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The Effects of Virus and Viruslike Diseaseson Productionin Florida

S. M. Garnsey

Summary

Florida hasnot experiencedas much devastatinginjury from virus diseasesas many citrus-growingareas; however, tristeza, psorosis, exocortis, and xyloporosis have all been damaging. Tristela continues to be a major problem, but psorosis, xylopo- rosis, and exocortis are now controllable. Sewral other viruses have been described, but are not widely distributed. New dis- eases,including a stem-pitting disorder of 'Milam' (Citrus jambhiri Lush. hybrid?) and some hybrids (Citrus paradisi Macf. X Citrus reticulata Blanco), are appearing. Several viruses haw been discowred by inoculating herbaceous plants, but relationship of these viruses to specific diseaseshas not been established. Introduction of new citrus varieties, rapid shifts in rootstocks, and rapidly changing production practices will contribute to future problems. Vigorous researdt and regulatory programs will be needed in responseto existing and future problems.

Introduction

Strictlydefined, virus diseasesare causedby small, infectious nucleoproteins.Commonly, however, infectious citrus dis- easesnot obviouslycaused by fungi, bacteria,or nematodesare called "virus" diseases,although me identity of the causal agentis often unknown. This broaderdefinition will be retainedin this paperso that the necessarytopics can be covered. Emphasiswill be placedon diseaseeffects, not on propertiesof the causalagent. Somediseases that will be covered,haw beencalled virus diseases,but are now known to haveother causalagents. Examplesare exocortis, caused by a small, naked, infectious nucleicacid, pro~sionally calleda viroid or pathogene(41,42), and stubborn,which hasbeen associated with a mycoplasmalike organismmuch largerand more complex than a true virus (3, 11). Young tree decline(YTO) and sandhill decline(SH 0), which are very similar to the older and alsounsolved decline problem called blight (30),are cowred specific- ally by other authorsand will be treated only briefly in this paper.

Regardle~ of nomenclature,virus and viruslike diseaseshave caused serious problems in most citrus-producingregions of the world. Thesediseases often limit the production of certaintypes of citrus in someareas. In addition, "latent" virus in- fectionscan causewidespread but lessobvious losses (43,44).

For severalreasons, Florida hashad lessserious virus problemsthan most major citrus areas. In Florida, orangesand (C.paradisi) havebeen grown mostly on rough (C.jambhiri) and sour (C. aurantium L.) rootstocks. Exocortisand xyloporosis,although common in most mature plantings,do not causeobvious symptoms on thesetrees. Psor- osishas been under increasinglyeffective control for a number of yearsby budwoodregistration. Tristezavirus hasbecome widespread,but severeinjury to sour orange-rootedtrees has occurred only in localizedareas. Natural infection of grapefruit treesby tristeza hasbeen uncommon.

Despitepast good fortune, concernabout citrus virus diseasesin Florida is increasing.Tristeza has spread rapidly into existingplantings on the eastcoast of Florida in recentyears, and evidenceof tristezadecline is increasingin this area. In spite of tristeza damage,sour orangeis beingwidely usedin new plantings.

New virus or viruslike diseasesare appearing.Some, such as the new stem-pittingdisease in 'Milam' (a probably hybrid) and in tangerinehybrids suchas 'Robinson', 'Page',and 'Lee' (C. reticulata XC. paradisi) (49), threatento be largeproblems in the future.

Damageto rough lemon-rootedplantings in recentyears by YTD and SHD is causinga rapid shift to other rootstocks. The~ are largely untestedcommercially in Florida, and manyare susceptibleto exocortisand/or xyloporosis. New scion varieties,which may harbor unsuspectedsusceptibility to virus di~ases,are alsobeing propagated.

Production practicescontinue to changerapidly in Florida. Many of thesechanges probably will not affect the citrus virus diseasesituation; but the effects of herbicides, permanent irrigation, hedging and topping, abscission agents, and mechanical harvesting devices are undetermined and may affect virus spread and diseasesymptoms. -78-

This paper will review the variousvirus and viruslikediseases in Florida andwill discuss,where possible,the problemsthat lie ahead. Some discussion of the possible beneficial use of citrus viruses for cross protection and tree-size control will also be included.

