CITRUS VIROIDS Distinct Disease Symptoms in Poncirus Trifoliata Induced by Three Citrus Viroids from Three Specific Groups

Distinct Disease Symptoms in Poncirus trifoliata Induced by Three Citrus Viroids from Three Specific Groups

C. N. Roistacher, J. A. Bash and J. S. Semancik

ABSTRACT. Observations on a Valencia/trifoliate planting inoculated with pure isolates of citrus viroids (CVds) showed the following specific reactions: 1) "Trifoliate deep pit" a symptom caused by citrus viroid Ia which induced deep pits in the wood of the trifoliate trunk with corresponding pointed pegs in the bark. 2) "Trifoliate mild bark cracking" a symptom caused by citrus viroid IIa which induced mild to moderate bark crackingin the trifoliate trunk. 3) "Trifoliate finger imprint9'a symptom caused by citrus viroid IIIb which induced grooving in the trifoliate trunk resembling a squeezing or strangling of the trunk leaving indented imprints with horizontal striations. These three distinct symptoms in Poncirus t~ifoliata,associated with representative members of the citrus viroid groups I, I1 and I11 are described and illustrated. In addition, viroids Ia and IIIb caused significant reduction of thetrunkcross sectioninValenciascionand trifoliate rootstock, andthecanopy volume ofinfected trees.

The association of severe bark shell- Subsequently, all the isolates in- ing on trifoliate orange rootstock with ducing a reaction in citron were ana- the citrus exocortis viroid (CEVd), and lyzed by sequential polyacrylamide gel the severe epinasty induced by this electrophoresis (sPAGE) and the spe- viroid in citron is well documented. cific citrus viroid content established. Beginning in the 1960's a collection of This paper presents the relationship of isolates, presumed to be mild CEVd specific citrus viroids, representative variants, was initiated in the Citrus of Groups I, I1 and I11 (3) to distinct Clonal Protection Program (CCPP) symptoms induced in the trifoliate virus bank at the University of Califor- orange rootstock and to effects on the nia, Riverside. Various mild to moder- Valencia scion. Analysis for the viroid ate symptoms on citron indicator plants profile in the scion and also in the root- were interpreted as the reaction of stock was made to confirm the presence CEVd isolates. A range of symptoms of the specific viroids. in citron from a distinct twisting of leaves, petiole wrinkle, petiole brown- MATERIALS AND METHODS ing, leaf tip browning, midrib browning with mild leaf epinasty or bending, In March, 1983, Campbell Valencia cracking or browning of the leaf midrib orange as the scion was budded on Ru- were described and illustrated (9). bidoux trifoliate seedlings. The scion The association of these mild symp- source was a virus-tested candidate toms induced in citron to specific symp- (V.I. 176) free of all known graft- toms in trifoliate orange was never transmissible pathogens. At the time of tested to verify the presence of the scion budding, groups of six trifoliate "exocortis" disease. A test of some of seedlings were graft-inoculated with these presumptive mild to moderate two blind buds from each of four "exocortis" isolates for reaction to tri- sources which induced a mild to moder- foliate orange rootstock was initiated ate reactions in citron. in 1983. At that time, the identification Trees were grown for one year at and cataloging of the citrus viroids had the Rubidoux greenhouse in Riverside not as yet been reported (2, 3, 4). and in April, 1984, were transferred to 174 Twelfth ZOCV Conference the field at the Lindcove Field Station RESULTS in Central California. Temperatures at Lindcove are warm during the summer Association of symptom expres- months and are excellent for produc- sion in citron with citrus viroids. tion of exocortis and cachexia symp- Symptoms in citron and the viroids toms. Scions and rootstocks were ex- found by nucleic acid sPAGE analysis amined and measured in July 1992. are shown in Table 1. The citrus viroids Trunks were calipered 15 cm above and found in each of the four treatments 5 cm below the bud union. Height and were uniform for both the Valencia width measurements were used for es- scion and the trifoliate rootstock. The timation of tree volume as a cylinder. viroids included in treatments were Statistic analysis was by Duncan's mul- CVd-Ia, CVd-IIa, CVd-IIIb and amix- tiple range test, using the SAS sys- ture of CVds-Ib, -1Iaand -1IIb (Fig. 3). tem. Tree size and growth. Fig. 1shows To determine the viroid content of the relationship of the presence of vari- the scion, budwood was collected from ous CVds to the trunk section of the the trees in September 1989, and again trifoliate rootstock and Valencia scion in August 1991. Buds were graft-inocu- and to the canopy volume of the tree. lated to 861-S1 Etrog citron on rough The presence of CVd-IIIb and mixtures lemon rootstock. Approximately 3-4 of CVds-Ib, -1Ia and -1IIb significantly months after inoculation, the young reduced both the trunk section of the tips of the citron were collected for scion and rootstock (Fig. la), and re- nucleic acid sPAGE analysis (8,12). To duced the canopy volume (Fig. lb). verify the viroid content of the root- The presence of CVd-IIIb alone stock as identical to the scions, bark caused a 47% reduction of scion cross samples were collected in May, 1992 sectional area and a 50% reduction in and graft-inoculated to citronlrough tree volume. The mixed infection of lemon for subsequent nucleic acid CVds-Ib, -1Ia and -1IIb induced a simi- sPAGE analysis of the citron. lar reduction of tree volume and scion

