.168.

Background Material For A Study of Rootstocks

A. H. Krezdorn

The basicelement in any attempt to advanceknowledge is the ability to communicate. Eacharea of study requiresa speciallanguage of definitions and facts that enablethose involved to discussa subjectintelligently. The purposeof the following materialis to presenta readyreference of facts and terminology for usein this course. The materialis presented in brief to servethe immediateinterests of the participants.

Classification and Nomenclatureof

Classification. - Scientistshave classified plants, (placedthem into groupsbased on their relationshipto eachother) and then affixed names to these groups according to accepted rules of nomenclature. Classification is inexact becausethere is no way to determine the precise relationship of plants. Thus, plants may be grouped differently by equally able scien- tists who interpret available information differently. Perhapsfortunately, not many scientists are inclined to undertake the tremendous task of classifying large groups of plants. Walter T. Swingle, a U.S. Department of Agricultu re scientist wf10worked in , is the universallyaccepted authority for the largefamily of plants that includesthe genusCitrus.

All is divided into that which is animaland that which is and then into groupsdecreasing in sizeand increas- ing in closenessof relationship.

The position of in the Plant Kingdomis:

Division . Embryophyta Siphonogama Subdivision. Angiospermae . Dicotyledoneae Subclass . Archichlamydeae . Geraniales Suborder. Geraniineae . .

The subfamily Aurantioideae is a ramer large category but wormy of mention becauseit contains not only the various kinds of commercial citrus but wild relatives, some which hybridize with citrus and some which have potential as rootstocks. Swingle has divided the subfamily Aurantioideae into tribes, subtribes, subtribal groups and genera as follows:

Subfamily - Aurantioideae TribeI - Subtribe 1 . Micromelinae GenusI - Subtribe2 - Clauseninae GenusI - GenusII . GenusIII - Subtribe3 - Merrillinae GenusI . TribeII . Subtribe1 - Triphasiinae GenusI - Wenzelia GenusII . GenusIII . GenusIV . Merope GenusV - GenusVI . GenusVII - .169-

GenusVIII. Subtribe2 . Citrinae GenusI . GenusII . Pleiospermum GenusIII . GenusIV . GenusV - GenusVI . GenusVII. GenusVIII - Fortunella GenusIX . Eremocitrus GenusX . Poncirus GenusXI . Microcitrus GenusXII. Citrus Subtribe3 - Balsamocitrinae GenusI - GenusII - Aegle GenusIII . GenusIV - GenusV . GenusVI - Feronia GenusVII. Feroniella

Nomenclature. - The ordinary laymandefines citrus asthe sweetoranges, , , lemonsand , commonly found on the grocery shelf under the citrus label. Growers, more knowledgeable of citrus technology, include unpalatable speciesand close relatives such as sour , trifoliate oranges, , and others in their definition of citrus. Scientists use the term citrus in several ways as follows:

A. The subtribe C;tr;neaehas been divided into 3 citrus groups.

Subtribe - Citrineae 1. Primitive Citrus Fruit GenusI - Severinia GenusII . P/eiospermum GenusIII. Burki//anthus GenusIV. Limnocitros GenusV. Hesperethusa 2. Near-CitrusFruit Trees GenusI. Citropsis GenusII - Ata/antia 3. True Citrus Fruit Trees GenusI - Fortune//a (kumquats) GenusII. Eremocitrus GenusIII. Ponciros() GenusIV. C/ymenia GenusV. Microcitrus GenusVI - Citros (includescommon commercialcitrus types)

Thus,the scientistmay usethe tenn citrus in a broad senseto meanthe commercialtypes of the genusCitrus, the inedibletypes of Citrusand certain Citrus relatives.

B. The term Citrus; ie, citrus spelledwith a capital C and underlinedor italicized indicatesthe genericname of that group. All of the fruit marketedcommercially as citrus is includedin this .The , in the genusFortunella, is a minor exception. .170.

Scientists and well-informed growers often use scientific botanical names. such as Citrus sinensis (L.) Osbeck. The .:ientific name can be divided into 3 parts the genus, Citrus, the speciesepithet sinensis and the author citation, (L.) Osbeck. The common name of this is sweet orange or orange.