MajorViruses

Tristeza. Tristeza remains a major threat to Florida citrus growers, in spite of its past failure to cause more widespread damageto sour orange-rooted trees. The presenceof tristeza in Florida was confirmed in 1951 (34), but it was un doubted- Iy introduced much earlier (20). Significant natural spread occurred first in central Florida, and then later in all major citrus areas in the State (5,34). It has just recently spread rapidly into existing plantings on the east coast and IMer interior (5). These plantings are largely on sour orange and were nearly free of tristeza until recently.

The mostefficient vector of tristeza, Toxopteracitr;c;dus Kirk., hasnot ooenreported in Florida; but the 3 aphid vectors present(35) can causeconsiderable natural spread when inoculum is plentiful and vector populationsare high.

The effect of tristeza on sour orange-rooted trees in Florida has been variable. Quick decline symptoms have been ob- served (Fig. 1), but a slower decline is more typical. Even more common are large numbers of tristeza-infected trees on sour orange that show no obvious signs of infection. The absence of symptoms is probably due to a predominance of mild strains of tristeza and mild climatic conditions.

Florida isolatesof tristezaoften producemild vein-clearingand stem-pittingsymptoms in 'Mexican' (C.aurantifolia [Christm.] SNing.) indicators, but some isolates causesevere symptoms. Some isolates, especially from 'Meyer' lemon (C. limon [t.] Burm. f. hybrid), can causeseedling yellows in 'Eureka' lemon (C. limon) (20). Tristeza stem1Jitting in grape- fruit or sweet orange has been rare. Natural infection of grapefruit trees has been uncommon, but does occur and may be increasing (5). Tristeza decline in mature grapefruit trees on sour orange has recently been observed in central Florida.

Fig.1. Tristezaquick decline symptoms in 'Valencia'tree on sourorange root. Treeis approximately20 yearsold. Phototaken an n3, at WinterGarden, Fla. .79.

Tristezawill continue to be a problem in Florida citrus for a long time. The useof sour orangerootstocks is increasing, partly becauseof its VTO tolerance. Somelocally severedamage seems certain to continue as it hasin the past. The ex- tent of future damageprobably dependson the proportion of nM treesthat becomeinfected with severeand mild strains and whether treesinfected with mild strainswill resistsubsequent inoculation by severestrains (33,47). It is probable that there is somenatural cross-protectionat work in Florida, althoughthis is difficult to prove. There is considerablein- terest in deliberateuse of cross-protection,and this approachis beingstudied here. Onedifficulty with cross-protection testsin Florida is lack of rapid meansto subjectplants infectedwith mild strainsto natural challengeby severestrains to determinethe "protection" given by the variousmild strains. While naturalspread is occurring,young plants in the field do not all becomerapidly infectedand do not necessarilyget challengedby severeisolates.

Some strainsof tristeza aretransmitted more readily than others in Israel(2). The relativetransmissibility of Florida strainsof tristeza hasnot beenstudied, but this factor could havean effect on the relativedistribution of different tris- tela strainsin new plantings.

The largenumbers of uninfectedgrapefruit trees on sour orangein Florida appearvulnerable to further and more rapid spreadof tristezaisolates capable of causingdecline. Our grapefruittrees, regardless of stock, are alsovulnerable to severe stem-pittingforms of tristezathat exist in many parts of the world. Pitting hasbeen induced in experimentallyinoculated plants (24) and could be a problem if pitting strains becomewidespread.

Reports of tristeza-related problems with (poncirus trifoliata [L.] Raf. X C. sinensis Osbeck) and (P. trifoliata) from other areas(8,36) suggestthat tristezawill be a continuing problmn in rootstock selection. However,no tristezaproblems have been reported to date on 'Carrizo' -or trifoliate orange-rootedtrees in Florida.

£xocortis. Citrusexocortis virus (CEV) is presentin most old-line Florida citrus trees. However,typical CEV symptoms, ar9 rare, becauseCEV-sensitiw rootstocks have been used sparingly until recently. The CEV isolatesfound in Florida vary widely in their effect on treesgrafted to trifoliate-orangerootstocks and on '' (C. medica t.) indicators. Many CEV isolatescause severe tree stunting and bark scalingon trifoliate-orangerootstocks. Thesealso cause severe symptoms in citron indicators. Other isolatescause little stunting or scalingin treeson trifoliate orange,but causedefinite, although mild, symptomsin citron indicators. Intermediateforms are alsofound.

