search of tristeza virus- tolerant rootstocks. Results (11, 12, 13) showed that 'Swingle' citrumelo was tolerant of the tristeza virus. Further studies in Brazil (14, 17) on the orchard performance of the tristeza-tolerant rootstocks indicated that 'Swingle* citrumelo was one of the best-yielding rootstocks for 'Valencia' sweet orange. Fig. 2. Upper row, leaves of Poncirus trifoliata. Lower row, leaves of 'Swingle' citrumelo. Information to date on the 'Swingle' citrumelo hybrid warrants its release for further trial plant ings as a potential commercial rootstock. 'Pineapple* orange as the scion variety, 'Swingle' citrumelo and sour orange rootstocks have been observed to be about equal in total fruit production, Literature Cited soluble solids, and total acids. 1. Carpenter, J. B. and J. R. Furr. 1962. Evaluation of tolerance to root rot caused by Phytophthora parasitica in Gardner and Horanic (9) reported on cold seedlings of citrus and related genera. Phytopathology 52:1277-1285. injury of 41 rootstocks with 4-year-old /Valencia* 2. Cooper, W. C. 1961. Toxicity and accumulation of salts orange tops. They showed that 'Swingle* citrumelo in citrus trees on various rootstocks in Texas. Proc. Fla. State Hort. Soc. 74:95-104. was intermediate between sour orange and 'rough 3. and B. J. Lime. 1960. Quality of red grape lemon* (C. limon (L.) Burm. f.) in its cold- fruit on old-line grapefruit varieties on xyloporosis- and exocortis-tolerant rootstocks. J. Rio Grande Valley Hort. Soc. hardiness. Recently, Yelenosky et al. (19) showed 14:66-76. 4. and E. O. Olson. 1959. Yellow-vein chlorosis that 'Swingle* citrumelo seedlings are relatively of old-line and young-line red grapefruit trees on various tolerant of freezes. In Florida nurseries, 'Swingle* rootstocks. J. Rio Grande Valley Hort. Soc. 13:81-88. 5. , E. O. Olson, N. Maxwell and G. Otey. 1956. citrumelo has been observed to have a high germi Review of studies on adaptability of citrus varieties as root stocks for grapefruit in Texas. J. Rio Grande Valley Hort. nation percentage, produce uniform and vigorous Soc. 10:6-19. seedling populations with an extensive root system, 6. , , and A. Shull. 1957. Orchard performance of young trees of red grapefruit on and be easily maintained and propagated. various rootstocks in Texas. Proc. Amer. Soc. Hort. Sci. 70:213-222. Extensive field trial plantings in Texas have 7. and A. Peynado. 1955. Boron accumulation used old-line and nucellar red-grapefruit budwood. in citrus as influenced by rootstock. J. Rio Grande Valley Hort. Soc. 9:86-94. The results showed that 'Swingle* citrumelo con 8. . 1959. Chloride and boron tolerance of sistently outyielded sour orange and approximately young-line citrus trees on various rootstocks. J. Rio Grande Valley Hort. Soc. 13:89-96. equaled sour orange in tree size, total acids, and 9. Gardner, F. E. and G. E. Horanic. 1958. Influence of various rootstocks on the cold resistance of the scion variety. total soluble solids with old-line and nucellar bud- Proc. Fla. State Hort. Soc. 71:81-86. wood (3, 18). The highest yielding trees in Texas 10. , D. J. Hutchison, G. E. Horanic and P. C. Hutchins. 1967. Growth and productivity of virus-infected of 9-year-old 'Marrs* early orange and 'Orlando* Valencia orange trees on 25 rootstocks. Proc. Fla. State Hort. Soc. 80:89-92. tangelo (C. paradisi X C. reticulata Blanco) were 11. Grant, T. J. and A. S. Costa. 1948. A progress report those on 'Swingle' citrumelo rootstock (Wutscher, of studies of tristeza disease of citrus in Brazil. J. Rio Grande Valley Hort. Soc. 3:113-127. unpublished data). 12. and . 1948. A progress report on studies of tristeza disease of citrus in Brazil. I. Behavior Other experimental data (2, 4, 5, 6, 7, 8, 15, of a number of citrus varieties as stocks for sweet orange SMITH: BLIGHT AND ZINC ACCUMULATION 91