TABLE 1 SYMPTOMS OBSERVED ON 861-S1 ETROG CITRON AND IN THE TRUNKS OF THE TRIFOLIATE ROOTSTOCKS IN ASSOCIATION WITH CITRUS VIROIDS. -- Citrus Symptoms on Trifoliate Viroids Symptoms on Citron Rootstock - Ia Foliage: Mild and variable bent- Deep pits withcorresponding leaf induced by point necrosis. pegsin the bark. Size: Little tonoreduction. (Fig. 2a).

IIa Foliage: Leaftwist, verymildtip Mild bark cracks. browning, petiole wrinkle and petiole browning. Size: Noreduction. (Fig. 2b).

IIIb Foliage: Tip browning, petiole Finger imprint and bark wrinkle and browningplus leaf striations. twist. Leaf droopandbent leaf inducedby point necrosesofthe midvein. Size: Little tonoreduction. (Fig. 2c)

Ib Foliage: Moderate to strongleaf Finger imprint and bark IIa epinasty, petiole browningand striations plusmild barkcracks IIIb stemlesions. Size: Significantreduction. (Fig. 2d). Citrus Viroids 175

1 Fig. la

I Rootstock SC~O?~

I i Fig. 1b I

Fig. 1. The relationship of presence of various citrus viroids to growth of Valencia orange trees grafted on Poncirus trifoliata. la) Effect of viroids on the cross sectional area of the trifoliate rootstock and the Valencia scion. lb) Effect of the viroids on canopy volume. Different letters at the top of bars with the same marking indicate significant differences at P 5 0.05 (Fig. lb) or P 5 0.01 (Fig. la), based on Duncan's multiple range test. Twelfth ZOCV Conference and rootstock sections. Although pres- ogy defined by molecular hybridization ence of CVd-Ia significantly reduced studies. They observed an association the trunk section of the scion by 32%, of the various groupings to specific the reduction of the rootstock section reactions in citron as well as alternate was not significant. However, as shown hosts. When viroids from different in Fig. lb the presence of CVd-la re- groups were mixed or found in combi- duced tree volume by 33%. Presence nation, the effect on citron could be as of CVd-IIa had no significant effect on severe as that of CEVd (11). tree volume and cross sectional area Data presented here show a signif- relative to the control. icant stunting reaction induced by the Symptoms in the trifoliate individual citrus viroids Ia and IIIb, rootstock. Three distinct symptoms and in the combination of CVds-Ib, -1Ia appeared in the trifoliate rootstock as- and -1IIb (Fig. 1).The stunting of scions sociated with specific citrus viroids. associated with presence of these vi- Deep pitting of the trifoliate rootstock roids was shown in previous studies was consistently observed in the pres- and reports (6, 10). ence of CVd-Ia (Fig. 2a); typical mild The results also indicate that each to moderate bark cracks appeared on of these isolates (CVd-Ia, CVd-IIa and all trees inoculated with sources con- CVd-IIIb) respectively induced spe- taining only CVd-IIa as shown in Fig. cific symptoms in trifoliate orange as 3b, and symptoms of grooves or finger shown in Fig. 2. When two of the three imprints resembling a squeezing or viroids (CVd-IIa, and CVd-IIIb) were strangling of the trunk appeared on all present together, both the symptoms trifoliate trunks inoculated with CVd- of bark cracking associated with CVd- IIIb (Fig. 2c). Associated with these IIa and the finger imprint with stria- finger-imprint impressions were light tions associated with CVd-IIIb were horizontal lined striations also shown evident (Fig. 2d). in Fig. 2c. Symptoms of finger imprint We do not know the effects of CVd- with striations was noted when CVd- Ib