VIRUSES IN FLORIDA'S 'MEYER' LEMON TREES AND THEIR

66 FLORIDA STATE HORTICULTURAL SOCIETY, 1970 VIRUSES IN FLORIDA'S 'MEYER' LEMON TREES AND THEIR EFFECTS ON OTHER CITRUS S. M. Garnsey Horticultural Field Station U.S.D.A. Orlando Abstract 'Meyer' lemon (a probable hybrid of Citrus limon [L.] Burm. f) trees in Florida often con tain both the tristeza-seedling yellows and the tatter leaf-citrange stunt virus (V-V) complexes, but trees with only one or the other virus complex also occur. Graft inoculations with the V-V com plex caused a bud-union groove, stunting and decline in young sweet orange (C. smensis [L.] Osb.) trees budded on 'Rusk', 'Troyer' and 'Carrizo' citranges (Poncirus trifoliata [L.] Raf. X C. sinensis) and trifoliate orange (P. trifoliata) rootstocks. Graft inoculations with tissue from tristezainfected 'Meyer* lemon trees caused severe stunting in sweet orange trees on sour orange (C. aurantium L.) rootstock. However, graft inoculations with an aphid-transmitted isolate of tristeza from 'Meyer' lemon caused only mild stunting. Introduction The present study was undertaken to explore more fully the virus content of 'Meyer' trees in Florida and to measure the effect of these vi ruses on certain other citrus plants. Most 'Meyer' trees in Florida are probably vegetative descendents of the original introduc tions. Six plants were introduced from China in 1908 and propagated at Chico, California (11, 15). There was immediate interest in these trees because of their cold hardiness and fruitfulness. Propagations were widely distributed to most citrus areas in the U.S., including Florida, by 1925 (11). 'Meyer' lemon became a desired variety for dooryard plantings, but tenderness of the fruit limited commercial acceptance (10). Some commercial plantings were made in Texas during the 1930's (6), and scattered commercial plantings still exist in Florida. During the 1950's, many 'Meyer' trees were found to be infected with tristeza virus (2,9,13, 17,21,22). Since earliest attempts to propagate 'Meyer' lemons on sour orange rootstock often failed (11,16), it is likely that some of the six 'Meyer' plants introduced were infected with tristeza (17). Tristeza-infected 'Meyer' trees were probably introduced into Florida prior to 1920, but the virus was not discovered until the early 1950's (9,12). Twenty-two of 24 'Meyer' trees indexed for tristeza in 1964 were infected (7). Wallace and Drake (18) reported tatter leaf symptoms in plants of C. excelsa Wester graftinoculated with 'Meyer' tissue, and subsequently reported (19) foliar and stem symptoms of tatter leaf virus (V) in graft inoculated 'Troyer' citrange and citremon plants (P. tri foliata X C. limon). Yarwood found a sap trans missible virus in 'Meyer' lemon which he called 'Meyer' lemon latent virus (23). Calavan et al. (1) reported that satsuma mandarin (C. reticulata Blanco) trees on 'Troyer' citrange, graftinoculated with 'Meyer' tissue, developed a groove at the bud union, vertical corrugations in the rootstock and then declined. They asso ciated these symptoms with V infection. Later, Wallace and Drake (20) presented evidence that there are actually two viruses present in 'Meyer' lemon which cause symptoms described for V. One virus causes symptoms in C. excelsa and is not sap transmissible. The name tatter leaf virus (V) was retained for this virus. The other virus causes symptoms in citrange and citremon plants and is sap-trans missible to non-citrus hosts as well. This virus has been named citrange stunt virus (V). Differentiation of V and V was recently discussed by Catara and Wallace (4) and their terminology will be used in this report. Twenty-four 'Meyer' trees in Florida were indexed on 'Rusk' citrange in 1964. Fourteen were infected with V (identified as V at that time) (7). Subsequently, tests have been conducted on additional trees, and a more ex tensive range of hosts was used to determine the virus content of selected trees. The effect of the V-V complex on sweet orange trees budded to 'Troyer', 'Carrizo', and 'Rusk' cit ranges and trifoliate orange was investigated in several tests. The virulence of an aphid-trans-

