Breeding Citrus for Cold Hardiness

66 FLORIDA STATE HORTICULTURAL SOCIETY, 1959

LITERATURE CITED liminary studies on cold hardiness in citrus as related to cambiai activity and bud growth. Proc. Ann. Rio Grande 1. Camp, A. F., H. AAowry and K. W. Loucks. The effect Valley Hort. Inst. 9:1-15. 1955. of soil temperature on the germination of citrus seeds. Am. 9. Fawcett, H. S. Temperature experiments in germinat Jour. Bot. 20:348-357. 1933. ing orange seed. Calif. Citrog. 14:5-15. 1929. 2. Cathey, H. AA. Mutual antagonism of growth control 10. Girton, R. E. The growth of citrus seedlings as in of Chrysanthemum morifolium by gibberellin and Amo-1618. fluenced by environmental factors. Calif. Univ. publication Proc. Plant Physiol. meetings 33:43. 1958. Agr. Sci. 5:83117. 1927. 3. Cooper, W. C. Periodicity of growth and dormancy , 11. Lawless, W. W. Effect of freeze damage on citrus in citrus—a review with some observations on conditions in trees and fruit in relation to growth practices. Proc. Fla. the Lower Rio Grande Valley of Texas. Jour. Rio Grande State Hort. Soc. 54:67-74. 1941. Valley Hort. Soc. 11:3-10. 1957. 12. Lawless, W. W. and A. F. Camp. Preliminary report 4. Cooper, W. C. Influence of rootstock on injury and on various fertilizers and other factors as influencing cold recovery of young citrus trees exposed to thefreezes of resistance in citrus. Proc. Fla. StateHort. Soc. 53:120-125. 1950-51 in the Rio Grande Valley. Proc. Ann. Rio Grande 1940. Valley Hort. Inst. 6:16-24. 1952. 13. Peltier, G. L. Influence of temperature and humidity 5. Cooper, W. C, B. S. Gorton, and Sam Tayloe. Freezing on the growth of Pseudononas citri and its host plants and tests with small trees and detached leavesof grapefruit. on infection and development of the disease. Jour. Agr. Proc. Am. Soc. Hort. Sci. 63:167-172. 1954. Res. 20:437-506. 1920. 6. Cooper, W. C. and Ascension Peynado. Effect of 14. Peynado, Ascension. Devices for observing root gibberellic acid on growth and dormancy in citrus. Proc. growth and calipering tree trunks. Jour. Rio Grande Valley Am. Soc. Hort. Sci. 72:284-289. 1958. Hort. Soc. 12:61-66. 1958. 7. Cooper, W. C, Ascension Peynado, and Geo. Otey. 15. Stewart, I. Inducing dormancy in citrus trees by Effects of plant regulators on dormancy, cofd hardiness, and theuse of chemical sprays. Unpublished paper read before leaf form of grapefruit trees. Proc. Am. Soc.Hort. Sci. meeting of Fla. Cit. Prod. Managers Assoc. Oct. 7, 1959. 66:100-110. 1955. 16. Young, Floyd D. Frosf and the prevention of frost 8. Cooper, W.C, Sam Tayloe and N. Maxwell. Pre damage. U. S. Dept. of Agric. Bull. 1588. 62 pp. 1929.

BREEDING CITRUS FOR COLD HARDINESS

J. R. Furr and W. W. Armstrong, Jr. was renewed in 1896, one of the primary objectives was the production of cold-hardy Crops Research Division varieties. U.S.D.A. Agricultural Research Service In the early work by Swingle and Webber and the later work by Swingle, Robinson, and Indio, Calif. Savage described by Traub and Robinson (6), the trifoliate orange was used as the principal The possibility of severe loss from cold in source of cold resistance in crosses made for jury is an ever-present hazard for the citrus the production of cold-hardy varieties. These grower in all of the major citrus-growing hybrids between various kinds of citrus and regions of this country. The severe freezes of the trifoliate orange proved to be of outstand 1949 and 1951 in Texas, the equally severe, ing cold hardiness, but their fruits were prac but less damaging, ones in California and Ari tically inedible. These results seem to have zona in 1949 and 1950, and the freezes of discouraged much further use of the trifoliate 1957-58 in Florida have stimulated renewed orange and its hybrids in the breeding of scion interest in research work on cold hardiness of varieties because of the disagreeable flavors citrus. In recognition of this need, the horti they transmit to their progeny. cultural laboratories of the United States De partment of Agriculture at Orlando, Florida; Swingle and Robinson (5) also used kum- Weslaco, Texas; and Indio, California, were quats as sources of cold hardiness in crosses recently provided funds for increasing work made with lemons and limes. From these related to cold hardiness of citrus. At these crosses the limequats resulted; and several of three laboratories investigations are being plan these, the Eustis and Lakeland limequats in ned, or are already under way, with the particular, have been widely used in door- ultimate object of reducing losses from cold yards as hardy substitutes for limes. injury to citrus. The current citrus-breeding work in Florida, One of the most obvious methods of re Texas and California is being closely coordin ducing losses from frost is the production by ated. The large and extremely valuable col breeding and selection of cold-hardy varieties. lection of citrus species, varieties and relatives Citrus breeding was begun in this country in at Orlando and the smaller but very useful 1893 by Webber and Swingle (7) just before ones at Weslaco and Indio are available for the disastrous freezes of 1894-95 in Florida, hybridization and selection work at all three and most of the hybrids produced were lost stations. Also, materials from the extensive in the freeze. Consequently, when this work citrus collection of the University of California FURR AND ARMSTRONG: COLD HARDINESS 67

