sign in sign up
<<
Home , Rangpur (fruit)

Further Studies on Exocortis Disease of

Ary A. Salibe

Exocortis, as a gum disease on Rangpur 1975) and extensive spread of CEV rootstocks, is known to have oc- might take place if an insect vector curred in Brazil long ago (Moreira, should appear, thus threatening an 1946). Extensive losses due to citrus industry based almost entirely on a root- exocortis viroid (CEV) occurred in the stock intolerant to CEV. 1950's, following the tristeza disaster, Trees of eight nucellar scion varieties, because replanted trees were mostly on budded on Rangpur lime rootstock exocortis-susceptible Rangpur lime eight years old were used. Scions were: (Moreira, 1955; Rossetti, 1955; Monte- Dancy and Cravo , negro and Salibe, 1957; Salibe, 1961). ; Hamlin, Barao, Baianinha Inarching declining trees with Navel and Valencia oranges; and Tahiti exocortis-tolerant rootstocks then lime. Inoculation was done in February became a common practice and the 1974 by budding 10 young branches of general use of CEV-free nucellar clones each tree with a bud carrying a severe soon occurred. Currently, of the 150 CEV strain. Four trees of each scion million citrus trees in Brazil, about 90 variety were inoculated at random and per cent are on Rangpur lime, so another four maintained as healthy exocortis, though now a rare disease, controls. No visible difference between remains a permanent threat to the citrus infected trees and healthy controls was industry. Some instances of CEV con- observed in a five-year period, except tamination on supposedly healthy for some occasional leaf yellowing of nucellar trees have been found recently the inoculated Tahiti lime trees. No in commercial orchards, causing cracking, bark scaling, or gumming concern among growers. Mother trees developed in the Rangpur lime used by growers are exocortis free and rootstock of any tree. Comparisons of the seed transmission reported by average tree growth and production in Salibe and Moreira (1965a) is now con- the five-year period, and of tree height sidered to have been mechanical trans- in 1979, are shown in table 1. mission. Spread of CEV must be The experimental trees will be occurringby mechanical means or by an followed for years to determine the inefficient insect vector. This paper incubation period of CEV in large trees reports the results of CEV inoculations and the extent of its detrimental effect in large citrus trees and noncitrus on growth and production. trees, mechanical spread, and some Indexing these trees for CEV as relationships between Tahiti lime and described by Calavan et al. (1964), re- CEV. vealed that all are infected with a severe strain of CEV. Mechanical spread. A study was EXPERIMENTS AND RESULTS made to find out if some citrus varieties Late infection. An experiment was provide more highly infectious conducted to determine the effect of late inoculum than others in mechanical infection with CEV on the growth and transmission of CEV. If this is the case, production of large field trees. it might explain the negative results in Exocortis infection due to cuts, previous mechanical transmission pruning, or machine hedging is known experiments by Salibe (1961). Since the to occur in the greenhouse (Garnsey, presence of viruses in the source 1967) and field (Wutscher and Shull, might affect CEV transmission, this 216 Eighth IOCV Conference possibility was considered. Trees of four ber 1975, using the technique described citrus varieties were selected as donor by Garnsey (1967). Twenty plants of plants for the transmission experiment: each indicator variety were mechani- a tree of Moro carrying CEV cally inoculated from each donor tree. and psorosis and tristeza viruses; a tree Control plants were budded with blind of Salustiana orange carrying CEV plus buds from the same donor trees. a severe strain of tristeza virus; a tree of Readings made up to one year after in- Eureka infected only with CEV; oculation showed that most (90-95 per and a of previously cent) of the mechanically inoculated infected with CEV plus mild tristeza Etrog citron plants developed mild or virus. All field trees of citrus in Brazil severe symptoms of CEV independently are infected by tristeza virus, except true of donor variety and the presence of , , and a few tristeza virus. However, the situation other citrus types having tissues was different in Rangpur lime plants; resistant or highly intolerant to the only 45 per cent (9 of 20) plants with virus. inoculum from Eureka lemon and 50 Indicator plants were Etrog citron 60- per cent (10 of 20) plants with inoculum 13 and Rangpur lime, free from tristeza from Etrog citron developed exocortis virus. Inoculations were made in the symptoms. None of the Rangpur lime greenhouse in November and Decem- plants mechanically inoculated from

TABLE 1

AVERAGE TRUNK SIZE AND GROWTH AND FRUIT PRODUCTION IN A 5-YEAR PERIOD AND TREE HEIGHT IN 1979 OF LARGE FIELD TREES ON RANGPUR LIME ROOTSTOCK INOCULATED LATE WlTH CEV COMPARED WlTH UNINOCULATED HEALTHY TREES Trunk Trunk circumference growth Tree Scion 1974 1979 1974-1979 height varieties c m c rn no. m

