Virusand -LikeDiseases of Pome andStone Fruits

Editedby

Ahmed Hadidi U.S.Department of Agriculture AgriculturalResearch Service Beltsville,Maryland 20705, USA

MarinaBarba AgriculturalResearch Council PlantPathology Research Center 00156Rome, ltaly

Thierry Candresse NationalInstitutefor AgriculturalResearch INRAResearch Center 33883Villenaved'Ornon Cedex, France

WilhelmJelkmann JuliusKuehn Institute, Federal Research Center for CultivatedPlants Institutefor PlantProtection in FruitCrops and Viticulture 69221Dossenheim, Cermany

f-\Vry HT'E The AmericanPhytopathological Society St.Paul. Minnesota Coverimages courtesy M. Barba,T. Candresse,J. C. Desvignes,J. Dunez, P.Gentit, A. Hadidi,T. lto, D. James,W. Jelkmann,G. Jesperson,E. Seemüller, and J. K. Uyemoto

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The AmericanPhytopathological Society 3340Pilot Knob Road St. Paul,Minnesota 55121, U.S.A. CHAPTER24

Cherry leaf roll virus

C. Büttner, S. von Bargen,M. Bandte,and A. Myrta

lntroduction have not yet been determined.The sizesof RNA-I and RNA-2, Cherry leaf roll virus (CLRV) was reported for the first estimated by denaturing electrophoresis of viral RNA prepa- il time in 1933 in English walnut ( L., Schusterand rations,ranged between 1.02-8.2 (RNA-l) and 6.33-6.8 (RNA- Miller, 1933)and sweetcherry ( L.; Posnetteand 2) kilobases(Murant et al., 1981,Pallas et al., 1991).CLRV Cropley, 1955).Since then numerous hosts have been recorded belongs to the subgroup C of .Species within this revealing its wide natural host range which includes 17 genera cluster are characterized by a large RNA-2 with a 3' NCR of woody plants and a variety of herbaceousplants. Some of the which is identical or nearly identical to that of RNA-I (Scott most reported and common natural hosts of CLRV are com- et al., 1992; Borja et al., 1995). The 3' NCR region is among nron birch species (Betula sp.), black elderberry ( the longest known for nepovirusesas estimated for six CLRV nigra L.), English walnut, and sweetcherry. The virus has been isolatesexhibiting lengths between 1,557and 1,602nucleotides detected worldwide, for example, throughout Europe, the for- (Langer et al., 2010). Although no evidenceof transmissionby mer USSR, North America, Chile, New Zealand, and Japan nematodes or other animal vectors has been found to date, ge- (Jones,1986). nome organization of CLRV seems to be in accordance with CLRV belongs to the genus within the family that of Tobacco ringspot uirzs (TRSV), the type speciesof the (Wellink et aI.,2000). Unlike the majority of other nepoviruses. The CLRV genome codes for a single coat protein membersof this genus,CLRV is not consideredto be transmit- (CP), the coding sequenceof which is located 3' proximal of ted by nematodes.However, reliable investigations on nematode the putative movement protein sequence on genomic RNA-2. transmissionare still lacking. CLRV belongsto the subgroupC Coat protein sequencesfor one birch isolate and for four walnut of the nepoviruses, which are characterized by a large RNA-2 isolates of CLRV respectively,have been published (Scott et with a long (1.2-1.6kb) 3' non-codingregion (3'NCR), which al., 1993, Zhou et al., 1998). Comparison of this CP coding is almost identical to that of RNA-I (Borja et al., 1995). The region with additional sequencesobtained from seven CLRV bipartite, single-stranded,positive-sense RNA genome is es- isolates varied between 1,539 and I,542 nucleotides in length timated to be about 15 kb, with RNA-1 and RNA-2 at about (Langer et al.200l ), but substantiated the serological relation- 8 and 7 kb, respectively.Both RNAs are encapsidatedsepa- ships found by Rebenstorfet al. (2006). Additionally, a719 bp rately in isometric particles 28 nm in diameter (Jones, 1986). fragment of the coding region of viral RNA-1 for a virus isolate Rebenstorfet al. (2006) assessedthe serologicaland molecular originating from ash ( excelsior) has been sequenced diversity of CLRV using a collection of isolatesand samplesre- by Maliogka et al. (2004). Phylogenetic comparison of the de- covered fiom woody and herbaceoushost plants from different rived amino acid sequence,corresponding to part of the viral geographical origins. Serological and molecular phylogenetic RNA-dependent RNA polymerase (RdRP) with other taxa of reconstructionswere strongly correlated. Remarkably, the di- the picorna-like plant , supported the taxonomic clas- versity of CLRV is defined, to a large extent, by the host plant sification of CLRV within the genus Nepovirus. from which the viral sampleswere originally obtained. Economiclmpact and DiseaseSymptoms TaxonomicPosition and NucleotideSequence As CLRVis transmissiblebyseed, it is a threatto genebank Family: Secoviridae,' genus: Nepovirus; species: Chercy contamination. Such ClRV-contaminated propagative mate- leaf roll vlrus (CLRV). CLRV is an establishedspecies within rial is of major importance for human-mediated propagation the genus Nepovirus that belongs to the family Secoviridae. and dispersal. Latent virus contamination of mother planta- In accordance with the currently acknowledged criteria for tions also has to be taken into consideration. Therefore, CLRV classificationwithin this family, CLRV consistsof two single- is included in the list of plant viruses that should be closely stranded RNAs, encapsidatedseparately in icosahedral, non- monitored during sanitary production of propagationmaterial, envelopedparticles, measuring 28 nm in diameter (Wellink et especiallyfor walnut and trees(Bassi and Martelli, 2003). al., 2000). Both particles are required for infectivity (Jonesand Kegler et al. (1972) reported on crop losses in sour cherry Duncan, 1980). Genomic RNAs, each coding for a polyproG (Prunus cerasus L.) of 9I to 98Va.In Italy approximately 5Vo ein, have a genome encoded protein (VPg) covalently linked of the tested olive trees grown in areas in which national and at their 5'end and are polyadenylatedat the 3' terminus (Jones local olive tree cultivars and selectionsare grown were CLRV and Mayo, 1972; Walkey et al., 1973: Hellen and Cooper, infected. The percentage of infection by CLRV in olive in 1987).The full length sequencesof the CLRV genomic RNAs Italy was similar to that in Spain (Faggioli et al., 2005). CLRV

