Citrus exocortis viroid
J. S. Semancik
Department of Plant Pathology and Cell Interaction Group, University of California, Riverside, California 92521, USA
Disease described by
Fawcett & Klotz (1948)
and transmission described by
Benton et al. (1950)
The infective agent is a small naked single-stranded RNA (c. 105
daltons) which associates with host nucleus and membranes. There is no evidence for
vector transmission. The agent is stable in sap, resistant to thermal inactivation
and easily transmitted mechanically. It infects many species in the Rutaceae and a
few in the Solanaceae and Compositae. Widespread in citrus-growing regions of the
In citrus the agent causes shelling, scaling and splitting of the bark of
hence the name exocortis (exo
= pertaining to the bark;
Other names for the disease
are scaly butt
(Benton et al. 1949
and Rangpur lime disease
Trees on susceptible rootstocks are stunted and yield poorly
Leaf epinasty and severe stunting is common in indicator species
(Weathers, Greer & Harjung, 1967
The disease is present in most citrus-growing areas where susceptible
rootstock is used. It is widespread in S. America (especially Brazil and Argentina),
Australia and the Mediterranean region (especially Spain) but is of limited
occurrence in the USA and N. Africa. Commercial plantings in Japan are essentially
free from infection
Host Range and Symptomatology
The host range is largely restricted to the citrus family (Rutaceae) although
some species in the Solanaceae (Solanum tuberosum, Lycopersicon esculentum,
) and Compositae (Gynura aurantiaca, G. saramentosa
are susceptible. Transmitted easily by mechanical inoculation of sap from
(Weathers & Greer, 1968
but with difficulty from
(Garnsey & Jones, 1967
- Citrus sinensis (sweet orange) on Poncirus trifoliata
(trifoliate orange) rootstock. Between 1 and 2 years after graft inoculation
the plants show classical symptoms of bark shelling and scaling of the
rootstock and a decrease in tree vigour leading to stunting and yield reduction.
- Citrus medica (Etrog citron). Leaf epinasty and rugosity
cracking and browning of the underside of the veins and severe stunting visible
from 3 weeks to 6 months after graft or razor slash inoculation. There are
preferred sensitive clones such as Arizona 861 or USDCS 60-13
(Calavan et al., 1964).
- A wide range of citrus species and varieties respond but differ in the time
of appearance and severity of symptoms.
- Gynura aurantiaca (velvet plant). Principal herbaceous indicator
The symptoms are similar to those in Etrog citron but are visible from 10 to
30 days after inoculation with sap
(Weathers & Greer, 1968).
- Lycopersicon esculentum (tomato) cv. Rutgers. Same symptons as Etrog
citron; visible from 10 to 30 days after inoculation with sap
(Semancik & Weathers, 1972c).
- Gynura aurantiaca or Lycopersicon esculentum. Citrus medica
(Etrog citron) is a good host for some field isolates that cannot be transmitted
to herbaceous test plants.
- Citrus medica (Etrog citron) and Gynura aurantiaca.
Differences among field isolates in the severity of symptoms induced,
incubation period and degree of stunting were attributed to the existence of
(Fraser & Leavitt, 1959
Calavan & Weathers, 1961
Weathers & Calavan, 1961
It has been suggested that the mild strains
occur in leaves
and cause stunting whereas the severe strains
cause bark shelling
(Vogel, Bové & Bové, 1965
exists for cross protection between mild and severe strains (E. C. Calavan,
unpublished data; J. S. Semancik & L. G. Weathers, unpublished data).
Transmission by Vectors
No evidence for transmission by a vector.
Transmission through Seed
Tests for seed transmission in citrus have been negative
(Bitters, Brusca & Dukeshire, 1954
except for a single report
(Salibe & Moreira, 1965
which might be reinterpreted as natural spread. The viroid can be transmitted
through the seed of Lycopersicon esculentum
cv. Rutgers (J. S. Semancik,
Transmission by Dodder
Transmission from citrus to citrus and from citrus to petunia was
accomplished by means of Cuscuta subinclusa
No positive application of serological techniques has been reported. The
naked-RNA structures of the pathogen would not be expected to be immunogenic,
and as no viroid-specified proteins have been detected
(Hall et al., 1974
any serological reaction could only result from the detection of an increase in
a host-specified protein
(Conejero & Semancik, 1977
Strains in citrus have been recognized on the basis of severity of symptoms
on various rootstock/scion combinations. No physical description of these
isolates nor any evidence for cross protection in citrus has been reported.