There are indicationsfrom the budwood certification programindexing tests that symptomsof stunting and bark scaling in trifoliate orange-rooted trees are not always linked (G. D. Bridges, personal communication). Severescaling is not always accompanied by severestunting and, conversely, severestunting may occur with little scaling. This suggeststhe presence of a "stunting factor ," perhaps distinct from CEV. CEV sources that cause stunting but little scaling have been reported in Australia (6). Further experiments with mechanically transmitted sources of CEV free of other viruses should help clarify this situation.

The recent widespread use of 'Carrizo' citrange and other CEV-sensitive rootstocks in new plantings has prompted grea. ter concern about CEV. Fortunately, CEV-free selections of all important scion varieties, except 'Temple' orange (C. reticulatahybrid?) and 'Thompson' grapefruit, are available through the budwoodcertification program.

CEV can be transmitted mechanicallyas a contaminanton cutting tools and hands(22). This hazardmust be carefully avoidedin nurseryoperations, where contamination of budwood-sourcetrees or young nurserytrees can be disastrous. Contaminationcan be readily avoidedby useof sterilantssuch ~ dilute (5.10010)solutions of householdbleach (23,38).

There is a hazardof spreadingCEV in the groveduring hedgingand topping operations,but the potential for damageto maturegroves on CEV-sensitive rootstocks may not be too great. First, somescion varieties,such as grapefruit and tan- gelos,have been resistant to infection via contaminatedtools in experimentaltests (22). Secondly,infection of mature treesis unlikely to haveany sudden,drastic effect. There may be reduction in growth after severalyears, but this would not be harmful where treesare pruned periodically to limit size. Pruningand/or removalof root sproutsfrom young trees might be more hazardousbecause of stunting beforetrees reach the desiredsize.

Natural spread of CEV (1) and apparent seedtransmission (39) were reportedbefore the discoverythat CEV can be spread as a contaminant. It nlM appears that contamination is the causefor the natural infections of CEV observed. -80-

Futuredamage from CEV can be largely avoidedby usingclean budwoodand simplesanitation procedures. However, increaseduse of sensitivestocks, coupled with inattention to selectionof budwood andcontamination, may result in more CEV damagethan we havepreviously experienced.

A beneficialuse of citrus virusesto control tree sizehas been proposed (32), and this is beingtested with CEV (6,11, 13,26). While CEV infection usuallyreduces tree sizeand yield of treeson sensitivestocks, the effect on fruit quality may be beneficial. Reductionin yield per tree can be compensatedfor by increasingthe numberof treesper unit area,if the sizingpotential of the tree is known at time of planting. This approachhas been on trial for sometime in Australia (6,26), and experimentsare beingconducted in Florida. It appearsthat accurateand successfuluse of CEV for tree-sizecontrol will ultimately requireinformation on the stock-scioncombination used,the virus isolate,and the t~e ageat the time of inoculation. The beneficialuse of a virus suchas CEV is intriguing, but obviously,this requiresa full understandingof the virus and its effects.

Xyloporos;s (cachex;a). Severesymptoms of xyloporosisare encounteredinfrequently in Florida. Although a consid- erableacreage of 'Orlando' (C. ret;culata X C. paradis;) and other mandarinhybrids sug:eptibleto xyloporosis(30, 48) is grown in Florida, these trees usually have been propagated from xyloporosis-free budwood. A common cause for the xyloporosis damageobserved has been topworking symptomless varieties to a sug:eptible g:ion (48).

Xyloporosis symptoms have been seen on satsumamandarins (C. reticulata) in several instal.:es(30). Thesesymptoms developslowly, becomingapparent at 8 to 10 yearsafter budding. Xyloporosiscan apparentlycause mild pitting and gum. pocket formation in rough lemon and affect tree growth and yield (44).

Fovea,a diseasesimilar to xyloporosisin '~urcott' oranges(C. reticulate hybrid), hascaused some injury in Florida (31), but is generallyavoided by the useof clean budwood.

Sweetlime (C. aurantifolia), C. macrophylla, and '' lime (C. reticulata var.austera hybrid) rootstocks are being used more widely. These are susceptible to xyloporosis, and budwood sources free of xyloporosis and fovea should be used in propagation. Topworking with sensitive varieties should be avoided, unless the virus status of the trees is known. Several unexplained, apparently natural infections of xyloporosis have been observed (G. D. Bridges, personal communication), but extensive natural spread is not indicated now (9).