The advantageof the scientific nameis that eachspecies, the basictaxonomic unit, can haveonly one nameand that nameis formed accordingto specificrules. The unfortunate fact, however,is that thosewho classifyplants may differ asto which plants should belongto a givengroup. This situation borderson the ridiculous in the genusCitrus which Swingledivided into only 16 species.Tyosaburo Tanaka from Japan,on the other hand,divided the samegenus or group of plants (Citrus) into over 145 species!Tanaka's classification is too unwieldy to be functional. Swingle'sis more usable than Tanaka'sbecause it is simple,but it is obviouslyin error in somecases. Scientists and professionalhorticulturists gen- erally useSwingle's system modified at times to include someof Tanaka'ssystem when modificationsappear appropriate. A briefsummary of Swingle'ssystem with occasionalcomparisons with Tanaka'ssystem is presentedbelow:

Genus- Citrus 1. SubgenusCitrus, includesthose species with edible fruit and pulp not containingdrops of acrid or bitter oil. This subgenuscontains the following species: a. C. medica(), a speciesvalued primarily for the useof its peel. The '' or is used in certain religiousceremonies by Hebrewsand someselections of the 'Etrog', such asArizona 861 and USDA 60-31, arecommonly usedas indicator plantsto detect the exocortisvirus. b. C.limon (),a speciesused by Swingleto include 811lemons, both acid and acidless. His classification is probably too inclusiveand certain exceptionsare commonly made,with C. limon primarily including the acid lemonsof commerce.Tanaka's classification includes the following types commonly usedas rootstocks: (1) C.jambhiri - rough (2) C. limonia . Rangpurlime (3) c. volkameriana - Volkameriana lemon The 'Meyer' and 'Ponderosa'lemons are considered hybrids by Swinglebut separatespecies by Tanaka. c. C. aurantifolia (acid limes), a speciesincluding all acid limesaccording to Swingle. Somecommonly used exceptionsby Tanakaare: (1) C. aurantifolia. the 'Key', 'West Indian', 'Mexican' or 'Gallego'lime. The 'Key' lime is usedas an indicator plant for the tristezavirus. (2) C. latifolia - the 'Tahiti', 'Persian'or 'Bearss'lime. (3) C. macrophylla - the Alemow. Tanakadescribed this speciesas a lime-like fruit. Somefeel it is lemon. like and othersthat it is a hybrid of C. celebica. This specieshas become an important lemon rootstock in recentyears. (4) C. limetta - the limmetas,a group of lemon-likefruit with both acid and acidlessforms. (5) C. limmetoides - the Indian or Palestinesweet lime that is widely usedas a rootstock in someparts of the world. The Columbiansweet lime is indistinguishablefrom the standardPalestine sweet lime. d. C. aurantium (sour orange),a speciesused widely asa rootstock wherethe tristezavirus doesn'tprevent its use. There are numerousforms of sour orangeincluding a low acid type called bittersweet. (1) C. taiwanica. is a sour orange-likefruit with resistanceto certain forms of tristeza. (2) Australian sour orange - is a hybrid of unknown origin that is reputedly tolerant of tristeza;however, this selectionhas few if any characteristicsthat resemblesour orange. Callingthis fruit a sour orangeis misleading. e. C. reticulata (mandarins),a specieswith loose-peeledfruit that includesthe commontangerines and satsumas (Unshiu,Mikan) oranges.Tanaka divides this speciesinto severalas indicatedin the table below.

CommonName Swingle ;1.f1;';',,~ Tanaka , c-..' C. unsh;u C. reticulata C. reticulata C. tanger;na C. reticulata C. ret;culata C. reticulata C. reshn; C. reticulata C. clement;na -171-

co~t. " , Tanaka CommonName Swingle - C. deliciosa 'Willowleaf C. reticulata 'King' ? C. nobilis b , -' tangor? C. temple 'Temple''.-- (;" :' """'c " C. reticulata Vir. 'Calamodin' C. madurensis Busters x C. ichangensis YOlO ~~' Vi' (tangerinex C.ichangensis C.junos ~_..~ ~