GARNSEY: VIRUSES 67 mitted isolate of tristeza from 'Meyer' lemon was also compared with the original source. The results of these tests are reported and discussed in this paper. Methods and Materials Greenhouse studies were conducted in a par tially shaded greenhouse cooled by an evapora tive cooling system. Light intensity at midday usually ranged from 800 to 1500 ft-c, and sum mer temperatures ranged from 22 to 32 C daily. From fall through spring, temperatures usually varied daily from 20 to 26 C. 'Etrog' citron (C. medico, L.), 'Eureka* lemon (C. limon) and C. excelsa plants were propagated from rooted cuttings while other test plants were grown from seed. 'Pineapple* sweet orange seedlings were the source of scion wood for grafted plants. Test plants were grown in steamed potting soil and received frequent fer tilization to promote vigorous growth. All pests were carefully controlled. Plants in field plots received good horticul tural care, and special attention for control of possible insect vectors. Graft inoculations were made by using con ventional T-bud and chip-bud techniques or by using a leaf-piece grafting technique (8). An aphid-transmitted isolate of tristeza was obtained by allowing several hundred aphids (Aphis gossypii Glover) to feed overnight on new growth of a tristeza-infected 'Meyer* plant and then transferring them to succulent new growth of 'Mexican' lime seedlings (C. aurantifolia [Christm.] Swing.). Results and Discussion Preliminary indexing. Over a period of sev eral years, 58 'Meyer' trees from 12 locations were indexed on 'Mexican' lime and 'Rusk' citrange seedlings. Most of these trees were in commercial plantings, but several were speci men trees in variety collections or experimental plantings. Most had been propagated as rooted cuttings, but some were budded on rough lemon (C. jambhi7'i Lush.) rootstock. Fifty-two of the 58 produced typical tristeza vein-clearing and stem-pitting symptoms on 'Mexican' lime. Thirty-four produced V symp toms in 'Rusk' citrange plants. Twenty-nine pro duced both tristeza and V symptoms while five produced only V symptoms. Tissue of one plant, obtained from H. C. Burnett, Florida Department of Agriculture, Winter Haven, pro duced no symptoms in plants of either indicator variety. Trees with a full virus complement usually appeared as vigorous and fruitful as any others, but no attempt was made to obtain precise com parisons of growth rate and yield. Comprehensive indexing of selected trees. Six Meyer' trees were selected for further test ing to determine: 1) If the V component was present with V; 2) If the V present would cause symptoms in sweet orange trees grafted on citrange rootstock; 3) If the tristeza-infected trees carried the seedling yellows component; 4) If the tristeza in these trees was virulent to sweet orange trees grafted on sour orange rootstock; and 5) If these trees carried other viruses. According to the preliminary tests, two of the trees selected carried both tristeza and V, 2 carried tristeza but no V and 2 car ried V but no tristeza. Tissues from these six trees were used to graft inoculate plants of the following: 1) 'Mexi can' lime, 2) 'Rusk' citrange, 3) 'Eureka' lemon, 4) C. excelsa, 5) 'Pineapple' orange, 6) 'Etrog' citron (Arizona 861), and 7) 