Psoro!i!. Psorosissymptoms are found in many older groves in Florida. Sometimes severebark-scaling symptoms are present, and the debilitating effect on the tree is obvious. More commonly, the pre~nce of the virus is marked only by scattered, ephemeral leaf patterns in the young, spring-flush leaves. The effect of the latter type of infection on tree con- dition or production is not obvious. Psorosis infection has been associatedwith a decrea~ in vitamin C content (44).

Psorosisis uncommonin youngerplantings, because one of the first dividendsfrom Florida's budwoodcertifi:ation programwas identification of many sourcesof major scionvarieties free from psorosis.

Psorosiscan be seed-transmittedin 'Carrizo' and 'Troyer' citranges(45), but this hazardhas been overcome by certi- fying rootstock seed-sourcetrees. No natural spread of psorosis has been documented in Florida, and future effects of this virus should be minor.

Other Viruses

Florida citrushas other virusesin addition to the 4 commonviruses already discussed. Generally, these have limited distribution or they affect varietiesof minor importance. Somecould causeserious damage if they should becomewidely distributed. Severalviruses have been discovered only recently,and their potential for damageto citrus hasnot been evaluated.

Tatterleaf-citrange stunt complex (TL-CSV). Plany'Meyer' in Florida are infectedwith a virus complex that causessymptoms in plants of citrangeand C. excelsa Westerand a disablingstock-scion incompatibility in orangetrees on citrangeor trifoliate orangestocks (18,20, 37,46). Theabove symptoms were observedin plants inoculatedexperiment- ally, andthe TL-CSVcomplex is knownto occurnaturally only in '~.eyer' lemon. It shouldnot becomea problemun- less a vector appears.'Nleyer' lemon sourcesfree of the TL-CSVcomplex (20,37,46) should be usedin new plantingsto limit the inoculum potential. .81.

Citrusvariegation virus (CVV). Citrusvariegation virus occursin Florida (25),but it is rare in the field. It is a hazard mostly to acid lemons,a relatively minor crop here. CVV causessevere variegation symptoms in 'Etrog' citron indicator plants (unpublished),and ~an be readily detectedin an indexingprogram using that indicator.

Citrus leaf rugose(CiR V). This virus, originally reportedas a crinkly-leaf-typevirus (19), is relatedto CVV, but has somedifferent properties. ClRV is limited in its presentdistribution, but hassome potential for damage.It causessevere stunting in young grapefruitplants under cool conditions,some stunting on lemonsand tangelosand a leaf distortion in 'Mexican'lime (19). Naturalspread in the field is indicated (21).

New Virus Problems

New stem-pitting disease. Recently, stem-pitting symptoms of unknlmn cause have been observed in severalcitrus varieties in Florida (49). These symptoms vary in severity from scattered, inconspicuous pits, visible only when the bark is removed, to severe pitting that deforms even the external appearanceof trunks of affected trees. The most severe symptoms have been observed on 'Milam' (Fig. 2), 'Robinson', and 'Page' trees. Symptoms are most severe on the trunk and lower limbs. Pits are rare in limbs lessthan 1 inch in diameter.

Pitting symptomswere first detectedin June 1972, ~d full information on host rangeand effectsis not yet available. Severelypitted 'Milam' treescontinue to grow vigorously,although these are still lessthan 10 yearsold, and no controlled comparisonswith healthy plantsare available. Pitting on 'Milam' is especiallysevere, and there may be somedebilitation eventually. However,pitting symptomsdo not extend belowgroundline, and there may be lesseffect on this variety asa rootstock. Natural spreadis indicated,but the meansis unknown. Pitting symptomshave been reproduced in greenhousepropa- gationsof pitted 'Milam' trees. 'Milam' plants inoculatedwith tissuefrom a pitted 'Robinson' tree haveshown some gum- impregnatedstreaks in the cambialregion, but distinct pits havenot formed yet. A mechanicallytransmissable virus was discoveredin 1 pitted 'Robinson' tree, but this viruswas not found in other pitted 'Robinson'trees or pitted treesof other varieties;and its causalrole in the pitting diseaseis questionable(unpublished). Some, but not all, pitted treesare infected with tristeza (49). While tristeza apparentlyis not responsiblefor the pitting observed,severe isolates of tristezacan cause pitting on 'Milam' andcomplicate diagnosis in that variety.