and grapefruit, and as scions over sour orange rootstock irrigated with saline and boron-contaminated well water over when inoculated with the tristeza virus. Proc. Fla. State a 3-year period. J. Rio Grande Valley Hort. Soc. 16:52-58. Hort. Soc. 61:20-33. 17. Rodriguez, O., S. Moreira, A. A. Salibe and J. T. 13. , S. Moreira and A. A. Salibe. 1961. Citrus Sobrinho. 1972. Further study of the tolerance to tristeza variety reaction to tristeza virus in Brazil when used in virus of citrus varieties suitable for rootstocks in Brazil. various rootstocks and scion combinations. Plant Dis. Reptr. Pages 121-124 in W. C. Price (ed.) Proc. 5th Conf. Intern. 45:416-421. Organization Citrus Virol., University of Florida Press, 14. Moreira, S., T. J. Grant, A. A. Salibe and C. Roes- Gainesville. sing. 1965. Tristeza tolerant rootstocks—their behavior after 18. Wutscher, H. K. and A. V. Shull. 1972. Performance twelve years in orchard. Pages 18-24 in W. C. Price (ed.) of 13 citrus cultivars as rootstocks for grapefruit. J. Amer. Proc. 3rd Conf. Intern. Organization Citrus Virol., University Soc. Hort. Sci. 97:778-781. of Florida Press, Gainesville. 19. Yelenosky, G., R. T. Brown and C. J. Hearn. 1973. 15. Olson, E. O., W. C. Cooper, N. Maxwell and A. V. Tolerance of trifoliate orange selections and hybrids to freezes Shull. 1962. Survival, size and yield of xyloporosis and and flooding. Proc. Fla. State Hort. Soc. 86:99-104. exocortis infected old-line red grapefruit trees on 100 root 20. Young, R. and E. O. Olson. 1963. Freeze injury to stocks. J. Rio Grande Valley Hort. Soc. 16:44-51. citrus trees on various rootstocks in the lower Rio Grande 16. Peynado, A. and R. Young. 1962. Performance of Valley of Texas. Proc. Amer. Soc. Hort. Sci. 83:337-343. nucellar red blush grapefruit trees on 13 kinds of rootstocks

ZINC ACCUMULATION IN THE WOOD OF CITRUS TREES AFFECTED WITH BLIGHT

Paul F. Smith spread radially until the entire canopy is affected. Foliar sprays of Zn cause the chlorosis to disap Agricultural Research Service pear, thereby often masking a relation between the U.S. Department of Agriculture deficiency and the disease. Orlando In 1966, blight started on 9-year-old orange trees in one of my Zn fertilization experiments (5, Abstract In Florida, blight is primarily a wilt 6). These trees had been specifically propagated disease of citrus with an unknown cause. Zinc for the experiment from nucellar * Valencia' (Cit deficiency symptoms are frequently found in the rus sinensis (L.) Osbeck) budwood and planted on foliage of trees affected with blight. Accumulation virgin soil in Lake County. The experiment had of Zn in the wood of the trunk, large limbs, and been underway for several years and there was roots indicates selective absorption from the detailed knowledge available about the status of transport stream. Zn concentrations in the outer Zn and other minerals from frequent leaf analysis. layers of wood were about 8 times higher than in Unexpected patterns of Zn deficiency on a few healthy trees. There was a slight copper accumu scattered trees attracted my attention in the sum lation also, but 9 other minerals were not affected. mer of 1966. In the spring of 1967, symptoms of Interference in Zn transport is assumed to be a wilt and delayed flush were obvious. The chlorosis result of the affliction and not the cause, since symptoms disappeared rapidly following a Zn foliar applications of the mental cured the de spray, which left no doubt as to the diagnosis of ficiency but did not alleviate the blight. Zn deficiency on the declined trees. In the 8 years between 1966 and 1974, about 15% of 800 trees The term "Blight" has long been applied to a on rough lemon (C. limon (L.) Burm. f.) root- specific chronic wilt and decline of citrus trees in stock became blighted. However, none of over 200 Florida (1, 3, 7, 8, 9). The cause is still unknown. trees on 'Cleopatra' mandarin (C. reticulata Various names have been applied to describe this, Blanco) rootstock were affected. or similar, diseases. Two of the most common are The incidence of decline was not related to the "young-tree decline" and "rough-lemon decline". soil Zn status. High and low rates of Zn had previ Blight has precedence in usage and is therefore ously been incorporated in the soil in many plots of used here. 12 trees each. Once a tree started to decline, One of the earliest symptoms is induced zinc chronic Zn deficiency symptoms developed on all (Zn) deficiency. Patterns of Zn deficiency often subsequent flushes of growth. Application of sev show up in the foliage of one sector of the tree eral Zn sprays per year for 6 years prevented before or at the same time as the symptoms of foliar Zn deficiency symptoms, but the declined wilt. The symptoms of Zn deficiency and of drought trees did not recover.