Citrus Experiment Station at Riverside are at one station are made wherever the materials available for our use. Crosses that are wanted are available but mostly at Orlando and Indio,

^ - J* Vari,ation in «oW injury of citrus varieties in freezes of November 17 and 18, 1958, at Indio, California. A- Changsha mandarin on Cleopatra rootstock. B-Williow Leaf mandarin on Troyer rootstock. Both planted 1 year and grow- l?9« feet ,aParf- C-Calamondin x Eureka lemon hybrid seedlings, 3 years old; showing segregation for cold tolerance. D-Rangpur lime x Brazil sour orange hybrid seedlings, 3 years old; showing segregation for cold tolerance. FLORIDA STATE HORTICULTURAL SOCIETY, 1959 and the seeds are sent to the places where the Leaf, and Honey tangerines; the Hamlin, Sha- trees are to be grown and tested. mouti, Tarocco, Moro and the navel oranges; the Orlando and Pearl tangelos. The Salusti- Sources of Cold Hardiness ana, an early orange recently introduced to our In Scion Varieties collection from Spain, should soon come into flower and be available for breeding. This is The most critical problem in breeding for an excellent, early, seedless, round orange that, cold-hardy scion varieties is that of finding in Spain, ripens with the navel oranges. satisfactory sources of cold hardiness. The trifoliate orange seems like the most promising Sources of Cold Hardiness In source from which to make large improvements Rootstock Varieties in the cold tolerance of citrus, but because it and its first-generation hybrids impart dis In the breeding of cold-hardy citrus we are agreeable flavors to their progeny, it is likely chiefly concerned with the scion variety be that many years of back-crossing and selection cause usually it is the dominant factor in will be required to transfer to citrus varieties determining the cold tolerance of a citrus tree, a high degree of cold tolerance and to elimin but the rootstock may be of considerable im ate from their inheritance the factors that portance, and under some conditions, as for produce the pungent and disagreeable flavor example, in very light sands, highly calcareous ing compounds. While it seems highly desir or saline soils, it may have a profound influence able to pursue such a long-term program of on the cold tolerance of the tree. transferring cold tolerance from the trifoliate We already have some very cold-hardy orange to citrus in the hope of eventually ob rootstocks: the sour orange, the trifoliate orange taining varieties with good quality and much- and its hybrids—the citranges, citrumelos, cit- improved cold tolerance, other less cold-hardy randarins, citrangequats, etc. They are, how varieties are of more immediate promise for ever, suited neither to all scion varieties nor producing varieties suitable for commercial to all soil conditions. We need to obtain root- production. stocks that combine cold hardiness with other Possibly the most promising immediate desirable traits such as that of inducing in sources from which to introduce cold hardiness the scion vigorous growth and high yield or into mandarins, sweet oranges, tangors, and high quality of fruit, tolerance to highly cal grapefruit are the satsuma oranges, the Chang- careous or saline soils, and resistance to root sha mandarin (possibly the hardiest of the rot. There are fairly large numbers of root- tangerines) and to a lesser extent, the Dancy stock varieties with special qualities that should tangerine. In the freezes of November 17 and be hybridized with the most cold-hardy varie 18, 1958, at Indio several young Changsha ties and selected for testing as stocks. Among trees were only slightly injured, whereas these Rangpur, Cleopatra and Sunki are salt Willow Leaf and several other tangerine var tolerant; the shaddocks, Seminole tangelo, cit- ieties of the same age and growing nearby rumelo 4475 and a Savage seedling are re were severely injured (Fig. 1, A and B). It sistant to foot rot; Rough lemon, Citrus macro- also seems worth while to try to introduce phylla, and Rangpur produce vigorous growth into citrus from the kumquat factors that con and high yields on light soils; and Sour orange, tribute to early and profound winter dor Rough lemon and Shekwasha mandarin are mancy. For breeding acid fruits some sources tolerant of calcareous soils. We especially need of cold hardiness are the Meyer, Gul-Gul, and to determine if any rootstock can be produced Kusner lemons, the kumquats, lemonquats and that will have an appreciable effect in inducing limequats, and possibly the citrangequats. winter dormancy of the scion variety in areas There is also considerable, possibility of re where winter temperatures are relatively high ducing losses of the fruit itself by breeding for long periods. The kumquats possess the and selection for earliness of ripening; that is, important faculty of remaining dormant by producing varieties that so far as the fruit through fairly long warm spells in winter. If is concerned escape most of the severe freezes, which are likely to occur from late December this characteristic is heritable, perhaps it can to mid-February. Some of the varieties avail be transferred to types more useful as stocks able for this work are the Clementine, Willow than the kumquats. FURR AND ARMSTRONG: COLD HARDINESS 69