ll9 120 j Chapter 24

infection correlates with the death of grafted English walnut propagation whereas only 20vo of the cuttings froni scions (Juglans regia) propagated 11{e_rmis1 on rootstocks of Northern clRV-infected trees survived. The annual increment of-CLRv- California black walnut (J. hindsii), ,paradox, hybrids U. hind_ infected seedlingscultivated under greenhouseconditions was sii x J. regia), Chinese wingnut (pteriocarya stenoprtera)or half that of healthy birch seedlinss. other Juglans spp. (Mircetich and Rowhani, r9g4; Iridmeth et Referring to the production ofwood as raw material and to al., 1990; Grant and McGranahan, 2005). The infection re_ the ecological value of forests,the extent of lossesdr.re to virus mains symptomlessin seedlingsof many cultivars of J. regia. infection is difficult to calculate as there is a strong impact of However, if the virus invades the graft union of suitable giaft other stressfactors in forest stands in a long periöd between combinations, it induces tissue necrosis at the scion-rootstock sowing in nurseriesand wood harvesting.In any case,it is sug- junction, known as blackline or brownline, which eventually gested to prevent virus dispersal by using clean planting ma_ girdles and kills the tree (Mircetich et al., l9g0; Rowhani and terial for the production of wood and to sustain the function Mircetich, 1988). The potential monetary value of developing of forest as recreational, cultural forest parks and suburban effective measuresof cLRV in walnut ii indicated by annual forests. losses due to blackline diseaseof 3vo of the total Cälifornia CLRV symptoms vary according to plant species,virus crop, as well as a l3vo loss in coastal orchards in this state strain. and season,and have been summarized.by Bandte ancl (Brooks and Bruening, 1995). Büttner (2001). For instance,CLRV-infected birc-h,elderberry. A rapid decline over a one or two year period was described dogwood, and blackberry show yellow vein netting, chlorotic by Nömeth (1986) when clRV-infected cherry trees were mix- ringspots, mottling, and leaf roll (Fig. 24.1). Diebick is often infected with Prunus necrotic ringspot virzs (pNRSV) and observed in clRV-infected sweet cherry, birch, and black- Prune dwarf viras (PDV). Furthermore, germination the rate berry. Susceptiblewalnut leavesmay deveiop chlorosis,discol- of seedsobtained from those trees was reducä bv 20 to 50vo. ored rings, or arabesks.Blackline diseaseaffects only English when compared with seeds from healthy (LO*, trees lggr. walnuts grafted onto non-regia rootstocks, and causestermi- According to cooper and Massalski (19g4), seedlings and nal shoot dieback in some cases.chlorotic leaf mottling and cuttings from naturally infected birch trees grow less iapidly spotting is associatedwith CLRV infection in European than thejr healthy counterparts. They arso reported (Fagus that 35vo of sylvativca L.) whereas chlorotic spots, ringspots, and the hardwood cuttings from hearthy trees bicame established line patterns are induced in European ash-(Fraxinis'excelsior