'Pineapple' orange grafted to sour orange rootstocks. All plants were observed in the greenhouse for 9 months. The grafted sweet/sour trees were grown for an additional 6 months in the greenhouse and 32 months in a field plot. There were five plants per treatment. In addition, alternate trees in two rows of 'Rusk' seedlings, spaced 20 X 20' in a field plot, were grafted with 'Pineapple' orange scions. After the scions were established, 2 grafted trees and 2 seedlings each were inocu lated with tissue from the 6 'Meyer' trees above, 42 months prior to the preparation of this manuscript. The results of these tests are summarized in Table 1. The two 'Meyer' trees which did not produce tristeza symptoms in 'Mexican' lime plants in preliminary tests, produced no symp toms in 'Eureka' lemon or in sweet/sour plants. The four trees which caused tristeza symptoms on 'Mexican' lime also caused seedling yellows symptoms (16) in 'Eureka' lemon and produced stunting on grafted sweet/sour plants. While direct inoculation with 'Meyer' tissue caused severe stunting on sweet/sour trees, an aphid transmitted isolate caused only mild stunting in another field test. After nearly 5 years, the trunk

68 FLORIDA STATE HORTICULTURAL SOCIETY, 1970 Table 1. Virus symptoms ' in citrus plants graft inoculated with tissue from six ^eyer* lemon trees Greenhouse Field 'Mexican fw 'Eureka' 'Rusk' 'Etrog1 citron Sweet orange Sweet orange grafted Tree No. lime lemon excelsa citrange Arizona 861 grafted to 'Rusk' to sour orange FS 58 16.32 FS 114 17.0 18.6 FS 54 14.5 Decline 3/5 dead 4.3 FS 84 22.9 Decline 1/5 dead 8.1 FS 73 26.0 Decline 2/5 dead 8.5 FS 96 24.7 Decline 1/5 dead 6.2 Control 24.6 16.4 a/ Following abbreviations are used: -tatter leaf; -citrange stunt; -bud-union groove; VC-vein clearing; -stem pitting, -seedling yellows, b/ Latin binomials are listed in text, c/ Increase in trunk circumference (cm) 3.5 years after inoculation. circumference of five trees graft-inoculated with 'Meyer' tissue averaged 16.5 cm, while the fig ures for the five trees graft-inoculated with the aphid transmitted isolate and the five healthy controls were 24.5 cm and 29.2 cm respectively. The four 'Meyer* trees that caused V symptoms in 'Rusk* citrange seedlings also pro duced V symptoms in C. excelsa, and budunion grooves (Fig. 1), stunting and decline in grafted sweet/'rusk' trees. The later symptoms only became obvious during the past 12 months, and differences between trees with and without the V-V will probably increase. Some ver tical corrugations, as described by Calavan et al. (1), were observed, but were not especially con spicuous. Trees inoculated with tissue from 'Meyer' trees without V-V had no groove and were not stunted. Although 'Rusk' citrange seedlings showed citrange stunt leaf and stem symptoms in the greenhouse, seedlings in the field failed to show conspicuous leaf symptoms. Trunk circumference increased an average of 23.2 cm in 'Rusk' seedlings without V-V, versus 22.6 cm for those with V-V. Arizona 861 citron plants did not show exocortis symptoms in this test, but mild exocortis symptoms were observed in. another citron selec tion inoculated with tissue from the same FS-58 tree. The tristeza symptons observed in citron were milder than those observed in comparably inoculated 'Mexican' lime seedlings. No symptoms were observed in any of the inoculated 'Pineapple' orange seedlings, indicat ing that psorosis virus was not present in the six trees indexed. Figure 1. Sweet orange tree on trifoliate orange rootstock with a groove at the bud union 3 years after inocu lation with V-V complex. Virtually identical symptoms occurred in sweet orange trees on 'Rusk/ 'Carrizo' and 'Troyer' citrange rootstocks.