Relation of Physiology of Cold view. An especial effort has been made dur Hardiness to Breeding ing the last few years to collect cold-hardy varieties for use in breeding both scion and The basic work on the physiology of cold rootstock varieties. A large collection has been hardiness in citrus and most of the testing for made of trifoliate orange strains and hybrids hardiness of varieties and hybrid progenies and the available kumquats and kumquat hy produced in the breeding work will be done at brids. We have been much interested in the Weslaco laboratory, where special facilities collecting, selecting and testing salt-tolerant are available. It is expected that information materials for use in breeding for salt tolerance obtained in the work on the physiology of cold in rootstocks. Results of the extensive investi hardiness in citrus will be of great value in gations on salt tolerance of citrus rootstocks breeding for cold hardiness. conducted during the last 10 years in Texas In fact, for most effective breeding work it by Cooper and his associates (2, 3, 4) have is essential that we know more than we now do provided us with a sound basis for breeding about the nature of cold hardiness in citrus for salt tolerance in rootstocks. That work and the citrus relatives. The breeder must also showed that the degree of salt tolerance know what hereditary characteristics to look has a profound influence on the cold hardiness for and how to make tests of seedling progenies of citrus trees growing on highly saline soils so that reliable selections of cold-tolerant in dividuals can be made. It is obvious that one In most of the hybridization work so far of the factors involved in the cold tolerance accomplished the objective was usually the of citrus is dormancy of the tree—a condition combination of several desirable characteristics in which little or no cell division and growth in the hybrid progenies. In some of these cold occur. We need to know more about the hardiness was a major consideration, but in conditions required to induce or maintain dor many it was secondary. An account of the mancy in different species and varieties. An hybridization work on scion varieties related other possible heritable factor that citrus or to cold hardiness follows. some hybrids of citrus and the trifoliate orange Lemons and Limes. In the interior desert may possess is the ability to undergo cold areas of Arizona and California, where sum hardening — the process that enables some mers are hot and winters are cold, the Meyer plants, for example, cabbage and wheat, after lemon has been found to be cold hardy and being subjected to near-freezing temperatures highly productive. With the objective of com for a few days to withstand for some days bining the hardiness of the Meyer and the afterwards relatively severe freezing without high fruit quality of the commercial types, injury. Possibly there are other heritable fac crosses were made in 1952 between the Meyer tors involved in cold tolerance of citrus that and the variety Messina, 2 strains of Eureka, should be taken into consideration. Clear and 2 strains of Lisbon. Over 700 seedlings definition of these factors and the develop were produced, and many of these have fruit ment of reliable and satisfactory test methods ed. The Messina, Eureka and Lisbon were the will no doubt call for physiological investiga seed parents, and as they produce many nucel- tions under carefully controlled conditions such lar seedlings, it was difficult to determine as may be obtained at the cold-hardiness labor whether some of the desirable seedlings are atory at Weslaco. hybrids, but a few that apparently are show Breeding Work In California promise. Further study will be necessary to Related To Cold Hardiness select the best of these. It will also be necessary to test them for cold tolerance. The breeding work in California is carried With the object of producing a small hardy on at two stations. The crosses are made at acid fruit of the lime type suitable for use as Indio, where the variety collections are main a dooryard fruit in cold locations, the Cala- tained, but most of the seedling progenies are mondin lime was crossed with Eureka and grown at the Southwestern Irrigation Field Lisbon lemons and with the Rangpur lime. Station at Brawley. Only a few hybrids were produced from each Much of the work related to cold hardiness cross and these are not yet in bearing. In has been done with rather broad objectives in the 1958 freezes the Calomondin x Jemon 70 FLORIDA STATE HORTICULTURAL SOCIETY, 1959 hybrids showed striking segregation for cold 2 or