on the growth of the scion and on IIIb was inoculated in a mixture with rootstock. The absence of deep pits in CVd-Ib and CVd-IIa (Fig. 2d). There trees inoculated with the mixture of was no pitting or bark cracking in the CVds-Ia, -1Ia and -1IIb inoculum sug- trunks containing only CVd-IIIb. How- gests that either CVd-Ib does not in- ever, there was amild to moderate bark duce this reaction or the symptom is cracking on trunks which contained the suppressed in the mixed infection. Al- mixture of CVds-Ib, -1Iaand-IIIb, but though CVds-Ia and Ib are related vir- no deep pits were observed. The exter- oids, Pires Leitao et al. (7) have re- nal cracks induced by inoculation with cently shown that CVd-Ia induces a the viroidmixtures were similar to those pathogenesis-related protein in citron observed with inoculation of pure CVd- of approximately 14 kd which is not IIa. present in citrons infected with CVd- All non-inoculated control trees Ib. This would suggest a distinct host- showed no symptoms of pitting, crack- viroid relationship for both viroids. ing or finger imprints and all trees ap- We have presented here three new peared fairly vigorous and generally symptoms in trifoliate orange associated larger than those treated with viroids with specific viroids. The trifoliate (Fig. 1). deep pit resembles the pittingof crista- cortis, however P. trifoliata is a toler- DISCUSSION ant host for this virus and budwood used in this experiment was virus- Duran-Vila et al. (2,3) classified the tested and virus-free. The trifoliate citrus viroids into five groups (CEVd, finger imprint resembles somewhat CVd-I, CVd-11, CVd-I11 and CVd-IV) the 'finger mark' symptoms on limbs based on electrophoretic mobility of of sweet orange, lemon, grapefruit and their nucleic acids and sequence homol- mandarin reported from Italy, Peru, Citrus Viroids

Fig. 2. Symptoms on trifoliate rootstock associated with specific citrus viroids. A) Deep pits in thewood ;md pegs in bark associated with CVd-Ia. B) Mild ciacking in the bark associated with CVd-IIa. C), Finner- imorint with striations in the bark associated with CVd-IIIb. D) Finner- imorint- and mild bark cracks bssociated with a mixture of CVds-Ib, CVd-IIa and CVd-IIIb. Twelfth IOCV Conference

Fig. 3. sPAGE of nucleic acid extracts obtained from citron plants following inoculation from Valencia scion (lanes 1, 2,4, and 5) or trifoliate rootstock (lane 3) containing CVd-IIIb (lane I), CVd-IIa (lanes 2 and 3), CVd-Ia (lane 4) or a mixture of CVds-Ib, -IIa, and -1Ilb (lane 5). Gels were stained with silver nitrate. and Argentina (5),which some authors the viroid. Bark cracks induced by have related to the concave gum virus. CVd-IIa resemble symptoms reported This 'finger mark' has never been trans- on trifoliate rootstocks grafted with mitted and no specific pathogen impli- the Atwood navel orange scion (1, 9). cated. The symptoms do resemble the The Atwood navel was subsequently effect of a tight wrap on young trees. confirmed to contain CVd-IIa (3, 4). However, wraps were removed very early in the experiment and there was ACKNOWLEDGEMENT no similar symptom present on any other tree in the experiment except We wish to thank C. K. Huszar, those inoculated with CVd-IIIb, thus Department of Statistics, University indicating that this finger imprint reac- of California, Riverside, for his invalu- tion was a response to the presence of able aid in statistical analysis.

LITERATURE CITED

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