GARNSEY: VIRUSES 69 Effect of V-V on different stock/scion combinations. In one test, seedlings of 'Rusk', 'Carrizo' and 'Troyer' citranges were grafted with 'Pineapple' orange. Grafted plants were inoculated with tissue from the FS-58 'Meyer' tree which carried V-V, but not tristeza (Table 1), and planted in the field along with uninoculated controls. There were four repelications for each inoculated combination, and three for the controls. Results of this test after 4 years are sum marized in Table 2. A groove was observed at the bud union of all inoculated trees beginning 30-36 months after inoculation. Grooving was consistently strongest in trees on 'Rusk' citrange. Trees with well-developed bud-union grooves generally are stunted and have abnormally heavy crops of fruit this year. Several trees with budunion grooves were pushed out with a tractor. The extent of the virus-induced, stock-scion in compatibility was shown when part of the main trunk broke cleanly at the bud union (Fig. 2). No groove or stunting occurred in uninoculated trees. Six small sweet orange trees on trifoliate orange rootstock were inoculated with FS-58 'Meyer' tissue in another test. Three years after inoculation, 2 of the 6 are dead and the remain ing tree;s have well-developed bud-union grooves (Fig. 1) and are stunted. No grooves occurred in the six uninoculated control plants. The symptoms in these tests seem consistent with those described earlier in California for satsuma/'troyer' trees inoculated with 'Meyer' tissue containing tristeza and the V-V complex (1). Symptoms were produced by the V-V complex with or without tristeza. It is too early to say whether the V-V will eventually kill more plants in our field plots, but the contrast between infected and healthy plants seems certain to increase. General Discussion The tristeza and V-V content of 'Meyer' lemon trees in Florida varies as it does also in Figure 2. Trunk of young sweet orange tree on 'Rusk' citrange rootstock inoculated with V-V complex from 'Meyer' lemon. When a portion of the trunk was torn loose, it separated cleanly at the union, revealing the extent of the virus-induced bud union incompatibility. Table 2. Effect of the tatter leaf-citrange stunt virus complex (V-V) on 'Pineapple1 orange trees grafted to three citrange rootstocks 4 years after inoculation 'Pineapple1/'Rusk1 'Pineai jple'/'trover' 'Pineai jple'/'carrizo' Budunion Treatment groove Increase Bud- Increase Bud- Increase in trunk union circumference groove (cm) in trunk union circumference groove (cm) in trunk c ircumference (cm) Inoc. V-V^/ + 20.5 + 21.6 + 24.0 Uninoculated - 27.0-33.3-33.9 Ratio 0.76 0.65 0.71 a/ Graft inoculated with tissue from 'Meyer* lemon carrying V-V, but not carrying tristeza.

70 FLORIDA STATE HORTICULTURAL SOCIETY, 1970 Texas (14) and California (19). The cause of this variation is difficult to establish. Since most 'Meyer' trees in the U.S. probably originated from vegetative propagations of the original introductions (17), the simplest way to account for variation in virus content is to assume that, of the s*x plants in the original introduction: some carried tristeza, some carried V-V, and some carried both. Propagation of seedlings similar to the original clone or chance propaga tion of virus-free buds from infected plants may be involved in some situations as suggested by Wallace (17). Later introductions may also be involved. For example, a thornless sport of 'Meyer* lemon in the USDA variety collection was imported from New Zealand in 1934 (PI- 106235). This plant does not carry V-V but does carry tristeza, which it may have con tracted in its present location. The V-V complex can cause severe injury to trees on. citrange and trifoliate orange rootstocks. Presumably, the V portion of the complex (4) is responsible for this reaction, but the V components was present in all our tests with V and its possible effect can not be completely discounted. Fortunately, natural in fections of this virus complex have been found only in 'Meyer' lemon and there has been no evidence of vector spread. However, all budwood sources for use on citrange or trifoliate orange should be checked for V-V, and any indica tion of vector spread should be rapidly investi gated. While many 'Meyer' trees carry a severe isolate of tristeza, there