Symptoms in tomato and potato
(Semancik, Magnuson & Weathers, 1973
identical to those caused by
potato spindle tuber viroid
The nucleotide sequences
of the two viroids are nevertheless distinct
(Dickson, Prensky & Robertson, 1975
Gross, Domdey & Sänger, 1977
Cross protection, defined as
an interference in symptom expression, was reported
(Niblett et al., 1978
when the two viroids were inoculated to tomato. However, when buds from
citron with the severe strain of citrus exocortis viroid were grafted to citron
containing the moderate strain, the severe strain became predominant, indicating
no cross protection (J. S. Semancik & L. G. Weathers, unpublished data). The
similarity in physical properties and in symptoms (stunting and epinasty) of all
viroids suggests that there are close biological relationships among these small
pathogenic RNA species.
Stability in Sap
The thermal inactivation point (10 min) is 90-100°C in sap of G.
and even higher in partially purified preparations of viroid
RNA. The agent remains infective on a contaminated cutting blade for at least
The agent can be recovered for biological tests or physical characterization
by employing techniques for purification of nucleic acids. Purified preparations
are highly infective. The commonest procedure is to blend infected leaves in a
two-phase phenol-buffer emulsion and precipitate the RNA from the aqueous phase
with 75% ethanol. The material soluble in 2 M LiCl is then subjected to
polyacrylamide gel electrophoresis
(Semancik & Weathers, 1972b
Semancik et al., 1975
Properties of Infective Nucleic Acid
The infective agent is a small RNA of about 1.1 to 1.2 x 105
(Semancik & Weathers, 1970
Semancik et al., 1975
Sänger et al., 1976
The molecule is single-stranded even
though it elutes from methylated albumin and CF-11 cellulose in a manner
characteristic of double-stranded RNA. Partial phosphodiesterase resistance
(Semancik & Weathers, 1970
suggests that the RNA has a circular structure
similar to those of other viroids. Nucleotide composition of G 28.8: A 21.5:
C 29.4: U 19.9 along with a Tm
= 52°C and 27%
hyperchromicity in 0.15 M NaCl + 0.015 M Na citrate indicates a GC-rich
molecule with a high degree of GC base pairing.
Low field nuclear magnetic resonance spectrum, solubility in 2 M LiCl, as well
as resistance to inactivation by diethylpyrocarbonate further suggest a highly
structured self-complementing molecule. The buoyant density in
Cs2SO4 is about 1.64 to 1.68 g/cm3 and
s20,w = 6.7.
Relations with Cells and Tissues
The agent can be isolated from all plant parts including roots and fruit
(Semancik, Grill & Civerolo, 1978
Regions of meristematic activity such
as shoot apices or neoplastic tissue contain a high concentration of the viroid
RNA. Symptoms suggesting vascular disorders may indicate association with
vascular tissues. The subcellular distribution of the agent indicates that it is
localized in the nucleus and endomembrane complex
(Semancik et al., 1976
The only subcellular cytopathic change observed is an increase in plasmalemmasomes,
or membraneous endocytic vesicles in the paramural area
(Semancik & Vanderwoude, 1976
- Benton, Bowman, Fraser & Kebby, Agric. Gaz. N. S. W. 60: 31, 1949.
- Benton, Bowman, Fraser & Kebby, Sci. Bull. Dep. Agric. N. S. W. 70: 20 pp., 1950.
- Bitters, in The Citrus Industry, Vol. V, eds. W. Reuther, E. C. Calavan & G. E. Carmen, Berkeley: Univ. Calif. Div. Agric. Sci., 1981.
- Bitters, Brusca & Dukeshire, Citrus Leaves 34: 8, 1954.