(1) The Cleopatramandarin is important asa rootstock throughout the world and the is usedto someextent asa rootstock in Japan. (2) Most of the mandarinsare usedfor their fine quality asfresh fruit. (3) The tangerineshave been hybridized with grapefruit to form tangelossuch as 'Orlando', 'Minneola' and 'Nova'. (4) The ''and 'Temple' are usedwidely in breedingprograms because they are monembryonic and of high quality. f. C.grandis (pummeloor shaddock),is little usedin the United Statesbut important for its fruit in the Far East. The fruit and are similar to grapefruit in many respectsbut the flesh of pummelois sweeterand lessjuicy than that of grapefruit and the peel of pummelois generallythicker. The in this species are monoembryonicand therefore the pummelois usedin grapefruit breedingprograms. The Cubanshaddock is sometimesmentioned as a rootstock. g. C.paradisi (grapefruit),a speciesvalued more for its fruit in the Westernthan the Easternworld, is usedpri- marily asa fresh fruit Grapefruit hasbeen used occasionally as a rootstock in Florida but without any apparentadvantages. h. C. sinensis(sweet oranges), is the most widely grown speciesof Citrus. There are many horticultural or commercialtypes that can be placedinto 3 groups: the common or round oranges,the naveloranges and the blood oranges.This speciesis most important for its fruit but sweetorange is alsocommonly usedas a rootstock. Somesweet oran~s havetolerance or resistanceto the spreadingdecline nematode. The 'Ridge Pineapple'is the best known example. i. C. tachibanaand C. indica are 2 other speciesof little importanceincluded in the subgenusCitrus.

2. - ,includes those species with numerousdroplets of acrid oil that makethem inedible. Also, the petiolesof speciesin this subgenusare long and broadly winged. The Papedagroup is of little valueexcept for its potential in a breedingprogram. a. C. ichangensis,a cold-hardy,monoembryonic species that hybridizesreadily. b. C. cetebica,is thought by Swingleto be one parent of the Yuzu, which is usedas a rootstock in Japan. c. C. talipes, C. , C. macroptera,C. annamensisand C. hystrix are of little value.

Genus.Poncirus

The trifoliate orangeis in a separategenus with only 1 species,Poncirus trifoliata. Unlike Citrus, Poncirus is deciduous, inedible and very cold.resistant.The trifoliate orangehas many characteristicsthat make it useful as a rootstock, suchas resistance to sp., tristeza, xyloporosis, cold and the citrus nematode. It is, however, sensitive to exocortis and grows poorly on coarse sands. Trifoliate orange has been hybridized with several species of Citrus. A few of the more important hybrids are:

P. trifo/iata x C. sinensis (sweet orange) . P. trifo/iata.x c. reticu/ata (tangerines) - citrangedin P. trifo/iata x C. paradisi (grapefruit) . P. trifo/iata x C. aurantium (sour orange)- citradia. .1n.

The most important hybrids Ire thosewith sweetorange, such as 'Rusk', 'Carrizo' and 'Troyer'. 'Carrizo' and 'Troyer' are rootstocksof major importance. None of the citrangesare edible. At leastone of the citrumelosnow beingtested shows promise as a rootstock. Genus- Fortunella

The kumquatsare often calledcitrus and dley havebeen hybridized widl bodl Citrus andPonciros but neitherthe kumquatsnor its hybridsare of majorsignificance.

Nomenclatureof Horticultural Varieties- The abovediscussion has dealt primarily with speciesand categorieslarger than species.Horticulturists are interestedin plant materialwith minor botanicaldifferences but differencesthat in- fluencetheir commercialvalue. There areseveral terms of interestin this regard.

Variety. the horbcultural unit. suchas 'Valencia' sweetorange, 'Marsh' grapefruit or 'Dancy' tan~rine. Such plant material differs very little within the speciesbut the differenceof the 'Marsh' grapefruitfrom the 'Duncan' grapefruit is very important commercially. The horticultural variety differs from the botanicalvariola, a term sometimesuS9d to indicate a unit below the level of species.

2. Cultivar- is a term synonymouswith variety and is beingused quite frequently in professionaland scientific language.The term meansa cultivatedvariety. The cultivar or varietal nameis generallywritten with the first letter capitalizedand the entire word in singlequotes. Example,'Hamlin' sweetorange. Cultivarsmay also be written C. sinensiscy. Hamlin. The singlequotes are eliminated because the term Hamlin is precededby the abbreviationfor cultivar, cy.

3. Group names.such as satsuma and navelare often erroneouslyused. They should be written just asany common namewithout capitalization or quotes. The satsumasand navelseach include severalcultivars or varieties. Some feel the 'Valencia',commonly considercda cultivar, should be givengroup statusbecause there areseveral selec- tions of 'Valencia' that differ sufficient to be givencultivar status. Termssuch assour orange, and so forth arecommon namesthat require no specialdesignation.

Rootstock and RelatedNomenclature

1. Rootstock- is the rootsystemportion of a tree propagatedby buddingor grafting. The rootstock may furnish part of the trunk when buddedhigh, a practicefollowed in many areas,but not in Florida, to reducethe damage often occuringto the basalpart of the treetrunk by Phytophthorasp.