Fig' 24.1.Symptoms of Cherry teafrottvirus (CLRV) on leavesof: beech(A), birch (B and C) showing chlorosisand ringspots,respectivery, buckthorn (D), elderberry (E), ano oägwood (F). Cheruyleaf roll virus I lZt

^-.t.ClRV-infected (Rheunt rltaponticum L.), hoary Transmission .,llison(Berteroa incana L.), delphinium (Delphinium elatum Not all modes of CLRV transmissionhave been investigated . . t. and bitter dock (Rumex obtusiJoliusL.) remain symptom- and describedin the samedetail. Most studieshave been on the r's (Harris et al., 2002). seed and pollen transmissionof CLRV. The seed transmission Investigations over the last fbur years confirm the coun- rate of CLRV is highly variable and dependson whether male rr wide presence of virus-like symptoms in birch species in or female gametophytes originate from a ClRV-infected tree lrrnland (Büttner, unpublished datci). Leaves exhibited chlo- (Cooper. 1993). CLRV adheres loosely to the surface of an- :,rtic rirt.pots, leaf roll, and a loss of vi-eor.Initial testscon- emophilous pollen such as that of birch or walnut and strongly :rrrrredCLRV infection in 17 out of 20 samples taken in the to entomophilous pollen such as that of cherry (Massalski and Rtrvaniemi region close to the Arctic Circle (Jalkanen et al., Cooper, 1984). Up to 227o of seedscollected from open pol- lo07). It was shown that CLRV is widely distributed in B. pen- linated and naturally infected birch trees carried virus that tula and B. pubescens throughout the country. Furthermore, was transmitted to progeny seedlings (Cooper 1916). Pollen .i* arf birch, mountain birch, Kiilopää birch, and curly birch germination is presumably required for virus transmission. ',r ere confirmed to be previously unknown hostsof CLRV (von Furthermore, CLRV can be introduced into the embryo, mul- flargen et al., 2009). tiplied within the embryo, and distributed through seedsfrom infected birch trees (Cooper et al., 1984). In olive, CLRV was detectable rn 90Va of the seeds obtained from virus-infected Host Range trees. and the rate of seedling infection was 417c (Saponari et Table24.1 showsthe wide natural host rangeof CLRV which a1.,2002). rncludes17 generaof woody plants and a range of herbaceous CLRV is transmissibleby mechanical inoculation (Nienhaus hosts. et al., 1990) and by grafting (Jones, 1986). Schmelzer (1966) Natural herbaceoushosts include rhubarb (Rheum rhaponti- showed that a CLRV isolate from Sambucus racemosa was t umL., Tomlinsonand Walkey, 196l) garlic (Alliutn tuberosurtt pathogenicto 62 speciesin 24 families. The author found most Rottler ex. Spreng; Yamashita and Fukui, 2004), delphinium of the virus-susceptibleplants in the families Chenopodiaceae, 'Delphinum elatum; Ahmed and Bailiss, 1975),broad-leaved Compositae, and Solanaceae. Plants known to be susceptible Jock (Rumex obtusfolius L.), and hoary allison (Berteroa in- to many different plant virus species,such asAmaranthus cau- ,'una(L.) DC.;Jones,1985). datus, Datura stamonium, and Lvcopersictln esculentum, ,1979\ The experimental host range includes more than 36 plant were shown to be resistantto the virus. Horvath added tamilies (Jones,1985). 34 other plant speciessusceptible to the virus and 23 plants in eight families which seemedto be resistant. Rumbou et al. (2009) provided a model systemto study infec- tivity and seed transmissionof CLRV in Arabidopsis thaliona Table 24.1. Host range of CLRV. (L.) Heynh. Transmissibilityof CLRV by seedin A. thaliana Host range (species) Reference was shown by virus detection in two consecutivegenerations Birch (Betulu spp.) Schmelzer,1972a; Cooper and grown from seedsof infected plants. The results indicated that Atkinson,1975;Rebenstorf et al.. genetically diverse CLRV isolateshave different capability to 2006:Jalkanen et al.. 2007:von be verticallytransmitted in A. thaliana. Bargenet al.. 2009:Buchhop et al., Detailed information on transmissionby nematodesand in- 2009;Bandte et al., 2009 sectsis missing. Although nematodetransmission has been pos- Hombeam(Carpinus betultt.sL. Rebenstorfet al.. 2006 S tulated for CLRV due to its taxonomic statusin the Nepot,irus Dogwood (Cornu.s L.) Waterworthand Lawson. 