is little evidence for natural spread of tristeza from 'Meyer' lemons into adjacent plantings (3,5,17,21). Further more, our results, plus those obtained earlier (12), indicate that aphids do not always trans mit an isolate with full virulence from 'Meyer' lemon. It seems pointless, however, to maintain large reservoirs of two virus complexes which could be dangerous to the Florida citrus indus try. Budwood of the one 'Meyer' tree which in dexed free of both tristeza and V-V is now available from the Citrus Budwood Registration Bureau, Florida Department of Agriculture, Winter Haven (Personal Communication, G. D. Bridges). Use of this, or other virus-free clones of 'Meyer' lemon, is recommended for future plantings. Acknowledgments The author gratefully acknowledges the as sistance of Mr. P. A. Norman, Research Ento mologist, Entomology Research Division, Agri cultural Research Service, U.S. Department of Agriculture, Orlando, Florida, in conducting aphid transmission tests and the technical as sistance of R. Whidden, W. L. Dean and J. W. Jones, Agricultural Research Technicians, Crops Research Division, Agricultural Research Serv ice, U.S. Department of Agriculture, Orlando, Florida. LITERATURE CITED 1. Calavan, E. C, D. W. Christiansen, and C. N. Roistacher. 1963. Symptoms associated with tatter-leaf virus in fection of Troyer citrange rootstocks. Plant Dis. Reptr. 47: 971-975. 2. Carpenter, J. B. 1956. Identification of tristeza in Meyer lemon in Arizona. Plant Dis. Reptr. 40: 701. 3. Carpenter, J. B. 1957. Further studies on tristeza in Meyer lemon in Arizona. Plant Dis. Reptr. 41: 1014-1015. 4. Catara, A., and J. M. Wallace. 1970. Identification of citrange stunt as the mechanically transmissible virus from Meyer lemons doubly infected with citrange stunt and tatter leaf viruses. Phytopathology 60: 737-738. 5. Dean, H. A., and E. O. Olson. 1956. Preliminary studies to determine possibility of insect transmission of tristeza virus in Texas. J. Rio Grande Valley Hort. Soc. 10: 25-30. 6. Friend, W. H. 1964. History of the Meyer lemon in the Valley Proc. 8th Ann. Rio Grande Valley Hort. Inst. p. 32-33. 7. Garnsey, S. M 1964. Detection of tatter leaf virus of citrus in Florida. Proc. Fla. State Hort. Soc. 77: 106-109. 8. Garnsey, S. M., and R. Whidden. 1970. A rapid tech nique for making leaf tissue grafts to transmit citrus viruses. Plant Dis. Reptr. 54: 907-908. 9. Grant, T J. 1953. Aids in the detection of tristeza in Florida citrus. Proc. Fla. State Hort. Soc. 66: 67-73. 10. Hodgson, R. W. 1967. Horticultural varieties of citrus, p. 431-591. In W. Reuther, H. J. Webber and L. D. Batchelor (Eds.) The Citrus Industry, Vol. I. University of California Press. 11. McKee, R. 1926. Chinese dwarf Meyer lemon intro duced. USDA Yearbook of Agriculture, 1926, p. 218-221. 12. Norman, P. A., and T. J. Grant. 1956. Transmission of tristeza virus by aphids in Florida. Proc. Fla. State Hort. Soc. 69: 38-42. 13. Olson, E. O. 1964. Tristeza virus carried by some Meyer lemon trees in South Texas, p. 84-88. Proc. 8th Ann. Rio Grande Valley Hort. Inst. 14. Olson, E. O., and A. Shull. 1966. A Meyer lemon rootstock trial: Scion-rooting, tree growth, yield and tree survival after a severe freeze. J. Rio Grande Valley Hort. Soc. 20: 102-108. 15. U.S. Department of Agriculture, Office of Foreign Seed and Plant Introduction, Bureau of Plant Industry. 1923-24. Plant Introductions, Twelfth Annual list. Wash., DC 54 p. 16. Wallace, J. M. 1968. Tristeza and seedling yellows, p. 20-27. In Agriculture Handbook No. 333, Indexing Pro cedures for 15 Virus Diseases of Citrus Trees, ARS, USDA, Washington, D.C. 17. Wallace, J. M., and R. J. Drake. 1956. The tristeza virus in Meyer lemon. Citrus Leaves 35(1): 8-9, 23. 18. Wallace, J. M., and R. J. Drake. 1962. Tatter leaf, a previously undescribed virus effect on citrus. Plant Dis- Reptr. 46: 211-212. 19. Wallace, J. M., and R. J. Drake. 1963. New infor mation on symptom effects and host range of the citrus tatter-leaf virus. Plant Dis. Reptr. 47: 352-353. 20. Wallace, J. M., and R. J. Drake. 1968. Citrange stunt and ringspot, two previously undescribed virus diseases of citrus, p. 177-183. In J.F. L. Childs (ed.) Proc. 4th Conf. International Organ. Citrus Virol., Univ. Fla. Press, Gainesville. 21. Wallace, J. M., P. C. J. Oberholzer, and J. D. J.