Fig.2. Stem pitting symptomsin trunk of 8-year-old'Milam' tree near HainesCity, Fla. -82.

A stem-pittingfactor that causessymptoms in 'MadamVinous' ~eet orangehas been reported recently (15). Its relationshipto the stem pitting in 'Milam' and the tangerinehybrids is unknown a: this time.

Miscellaneous. Recentiy, additional viruses have been found in Florida by use of herbaceous assay plants (unpublish- ed). Theseviruses are sap-transmitted,but their identity and propertiesare undeter~ijled. Two were found in citrus introductions from Algeria. One is apparentlylatent and hasnot causeddetectable symptoms in citrus indicators. The other causessymptoms similar to those describedfor citrus ring spot virus (46). Inoculatedplants showsevere shock symptomsinitially, but later recover.

A mechanicallytransmitted virus hasalso beenfound in a tree affectedwith a severebark-scaling form of psorosis,but its relationship to psorosis has not been determined.

AssociatedProblems

Stock/scionincompatibilities. Somevirus-induced incompatibility problemshave already been discussed. Virus or viruslike agentsare suspectedin severalothers. Bud.union-creasesymptoms (4) are often encounteredin trees on rough lemon rootstock. The causeof this prohlem is not known, but apparentlyit is graft-transmissible,and not causedby the virusesdiscussed above or VTD. Podagra,a stock/scionproblem hetween'Nagami' (Fortunella margarita [Laur.] Swing.) and'Meiwa' (F. crassifoliaSwing.) and roughlemon (29), alsooccurs in Florida.

Recently,some apparent compatibility problemswith mandarinor mandarin-hybridscions on trifoliate-orangehybrid rootstockshave been observed. (Bridges, unpublished; Hutchison, Hearn, and Garnsey,unpublished.) Symptomsconsisted of creasingwith or without gum depositsat the bud-union. TheseSVtrptoms did not alwaysextend completely around the trunk and may be transitory and recurring. The causeand final effects are unknown, but further observationis warranted.

Viruslike diseases. Severalviruslike diseaseshave been described in Florida, includinglime blotch, measles,leprosis, crinkle scurf, cancroidspot, and sphaeropsoidknot, but nonehas been proven infectious (28).

Youngtree decline. The pattern of spreadin the field of VTD and associateddiseases suggests that an infectious agentis involved(14). However,all attemptsso far to transmit or perpetuatethe diseasehave failed. Many of the more recentex- perimentsare not old enoughyet to be conclusive,but they do indicatethat if an infectiousagent is involved,it is difficult to transmit and/or requiresa long incubation period to producesymptoms. The initial symptomsof declinesuggest a xylem dysfunction.This type of symptomis not ordinarilyassociated with virusor mycoplasmalikediseases.

Mycoplasmalike diseases

Symptomsof stubbornand greeningrtiseases have been described in Florida (10,27). However,presence of either stub- born or greeningin Florida hasnot beendemonstrated by graft transmission.These diseases, once considered virus diseases, are now thought to be causedby mycoplasmalikeor similar organisms(3, 17). Electron-rnicroscopicobservation of tissues from YTD-affectedtrees that showedleaf symptomssimilar to greeningsymptoms failed to revealmycoplasmalike agents (Purcifull, Garnsey,Storey, and Christie,manuscript in preparation). Attempts to culture mycoplasmalikeorganisms from treessuspected of heingstubborn andfrom YTD-affectedtrees have not beensuccessful.

If stubbornand greeningare present in Florida, they currently are not widespread,nor do they causereadily detectable symptoms. Pastreliance on old-line budwood sources,with emphasison vigor and heavyfruit production, hasprobably forestalledspread of tllese problemsby propagation. The psyllid vectorsof greeningalso do not occur in Florida.

Continuedavoidance of stubborn and greeningis a must in Florida . Greeningand relateddiseases are destructivewhere vectorsare present,and no tolerant or resistantscion varieties are available.

Need for Further Research and Control

Virus and viruslike diseaseswill continue to be a problemin Florida. Researchwork haseliminated or controlled some problems. However, some solutions, such as development of tristeza-resistant rootstocks, may not be permanent; and new diseaseshave arisen. .83.

Basic information in a number of areas is need to provide better control of citrus virus diseases. Some examples are cited below.