1973 .florida (Wang Spindle(Eut tnt' ntu s eu ntp ue us) Larsenet al.. 1990 genus. it has not yet been confirmed et al., 2002). Beech(Faga s svlt'atir:a L.) Winter and Nienhaus1989 CLRV has been detected by RT-PCR in the seed-feedingbug Ash(Fruxinus excelsiorL.) Nienhausand Hamacher,1990, Ford et Kleidocerl,s resedae(Werner et a1.,1997). Potentially, insects al.. 1972 may thereforeoccasionally contribute to the infection of plants Walnut (.Juglansspp.) Cooperand Edwards. 1980: Mircetich through wounds via contaminatedpollen. 1980;De Zotenet 1982: et al., al.. Investigationsby Bandte et al. (2007) showed that CLRV is Rowhaniet al., 198-5;Buchhop et al.. 2009 easily transmitted by water. The results were obtained when Privet (Llgnstrwn t;ulgareL.) Schmelzer,1912b:' Bandte and Buttner, pots with CLRV-infected Chenopodium quinoa were grown 2001;Obermeier et al., 2003 in hydroculture. Virions were releasedfrom roots of infected Olive (Oleu euntpaeaL.) Savinoand Gallitelli, l98l plants and transmitted through the nutrient solution. Healthy Cheny {Prunus spp.) Posnetteand Cropley. l9-5-5, Cropley. plants were infected within three weeks. Under natural condi- 1960;Cropley, l961. Schimanskiet tions, this type of transmissionhas not been reported for CLRV al., 1975a;Schimanski et al., 1975b and, if it happens, it would probably be less efficient than in Hoptree(Ptel e a t riJbliutaL.) Schmelzer.1972b greenhouseexperiments, due to an abatedinfection pressure. Buckthorn(Rhamnus .frangula L.) Werneret al.. 1997 Blackberryand raspberry Cropley,l96l; Cropleyand Tornlinson, The recentdetection of CLRV in birch trees abovethe Arctic (Rubusspp.) l97l: Jonesand Murant. l97l: Jones Circle (latitude 66' 34' N) brings up new questionson the trans- and Wood. 1978 mission and fast spread.The contaminated pollen by melting (.Sambuc tilderbemy us spp.) Schmelzer,I 966; Schmelzer,197 2b: water has to be verified and further evaluated. Jonesand Murant, l97l , Buchhopet a|..2009 Mountain ash (Sorbr.rs Rebenstorfct al., 2006 aucupuria L.) GeographicalDistribution and Epidemiology Lilac (Syrlrrgavulg,ari,s L.) Novak andLanzova, 1975 CLRV occurs throughout Europe, North America, Chile, (Ulmu,same ri<:ana L.) Varneyand Moore. 19521Jones and the former USSR, China, Lebanon, Syria, Australia, and New Murant, 1971,Schmelzer. 1972b Zealand (Jones, 1985: Herrera and Madariaga, 2001; Fadel et vine (.VitisvinifbraL.\ Herreraand Madanaga,200 I Crape al.. 2005: Al Abdullah et al.. 2005; Jalkanenet al. 2007). By 122 | Chapter24 analyzingthe geneticdiversity of CLRV isolatesRebenstorf et requiresa sarnple.ize of at least80 g of male catkinsfiom eacl: al. (2006) reported a strongrelationship between CLRV genetic tree to balancethe unevenand erraticdistribution of the pathrr- diversity and the original host plant species.The geographical genwithin inf'ectedtrees (Rowhani et al.. 1985). origin of the isolateswas found to be of minor influenceon their Studiesb1'Jones et al. (1990)and Rebenstorfet al. (2006 phylogeneticaffinities. This grouping is explainableby either have demonstratedsi-enificant serological variation betwecr: host specializationof the CLRV isolatesor by the existenceof CLRV isolates.Depending on the serologicalassay and anti- ecologicaltransmission barriers which limit or altogetherpre- serum used.not all CLRV isolatesare detectableusing a gir,'err vent host change.Pollen transmissioncould potentially repre- antiserumin DAS-ELISA or relatedmethods. This is evident sentsuch a barrier,which could resultin rapid geneticisolation in the investigationsof Gentkow et al. (2007),who showedthat of viral variantswithin given host populationsand, over time, only ll out of l9 CLRV isolatescharacterized by Rebenstorl result in evolutionary divergenceof these separatevirus popu- et al. (2006) were detectableby a polyclonal