possibly 3 hybrids, and these have not tolerance (Fig 1, C). yet fruited. These results suggest, however, that with sufficiently large scale pollinations of Loose-skin Types. Extensive hybridization navel oranges it may be possible to obtain in of tangerine, tangor and tangelo varieties was a few years' time fairly large numbers of accomplished in the period 1952 through 1954. navel orange hybrids. The seed parents most extensively used, Clem The Shamouti orange is not a commercial entine, King, Wilking, Honey, Umatilla and variety in this country, but in Israel it is con Temple, were selected because they produce sidered to be one of the world's finest varieties. a high proportion of hybrid seedlings. Among For several years we have pollinated large the pollen parents were Dancy, Kinnow, numbers of Shamouti flowers with Hamlin, Honey, Kara, Wilking and Willow Leaf tanger Tarocco and Joppa pollen, in the hope of ob ines; Orlando, Minneola, San Jacinto and Pearl taining an early high-quality and nearly seed tangelos; Umatilla and Temple tangors. These less orange. Though the yield of seeds has crosses were not made specifically to produce been small and the number of hybrids very cold-hardy varieties, but in the several thou few, it has been encouraging to obtain even a sand seedlings produced it is apparent that few hybrids. We hope to improve the con wide variation in hardiness exists; and cold ditions under which these crosses are made hardiness is one of the factors that will be so as to obtain a higher yield of seeds. Temple considered in the testing and selection of orange produces some early-ripening seedlings, seedlings for commercial trial. Many of these and as it produces only hybrids, it also has seedlings have fruited and about 50 have been been used as seed parent in crosses with. Ham propagated on a common rootstock for further lin, Tarocco and Shamouti oranges. In all of observation at Indio. Budwood of many of these crosses many of the hybrid seedlings them has been sent to Weslaco for observation are weak and worthless, so that rogueing is and testing for cold hardiness. necessary. Breeding Early Tangerines and Oranges to escape Early Frosts: With the primary object Combining Cold Hardiness and Salt Toler of obtaining early tangerines, tangors and ance in Rootstock Types: Since 1954 extensive oranges of high quality a series of crosses was hybridization work with rootstock varieties begun in 1953 and additional ones were made has been performed with the object of pro each year to the present time. If, as is ex ducing a series of rootstock types that combine pected, very early ripening fruits are obtained various desirable traits. Especial attention has from some of these crosses, the fruit itself been given the trifoliate orange and its hy should escape injury in all except the very brids because many of them are resistant to early frosts, which usually are not severe. root rot, induce in the scion variety the pro Seed produced from a cross of Clementine duction of high-quality fruits, and under some by Owari satsuma made at Orlando by Philip conditions improve the cold tolerance of the Reece were received in 1953. Of the 224 seed scion variety. Because of the prevalence of lings produced from these seeds a few trees saline waters in many of the irrigated citrus fruited at Brawley in 1958, and these indicate districts the production of salt-tolerant root- that this cross is likely to be a profitable one. stocks has been a major consideration in the Over 200 seedlings from a cross of Clementine hybridization work at Indio. by Hamlin were planted in 1954 at Brawley. With the object of combining the factors Some of these fruited in 1958; several were for cold hardiness and salt tolerance in citrus quite early ripening, but the only one selected rootstock types, crosses were made using as for second test was not as early as Clementine. seed parents the salt-tolerant varieties Rangpur Most of these seedlings have not yet come into lime, Cleopatra and Sunki mandarins and as bearing. An attempt was made in 1954 to pollen parents several strains of the trifoliate obtain hybrids between several strains of navel orange, Troyer, Savage and Carrizo citranges, orange as seed parents and Clementine and and the Brazil sour orange. Some of the Honey tangerines, with the object of securing hybrid seedlings, for example the Rangpur x early, seedless tangors of high quality. From Brazil sour orange hybrids, showed clear segre large numbers of flowers pollinated a few gation for cold tolerance in the 1958 freeze seeds were obtained and these produced only (Fig. 