We needmore rapid and pre.:isemethods of detectingcitrus viruses. Serologicaltechniques (21), electronmicroscopy (1), and chromatographictechniques (16,40) have been promising in somesituations, but widerapplication is needed.We needto know more about vectorsof citrus viruses. The physiologyof citrus virus diseasesalso needsmore intensivestudy. Virus-inducedstock/scion incompatibilities and stempitting are major problems;yet, we don't know how thesesymptoms developor how to dete.:tsensitivity to theseproblems in new citrus . Sourcesof resistanceto somevirus diseases havebeen identified, but inheritanceof thesefactors hasnot beendefined. Breedingfor virus-resistantvarieties is still hap- hazard. Therapyof plantsaffected by virus and mycoplasmalikeagents (38) also deservesmore attention.

Regulatory and certification programshave eliminated or avoidedmany seriousdisease problems, and their needwill be undiminishedin the future. Greening,stubborn, \voody gall, impietratura,and cristacortis,among others, should be exclu- ded from Florida. The efficient aphid vector of tristezaand the psyllid vectorsof greeningcould also be seriousproblems if introduced.

The budwood certification programin Florida hasdeveloped many valuablesources of scionsand rootstocksfree of damagingviruses for the grower. Information on virus disea~sdeveloped by the programhas also beeninvaluable to re- searchon virus diseasesand identifying and solving new problems. Continuationand wide useof this programby growers is essentialif virus diseasesin Florida areto becontrolled. .84.

UTERA TUllE CITED

1. Bar.Joseph,M., and G. Loebenstein.1970. Rapid diagnosisof the citrus tristeza diseaseby electronmicroscopy of partially purified preparations.Ph}'topathology 60: 1510-1512.

2. Bar.Joseph,M., and G. loebenstein. 1913. Effects of strain, sourceplant, and temperatureon the transmissibilityof citrus tristezavirus by the melon aphid. Phytopathology 63: 716-720.

3. Bow,J. M., and P. S~lio. Stubborn: 1969-1972. In L. G. Weathersand M. Cohen(ed.),Proc. 6th Con'. Intern. Organ. Citrus Viral., Univ. of Calif. Press,Berkeley. (in press).

4. Bridges,G. D., and C. D. Youtsey. 1968. Further studieson the budunionabnormality of rough lemon rootstockswith sweet orangescions. P.236-239. In J. F. L. Childs(ed.),Prac. 4th Cant. Intern. Organ.Citrus Viral., Univ. of Fla. Press,Gainesville.

5. Bridges,G. D., and C. D. Youtsey. 1972. Naturaltristeza infection of citrus species,relatives and hybrids at one Florida lata tion from 1961-1971. Proc. Fla. State Hart. Soc. 85: 44-47.

6. Broadbent,P., L. R. Fraser,and J. K. Long. 1971. Exocortisvirus in dwarfedcitrus trees. Plant 0;$. Rep. 55: 998-999.

7. Calavan,E. C., R. K. Soost,and J. W. Cameron.1959. Exocortis-likesymptoms on unbuddedseedlings and rootstocksof Poncirustrifoliata with seedling-linetops, and probablespread of exocortisin a nursery. Plant Dis. Rep. 43: 374.379.

Calavan, E. C., R. M. Pratt, B. W. lee, J. P. Hill, and R. l. Blue. 1972. T ristezasusceptibility of sweetorange on Troyer citrange rootstock. P.146-153. In W. C. Price (ed.),5th Cant. Intern. Organ. Citrus Viral., Univ. of Fla. Press,Gainesville.

9. Childs,J. F. L. 1968. A reviewof the cachexia-xyloporosissituation. P.83-88. In J. F. L. Childs (ed.),Proto 4th Cant. Intern. Organ.Citrus Viral., Univ. Fla. Press,Gainesville.

10. Childs,J. F. L. 1968. Symptomsof stubborndisease in Florida. P. 137-140, In J. F. L. Childs (ed.),Proto 4th Cont.ln- tern. Organ.Citrus Viral., Univ. Fla. Press,Gainesville.

11. Cohen,M. 1968. Exocortis virus asa possiblefactor in producingdwarf citrus trees. Proc. Fla. State Hart. Soc. 81: 115-119.