Murcott tangor Dancy Cravo tangerine Hamlin orange Barao orange Baianin ha navel orange Tahiti lime

AVERAGE 1 39.9 52.8 12.9 751 3.61 H 42.0 55.1 13.1 788 3.69

* I = infected trees; H = healthy controls. Exocortis and Gummy Pittinx orange tree donors showed symptoms. exocortis symptoms in their Rangpur All bud-inoculated plants developed lime rootstocks. Treeseof Eureka lemon typical exocortis symptoms beginning on Rangpur lime rootstock, of the same within 4 months. Two plants of age and inoculated with buds from the Rangpur lime mechanically inoculated same source, developed severe exocortis from the Moro orange tree developed symptoms, but were less stunted than psorosis leaf symptoms during the the Tahiti lime trees. Obviously, severe spring flush in September 1976. strains of CEV became established in Eureka lemon tissues but not in those of Exocortis in Tahiti lime trees. Tahiti Tahiti lime. lime is highly valued and extensively Alternative hosts. Attempts were grown in Brazil. It is estimated that over made to determine if certain fruit trees three million trees of this variety of other than citrus could serve as citrus have been planted in the last 15 alternative hosts for CEV. In the last 2 years. Five clones of Tahiti lime are years, young seedlings or rooted commercially grown and are CEV cuttings were inoculated with CEV by infected, except for one clone named budding with various infected citrus IAC-5 (Peruano). CEV is considered varieties. The experiment included responsible for a bark disease on Tahiti loquat, annona, guava, papaya, pear, lime trees (Salibe and Moreira, 19656). apple, pecan, persimmon, avocado, Observations in commercial plantings mango, and a few other local tropical of Tahiti lime with bark disease never fruits. Results were negative, except showed bark-scaling symptoms on surprisingly, when seedlings of Rangpur lime rootstocks. For this avocado, Persea americana Mill., were reason, it was suspected that Tahiti lime used. Citrus buds inserted into 2-3 tissues do not increase or maintain the months-old avocado seedlings re- more severe strains of CEV. This mained alive for several months (up to 1 hypothesis was confirmed in an experi- year). The avocado seedlings continue ment at the "Presidente Medic?' Experi- to grow and have developed yellowish ment Station, in Botucatu, starting in areas in the bark near the site of January 1973. budding, resembling symptoms on Rangpur lime seedlings in the nursery Rangpur lime seedlings infected with were inoculated with blind buds (three mild CEV. Percentage survival of citrus buds per seedling) from a Hamlin buds in avocado seedlings seems to vary orange tree known to be carrying a with the used, in the following severe strain of CEV. Twenty days later descending order: citron (80), lemon these seedlings were budded with (50), lime (50), orange (20), and healthy Tahiti lime, IAC-5. Twelve of tangerine (10). None of the citrus buds the 48 nursery trees obtained grew sprouted. Stems of Etrog citron infected poorly qnd remained stunted in the with CEV topworked onto young nursery, being under 40 cm high 1 year avocado seedlings survived for several after budding. Fifteen of the nursery months but also failed to s-prout. Bark trees that grew well were transplanted to rings of Etrog citron (3 cm diameter) the field. Five healthy uninoculated used to replace similar bark rings of Tahiti lime trees from the same nursery avocado seedlings also induced yellow were planted also, as controls. All CEV- blotching in the upper portion of the infected trees, but none of the controls, stem. developed bark disease symptoms in the DISCUSSION AND CONCLUSIONS branches. When tested for CEV on Etrog citron 60-13, all 15 inoculated The results obtained from the late trees were found to carry mild strains of infection experiment clearly show that CEV. Observations on the field trees, 6 CEV has a longer incubation period in years after planting, showed that large trees than in young plants, thereby inoculated plants were smaller than diminishing the danger of sudden heavy controls, but none had developed losses to growers if rapid field spread of Eighrh IOCV Conference