- Calavan & Weathers, Proc. 2nd Conf. int. Org. Citrus Virologists, 26, 1961.
- Calavan, Frolich, Carpenter, Roistacher & Christiansen, Phytopathology 54: 1359, 1964.
- Conejero & Semancik, Virology 77: 221, 1977.
- Dickson, Prensky & Robertson, Virology 68: 309, 1975.
- Fawcett & Klotz, Citrus Leaves 28: 8, 1948.
- Fraser & Leavitt, in Citrus Virus Diseases, ed. J. M.Wallace, 129, Berkeley: Univ. Calif. Press, 1959.
- Garnsey, Citrus Ind. 49: 13, 1968.
- Garnsey & Jones, Pl. Dis. Reptr 51: 410, 1967.
- Gross, Domdey & Sänger, Nucleic Acids Res. 4: 2021, 1977.
- Hall, Weprich, Davies, Weathers & Semancik, Virology 61: 486, 1974.
- Niblett, Dickson, Fernow, Horst & Zaitlin, Virology 91: 198, 1978.
- Olson, Proc. Rio Grande Vall. hort. Inst. 6: 28, 1952.
- Pujol, Proc. 3rd Conf. int. Org. Citrus Virologists, 128, 1965.
- Rossetti, Proc. 2nd Conf. int. Org. Citrus Virologists, 43, 1961.
- Salibe & Moreira, Proc. 3rd Conf. int. Org. Citrus Virologists, 139, 1965.
- Sänger, Klotz, Riesner, Gross & Kleinschmidt, Proc. natn. Acad. Sci. U.S.A. 73: 3852, 1976.
- Semancik & Vanderwoude, Virology 69: 719, 1976.
- Semancik & Weathers, Phytopathology 60: 732, 1970.
- Semancik & Weathers, Nature New Biol. 237: 242, 1972a.
- Semancik & Weathers, Virology 47: 456, 1972b.
- Semancik & Weathers, Virology 49: 622, 1972c.
- Semancik, Magnuson & Weathers, Virology 52: 314, 1973.
- Semancik, Morris, Weathers, Rodorf & Kearns, Virology 63: 160, 1975.
- Semancik, Tsuruda, Zaner, Geelen & Weathers, Virology 69: 669, 1976.
- Semancik, Grill & Civerolo, Phytopathology 68: 1288, 1978.
- Vogel, Bové & Bové, Proc. 3rd Conf. int. Org. Citrus Virologists, 134, 1965.
- Wallace, in The Citrus Industry, Vol. IV, eds. W. Reuther, E. C. Calavan & G. E. Carmen, 67, Berkeley: Univ. Calif. Div. Agric. Sci., 1978.
- Weathers, Phytopathology 55: 1081, 1965a.
- Weathers, Pl. Dis. Reptr 49: 189, 1965b.
- Weathers & Calavan, Phytopathology 51: 262, 1961.
- Weathers & Greer, Phytopathology 58: 1071, 1968.
- Weathers, Greer & Harjung, Pl. Dis. Reptr 51: 868, 1967.
The author wishes to recognize the valuable suggestions and consultations of
Professors W. Bitters, E. C. Calavan and L. G. Weathers in the preparation of
Bark scaling symptoms of exocortis disease on Morton citrange rootstock.
(Courtesy of L. G. Weathers.)
Foliar symptoms on Etrog citron caused by severe isolate (left)
and moderate isolate (right) about 6 weeks post inoculation.
(Left) Stunting and severe epinasty of Etrog citron,
(right) healthy plant. (Courtesy of L. G. Weathers.)
Field symptoms. The tree on the left shows general stunting and reduced
vigour 8 years after infection with the exocortis disease agent. The tree on the
right is not infected. (Courtesy of D. J. Gumpf.)
Electrophoresis (from left to right) on 5% polyacrylamide slab
gel of RNA extracts from tomato, (a) healthy, (b) infected with citrus exocortis
viroid, (c) as (b) but treated with DNase. Stained with ethidium bromide.
Gynura aurantiaca, (left) 8 weeks post inoculation with a severe
isolate, (right) uninoculated control.