antiserum raised lations.By comparing host plant origin and phylogeneticaffini- againstan elderberryCLRV isolate,whereas all isolateswerc ties of a rangeof isolates,these authors showed that the genetic detectedby molecularmethods. isolation of host-specificCLRV variants is partial and not com- The high sensitivity of molecular methods in comparison plete with some CLRV isolates appearing to have the ability with serologicaltechniques facilitates the detectionof viruse: to infect a broad range of potential hosts. Such isolates might in soil, water,vectors. mixed samples,and samplestaken dur- be transmitted with contaminatedpollen through wounds by ing early stagesof inf'ection.even with a small amount of sani- meansof insectvectors. Milder winter climatesdue to climate ple material or a low virus titer. To date, several polymerasc' changewill enablea longer period in which viruses may spread chain reaction (PCR)-basedprotocols have been described for and infect plants. the detectionof CLRV in woody hostsof economic importance. The distributionof CLRV is rnainly driven by pollen and by Reversetranscription PCR is applied either after extraction o1' human movementof infected seedsor plants.To date. CLRV, total RNAs from plant tissue(Fadel et al., 2005; Faggioli et al.. a quarantinepest in Rubus in the EPPO region, has only been 2005) or after direct binding of viral particles to reaction tubes found in a few plants in England. the Czech Republic. and (Rowhani et al., 1995).Nested PCR can also be used to in- Slovenia in these hosts (CABI/EPPO, 1997). In the case of creasesensitivity (Pantaleo et al.,2001). Furthermore, serologi- Rtrbus.crops.transmission by pollen from imported Rubu.s cal methodscan be combinedwith moleculartechniques, such to local plants or propagation of imported Rubus would be as in the Immunocapture-RT-PCR developedby Werner et al. the practical means of establishmentof CLRV in the EPPO (1991), which was further optin-rizedfor the screeningof large region. numbers of samplesby Gentkow et al. (2001) and Jalkanen et al. (2007). This protocol is suitablefor the detectionof the virus in leaves.buds, or fruit tissuesand can be successfullyapplied Detection to isolatesoriginating from walnut, birch, and black elderberry. CLRV can be detectedby biological assays,either in the It can also be used for all other characterizedCLRV isolates. form of mechanicaltransmission to herbaceousindicator plants including strains fiom sweet cherry and from herbaceous (suchas Chenopodiumquinoa, Cuc'umissativus, and Nicotiunct plants. According to phylogenetic clades, a method for differ- spp.),or in some casesto woody plants in their early physio- entiation of CLRV isolatesby a RFLP assaywas developedby logical stages(such as diversebirch species.sweet cherry, and examining restriction patterns of the partial 3' NCR (approx. blackberry) by graft transmission.Attention should be paid to 420 bp) genomic fragments. The method was successfullyap- the long latent period of severalmonths to a few years that may plied in an IC-RT-PCR-RFLP assay to differentiate samples be needed in some hosts for symptom development(Nienhaus from walnLrt.black elderberry, and birch and determine their and Castello.1989.1. For indexingof Prunus rnaterial,the peach geneticrelations. In the future, this method will facilitate quick seedling indicator GF305 shows rosetting and slight leaf roll- estimation of phylogenetic cluster of CLRV isolates detected ing after being chip-budded with CLRV-infected Pruntl.r spp. in certain host plants by the universal IC-RT-PCR (Gentkow, (Diekmann and Putter, 1996). 