CEV occurs. Growers could be advised inoculum sources than others. Infected to establish their orchards on more than lemon trees apparently represent a one rootstock, but under Brazilian better and more important source of conditions, no other rootstock is equal inoculum for CEV spread in nurseries to Rangpur lime. and orchards than some other citrus The delayed effect of CEV inoculated species. into field trees should be considered for Igwegbe (1967) found that cala- the production of dwarfed trees, as mondin is a poor host for CEV. My suggested by Samadi (1976). It should results indicate that Tahiti lime is also a be investigated when inoculation with poor host for CEV, for only mild strains mild CEV is needed to obtain the of CEV remained in the Tahiti lime trees desired stunting effect. Garnsey and and caused no bark symptoms in Weathers (1972) have indicated that if Rangpur lime rootstocks. Tahiti lime, trees on sensitive rootstocks are large for this reason, may be useful in when infection with CEV occurs separating mild from severe strains of subsequent stunting is less detrimental CEV. These results also suggest that than to trees that are small when severe strains of CEV are always infected. accompanied by mild strains, as is The results of the inoculum source known to occur with tristeza virus. study emphasized the importance of Avocado seedlings were found to be this factor in the mechanical trans- possible alternative hosts for CEV and mission of CEV. All inoculum sources to develop yellow blotching in the area were effective for knife inoculations to of bud inoculation. This deserves citron but not to Rangpur lime, further study. suggesting interrelationships between ACKNOWLEDGMENTS host donors, host receptors, and CEV. The presence of tristeza virus in host Grateful acknowledgment is made donors apparently did not affect to Dr. S.M. Garnsey for demonstrating transmission of CEV to citron, but may the mechanical transmission technique have prevented transmission to during my visit in his laboratory at Rangpur lime. Inoculum from the two Orlando, Florida. Acknowledgement is orange sources carrying tristeza failed also made to FAPESP - Fundacao de to transmit CEV to the lime indicator. Amparo a Pesquisa do Estado de Sao Garnsey and Weathers (1972) and Paulo for sponsoring my trip to Aus- Weathers et al. (1974) have pointed out tralia to attend the 8th Conference of that certain citrus varieties are poorer the IOCV and to present this paper. Exororris and Gummy pit tin^

LITERATURE CITED CALAVAN, E.C., E.F. FROLICH, J.B. CARPENTER, C.N. ROISTACHER, and D.W. CHRIS- TIANSEN 1964. Rapid indexing for exocortis of citrus. Phytopathology 54: 1359-62. GARNSEY, S.M. 1967. Exocortis virus of citrus can be spread by contaminated tools. Proc. Fla. State Hort. Soc. 80: 68-73. GARNSEY, S.M., and L.G. WEATHERS 1972. Factors affecting mechanical spread of exocortis virus, p. 105-1 1. In Proc. 5th Conf. IOCV. Univ. Florida Press, Gainesville. IGWEGBE, E.C.K. 1967. Studies on the unequal distribution of exocortis virus in young and old plants of calamondin, Citrus mitis Blanco. M.S. thesis, Univ. California, Riverside. 61 p. MONTENEGRO, H.W.S., and A.A. SALIBE 1957. A exocortis e a situacZo atual dos porta-enxertos para citrus. Rev. Agr., S5o Paulo 32: 271-79. MOREIRA, S. 1946. Cavalos para citrus em SHo Paulo. Rev. Agr., Piracicaba 21: 206-26. MOREIRA, S. 1955. Sintomas de exocortis em limoeira Cravo. Bragantia 14 (Nota 6): 19-22. ROSSETTI, V. 1955. A doenca de limoeiro Cravo nos laraniais de SZo Paulo. 0 Biolonico- 21: 1-8. SALIBE, A.A. 1961. ContribuicLo ao estudo da doenca exocorte dos citros. Doctorate Thesis, Escola Superior de Agricultura "Luiz de Queiroz," Univ. de SIo Paulo. 71 p. SALIBE, A.A., and S. MOREIRA 1965~. Seed transmission of exocortis virus, p. 139-42. In Proc. 3rd Conf. IOCV. Univ. Florida Press, Gainesville. SALIBE, A.A., and S. MOREIRA 19656. Tahiti lime bark disease is caused by exocortis virus, p. 143-47. In Proc. 3rd Conf. IOCV. Univ. Florida Press, Gainesville. SAMADI, M. 1976. Growth of exocortis-infected citrus on Dabeh rootstock, p. 98-99. In Proc. 7th Conf. IOCV. IOCV, Riverside. WEATHERS, L.G., S.M. GARNSEY, A. CATARA, P.R. DESJARDINS, G. MAJORANA, and H. TANAKA 1974. Mechanical transmission of citrus viruses, p. 147-56. In Proc. 6th Conf. IOCV. Univ. California, Div. Agr. Sci., Richmond. WUTSCHER, H.K., and A.V. SHULL 1975. Machine-hedging of citrus trees and transmission of exocortis and xyloporosis viruses. Plant Dis. Rep. 59: 368-69.