KOO AND DRISCOLL: GROVE RENOVATION 71 Hofmeyer. 1956. Distribution of viruses of tristeza and Winocour, 1956. The tristeza virus in Israel. Phytopathother diseases of citrus in propagative material. Plant Dis. ology 46: 347. m Reptr 40: 3-10. 23. Yarwood, C. E. 1963. Mechanical transmission of 22 Wallace, J. M., I. Reichert, A. Bental, and E. a latent lemon virus. Phytopathology 53: 1145 (Abstr.). RENOVATING OLD CITRUS GROVES IN INDIAN RIVER AREA R. C. J. KOO Florida Citrus Experiment Station Lake Paul J. Alfred AND Driscoll Swain Groves, Inc. Fort Pierce Abstract A 20-acre block of old and low-producing grapefruit trees in single beds was pushed and the soil leveled. The soil profile was modified by plowing to a depth of 30 to 36 inches prior to the construction of new beds in one 10-acre section. A dragline was used in the remaining 10 acres in 2 ways. In one 5-acre section, clay and subsoil materials were mixed with sand in the rows where the trees were to be planted. The soil was restructured in the other 5-acre section by mixing clay from the water furrow with sand on the beds to a depth of about 18 inches. Double beds were constructed for the new planting. Soil samples collected before and after the soil was restructured, showed marked differences in water-holding and cation-ion exchange capaci ties, ph, calcium, and magnesium contents where clary-marl material was mixed with sand. Little difference was found in areas where the mixing did not involve clay. Significant correlations were found between tree growth and available soil moisture, and extractable soil calcium and magnesium contents. Introduction Citrus groves are long-term investments. When a grove becomes unprofitable, the grower Florida Agricultural Experiment Station Journal No. 3731. can either make further investments to improve the productivity of the grove or he can get out of the business. It may even be necessary to re move the grove, restructure the soil, and replant. It is estimated that 25 to 30% of the old groves in the Indian River area could fit into this category (3). This paper reports the complete renovation of one 20-acre grove. Methods and Materials The grove involved in this study was 20 acres of mature 'Ruby Red' grapefruit on sour orange rootstock located near Fort Pierce. The soil profile resembled a combination of Adamsville and Keri series underlain with marl and shell at a depth of approximately 30 inches The trees were planted on single beds 1,320 feet in length. A tree spacing of 25 x 30 feet resulted in 58 trees per planted acre. Drainage appeared in adequate. The trees were unhealthy and fruit production in the past 5 years averaged only 77 boxes per acre per year. Management decided this grove should be completely renovated. All the trees were re moved and the beds leveled. The block was divided into two 10-acre sections by a drainageirrigation ditch. In one 10-acre section, the soil was plowed to a depth of 30 to 35 inches prior to the construction of the beds. A dragline -was used in the other 10-acre section to mix subsoil clay with sand. In one 5-acre section, subsoil materials in the beds were mixed with sand in the rows where the trees would be planted. In the other 5-acre section, the soil was restructured by mixing the subsoil materials from the water furrow with the sand on the beds to a depth of about 18 inches. Double beds 400 feet long were constructed perpendicular to the ditch. Construction of beds was completed in November 1968. 'Ruby Red* grapefruit trees on sour orange rootstock were