2010).It is also suitablefor studyof CLRV populationdiversity Electronmicroscopy has beenapplied fbr studiesof sizeand as well as investigationof geneticdrifl within virus populations structure of the particles, detection and identification of the (Buchhop et al., 2009). virus in infectedplants, and investigationson cellular changes Although more cumbersome and probably not as sensitive, causedby virus infection (Rubio Huertoset al.. 1985;Nilsson dorblot-hybridization(Borja and Ponz, 1992,Mas et al., 1993) and Tomenius,1987). Histological investigations are useful to has also been applied to CLRV detection, particularly when a understandalterations due to the CLRV-infection process and fast and simple test is desired.Simple detection tools are desir- in virus replication,but cannotbe appliedto virus diagnosis.A able to support breederswith fast and cheapmeans of detection number of diff'erenttechniques have been establishedto detect fbr selection of plant and seed material. Such a tool may be and identify virus particlesin suspensionsor in thin sections achievedby developinga lateral flow test. (Dijkstra and de Jager, 1998a,b). Lesemann (1982) reported that the detection by immunosorbent electron microscopy (ISEM) is in many casesas sensitiveas the enzyme-linked- Control immunosorbentassay (ELISA), which is limited to 0.1-10 ng/ CLRV seemsto be distributedmainly by movementof in- mL. The detection of CLRV in leaf tissue of woody plants by fectedplants and seeds,or by pollen. Pollentransmission gives this approachmay, however,be hamperedby a low virus titer. the virus the potential for rapid dissemination,making its The reliability of enzyme-linkedimmunosorbent assays de- control difficult. Water transmissionof CLRV in ecosystems pends on the homogenousvirus titer in the testedplant. For also has to be taken into considerationas a potential causeof instance,in cherry, the period of reliable serologicaltesting in spread. routine surveysis extendableby using the emergentflower and Regularly spacedpollenizer rows of walnut trees are recom- leaf tissue(Torrance, l98l). Therefore,dormant budstickscan mended by Polito et al. (2005) to supply uninfectedpollen at be cut in midwinter, stored at 4'C, and forced to sproutjust be- elevatedlevels, in order to out competepotentially infected pol- fore starting the assays.In walnut. reliable detection of CLRV len from outsidethe croppingsystem. This strategyis basedon Cherry leaf roll virus | 123 their investigations showing an influr of walnut pollen frorn Cooper.J. 1.,N{assalski, P. R., and Edwards,M. L. 1984.Cherry leaf sources outside the orchard. Therefbre. an inf'ection by CLRV- roll virus in the f'emalegametophyte and seed of birch and its rel- inducing blackline disease may occLlr even in orchards that evanceto vertical virus transmission.Ann. Appl. Biol.l05:55-64. are free of the virus if the virus is prevalent in neighboring Cropley,R. 1960.Serologicaldetection of a virus in cherry treeswith a leaf roll disease.Nature 188:857. orchards or in the surrounding environment. Cropley.R. 1961.Cherry leaf roll virus. Ann. Appl. Biol. 49:524-529. Early detection followed by seed eradication, sanitary selec- Cropley,R.. andTornlinson.J. A. I971.Cherry leafrollvirus. Commonw. tion, and the use of virus-free certified planting material is the Mycol. Inst.,Assoc. Appl. Biol. Descr.Plant Viruses 80:l-4. most important integrated strategy used to prevent the spread De Zoeten,G. A., Lauritis,J. A., and Mircetich, S. M. 1982.Cyto- of the disease. An example of a virus-free certification scheme pathology and propertiesof cherry leaf roll virus associatedwith was presented recently for olive trees and rootstocks by EPPO walnut blackline disease.Etiology 12:1262-1265. (EPPO. 2006). The scherne provides detailed -euidance on the Diekmann. N.. and Putter.C. A. J. 1996.StoneFruits. Page 20 in:FAO/ production of propagated cultivars to be grown on their own IPGRI Technical Guidelines for the Sat-eMovements of Germplasm roots, of vegetatively propagated or seedling rootstocks and of 16. grafted trees. Dijkstra. J.. and de Jager.C. P.. eds. 1998a.Serological techniques fbr detection and identilication of viruses and viral products. Pages 317-315in: Practical Plant Virology. Springer-Verlag,Heidelberg, REFERENCES Germanv. Ahmed, A. H., and Bailiss, K. 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