1, D). Several hundred of these hybrids RASMUSSEN AND SMITH: SUPERPHOSPHATES 71 are now under test in salt plots. Ap orange varieties with the object of combining parently, the factors responsible for salt toler the factors in the trifoliate orange that induce ance are heritable because the progenies of high quality of fruit and cold hardiness with several crosses have shown striking segre various qualities carried by the other parent, gation for tolerance to high salt. Eremocitrus such as the tendency to induce high yields, glauca is hardy and highly salt-tolerant but vigor of growth, disease resistance, or tolerance a slow, weak grower. Some of its hybrids, to high-lime soils. however, are vigorous and salt-tolerant and Most of these crosses have been made so are being selected and tested for salt tolerance. recently that few of the seedlings have fruited. The best of these will be used for hybridization Unless the seedlings of a variety are largely with other species and varieties with the nucellar, it is of limited value as a rootstock object of producing a cold-hardy and excep even though it has otherwise excellent quali tionally salt-tolerant rootstock variety. ties. Until the seedlings produced by these Combining Cold Hardiness with Resistance crosses are examined for nucellar embiyony to Root Rot and Other Qualities. In a search and tested in other respects, no prediction of for plants highly resistant to injury from their value can be made. Phytophthora parasitica, the chief cause of The detailed method of handling the test root rot or foot rot of citrus in the warm soils ing of seedlings produced at Orlando and of the desert citrus areas, seedlings of a large Indio in the breeding of scion varieties and number of trifoliate orange varieties and hy rootstock varieties is yet to be worked out, brids, sour orange varieties, and mandarin but present plans are to send much of this and tangelo varieties have been screened for material as seed or as budwood to Weslaco for resistance to P. parasitica. This work has been tests of salt tolerance in the field or for tests canied out during the past 3 years by John of cold hardiness in the controlled-temperature B. Carpenter, citrus pathologist at Indio. A rooms and greenhouses there. method that results in infection of all of the LITERATURE CITED 1. Cooper, W. C. Influence of rootstock on injury and seedlings was devised. Following infection recovery of young citrus trees exposed to the freezes of 1950-51 in the Rio Grande Valley. Proc. Rio Grande Valley most of the seedlings die and only the highly Hort. Inst. 6:16-24. 1952. resistant ones quickly recover and resume 2. Cooper, W. C. and B. S. Gorton. Toxicity and accumu lation of chloride salts in citrus on various rootstocks. Proc growth. Large numbers of resistant seedlings Amer. Soc. Hort. Sci. 59: 143-146. 1952. 3. Cooper, W. C, B. S. Gorton and C. Edwards. Salt are now under field test. Some years will tolerance of various citrus rootstocks. Proc. Rio Grande be required to complete the screening of these Valley Hort. Inst. 5: 46-52. 1951. 4. Cooper, W. C. and A. Peynado. Chloride and boron for cold hardiness and other qualities desirable tolerance of young-line citrus trees on various root stocks. J. Rio Grande Valley Hort. Soc. 13: 89-96. 1959. in a rootstock. 5. Swingle, W. f. and T. R. Robinson. Two important new types of citrus hybrids for the home garden—citrange- Crosses have been made between trifoliate quats and limequats. Jour. Agr. Res. 23: 229-238. 1923. 6. Traub, H. P. and T. R. Robinson. Improvement of sub orange varieties and the following: Suen Kat, tropical fruit crops: citrus. U. S. Dept. of Agr. Yearbook Cleopatra, Sunki and Shekwasha mandarins, 1937: 749-826 (Reprinted as Yearbook Separate No. 1589 (1938).) Sunshine tangelo, several shaddocks, Iran 7. Webber, H. J. and W. T. Swingle. New citrus crea tions of the Department of Agriculture. U. S. Dept. of Agr. lemon and Rough lemon, and several sweet Yearbook 1904: 221-240. 1905.

POT STUDIES ON THE EFFECT OF SUPERPHOSPHATES ON

THE GROWTH OF CITRUS SEEDLINGS

G. K. Rasmussen and P. F. Smith field plots depressed growth of feeder roots (2, 7, 8) and stunted growth of Valencia Agricultural Research Service orange and grapefruit trees (5, 8, 9). United States Department of Agriculture Considerable acidity is added to the soil through the use of superphosphates. On light Orlando ly buffered sands the pH may be depressed Phosphates (PO4) applied as superphos appreciably in spite of the use of dolomite for phates at relatively high rates to experimental pH control (7). With only annual applica-