12. Cohen,M. 1970. Rangpur lime asa in Florida. Proco Fla. State Hort. Soc. 83: 78-84.

13. Cohen,M. Effect of exocortis inoculation on performanceof Marshgrapefruit trees on variousrootstocks. In L. G. Weathers and M. Cohen(ed.), Prac.6th Canf.lnterff. Organ.Citrus Viral., Univ. of Calif. Press,Berkeley. (in press).

14. DuCharme,E. P. 1971. Tree lossin relation to youngtree declineand sandhilldecline of citrus in Florida. Proc. Fla. State Hart. Soc. 84: 48-52.

15. Feldman,A. W., and R. W. Hanks. Resultsof initial indexingtests on citrus treesaffected with young tree declineand sand- hill decline. Plant Dis. Rep. (in press).

16. Feldman,A. W., G. D. Bridges,R. W. Hanks,and H. C. Burnett. 1971. Effectivenessof the chromatographicmethod for de. tecting exocortis virus infection in Poncirustrifoliata. Phytopathology 61: 1338-1341.

17. Fudl-Allah, A. E. A., E. C. Calavan.and E. C. K.lgwegbe. 1972. Culture of a mycoplasmalikeorganism associated with stub- born diseaseof citrus. Phytopathology 62: 729-731.

18. Garnsey,S. M. 1964. Detectionof tatter leaf virus of citrus in Florida. Proc. Fla. State Hort. Soc. 77: 106.109.

19. Garnsev.S. M.1968. A citruscrinklv4eaf-tvoe virus recently discovered in Florida.Proc. Fla. State Hort. Soc. 81: 79.84. .85.

20. Garnsey,S. M. 1970. Viruse~ in Florida's 'Meyer' lemon treesand their effects on other citrus. Proc. Fla. State Hart. Soc. 83: 66-71.

21. Garnsey,S. M., and D. E. Purcifull. 1969. Serologicaldetection of a citrus virus in leaf extractsfrom field trees. Proc. Fla. State Hart. Soc. 82: 5660.

22. Garnsey, S. M., and L. G. W~thers. 1972. Factors affecting mechanical spread of exocortis virus. P. 105.111. In W. C. Price (ed.), Prac.5thCanf.lntern. Organ.Citrus Virol., Univ. Fla. Press,Gainesville.

23. Garnsey,S. M., and R. Whidden. 1971. Oecontaminatiorrtreatments to reducethe spreadof citrus exocortisvirus (CEV) by contaminatedtools. Proc. Fla. State Hart. Soc. 84: 63-67.

24. Grant, T. J. 1959. Tristezavirus strains in relationto different citrus speciesused as test plants. Phytopathology 49: 823-827.

25. Grant,T. J., and P. F. Smith. 1960. Infectiousvariegation of citrus found in Florida. Plant Dis. Rep. 44: 426-429,

26. long, J. K., l. R. Fraser,and J. E. Cox. 1972. Possiblevalue of close-planted,virus-dwarfed orange trees. P. 262-267. In W. C. Price(ed.), Proc. 5th Cont.Intern. Organ. Citrus Virol., Univ.Fla. Press, Gainesville.

27. Knorr,LoC. 1967. Greeningdisease: What is it, and is it present in Florida? Citruslndo 48(4):13-14,18-19,21.

28. Knorr, L. C. 1963. Transmissiontrials with virus.like disoosesof citrus in Florida. P. 325-331. In J. F. L. Childs (ed.),Proc. 4th Cont.Intern. Orgall. Citrus Virol., Univ.Fla. Pres$, Gainesville.

29. Knorr, L. C. 1970. The Florida citrus virus picture. Citrus l(1d. 51 (7): 13,15, and 17

30. Knorr, L. C. Dictionary of Diseasesand Disorders of Citrus in Florida. Univ. of Fla. Press, Gainesville. (in press)

31. Knorr, L. C., and W. C. Price. 1959. Fovea- a diseaseof the . Citrus Mag, 22(1): 16-19,26

32. ~ndel, K. 1968. Interrelationsbetween tree performanceand somevirus diseases.P.310-313. In J. F. L. Childs (ed.), Proc. 4th Cant. Intern. Organ.Citrlls Viral., Univ. of Fla. Press,Gainesville.

33. Muller. G.W.,and A.S. Costa. 1968. Further evidenceon protectiveinterference incitrustristeza. P.71-8Z. InJ.F.l. Childs(ed.), Proc. 4th Cant. Intern. O/yan.Citrus Viral., Univ. of Fla. Press,Gainesville.

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