2. Scionvariety - is plant material used8 the sourceof budwoodfor producingthe aerial,crop-producing portion of the tree.

3. Intemock or Splice. is a portion of a buddedtree betweenthe rootstock and the fruit producingtop. This is un- common in citrus but common in appleswhere the interstock materialand the length of the interstock are usedto dwarf or control the tree size.

4. Budding.is the processof placing a singlebud with a small pieceof bark or bark and into a speciallyprepared wound or the rootstock (the matrix) which then unites. The bud is forced or causedto grow by removingthe root- stock portion abovethe bud.

5. Budeye. is a tenn usedto indicate a bud and tfte areaimmediately surrounding it.

6. Grafting. is the processof propagatingby insertinga short pieceof stemwith 2 or more buds into the wound or matrix on the rootstock.

7. Graftaae.is a teRn sometimesused to include budding and grafting. -173.

8. Topworking- is a term restricted to replacing the canopy of a relatively large tree through budding or grafting as opposed to budding or grafting seedling rootstocks in the nursery.

9. Suckers. areshoots arising from the roots and are usedto identify the rootstock whereit is unknown. Citrus roots suckerreadily when the soil is removedand the roots exposedto light (seewatersprouts).

10. Watersprout- is a term usedto indicate a shoot arisingfrom donnant or adventivebuds of the trunk or framework branches.The tenns watersproutsand suckersare often usedsynonymously, resulting in someconfusion. Both watersproutsand suckersare often excessivelyvigorous and the leaf and may not be preciselytypical of the nonnal shoots.

11. Fluting. is a term appliedto ridgesand valleysin the trunk at the baseof the tree. The ridgesare extensionsof the major roots. Trifoliate orangeand its hybrids are often fluted when usedas rootstocks.

12. Crown. this term is used to indicate both the baseof the tree and the upper portion or crown of the leafy canopy.

13. Foot - indicatesthe baseof the tree, asin foot rot.

14. Scion Rooting. refersto root formation of the scion top and therefore rooting abovethe bud union.

15. Bud Union. is the plane of union of the scion top and the rootstock. The areaof union is often very noticeable asa result of an overgrowthor undergrowthof the stock or scionvariety. Somebud unions arevery smooth but there is alwayssome difference of growth disparity, bark texture or bark color.

16. Tap root. the primary, central root(s) that extend straight down into the soil. There are often severaltap roots.

17. FeederRoots - small, branchedroots that apparentlyform the absorbingsurface of the root system.

18. Root Hairs. unicellular extensionsnear root tips that form an important part of the absorbingsurface of the root system.

Nucellar Seedlings

Most plants produceonly seedlingsthat are the result of sexualunion and haveonly one embryo per seed. Su~hseed- lings resembleeach of the parentsto a certain degreebut are unlike either one. They result from a sexualcombination of geneticmaterial.

Thissame sexual union and seedlingsresulting from a combination of geneticmaterial also occursin citrus. In addition, however,most kinds of citrus produceadditional embryosfrom a tissuecalled the nucellus,a tissuewhich surroundsthe embryo saccontaining the sexualor gameticembryo. The nucellarembryos arise solely from maternaltissue and the seed. lings that arisefrom them areessentially the sameas the mother plant. Suchseedlings are called nucellarseedlings. The nucellarembryos generally develop faster than the sexualembryos and often crowd them out. Thus, most of the seedlings of a 'Valencia' sweetorange or a 'Marsh' grapefruit are geneticallythe sameas the parent tree. Suchvarieties are alsocalled polyembryonicvarieties because they contain more than one embryo per seed. A few varieties,such as 'Temple', 'Clementine' and the pummeloare monoembryonicand produceonly sexualseedlings.

The nucellarseedlings are usedadvantageously in 2 ways.

1. Very uniform rootstocksare obtainedby roguingout or discardingthe very small and unusuallylarge seedlings of a givenrootstock variety, such asrough lemon, in the nurseryrow. The remainingseedlings will be essentiallythe samegenet- ically, with all the good and poor characteristicsof the species.This is not possiblewith most fruit species,such asapples, pears,grapes and pecans,and one must resort to vegetativepropagation, such as stem cuttings to get geneticallyuniform rootstocks. Vegetativepropagation is alsocalled asexualpropagation. .114-

2. Viruses,witt! rareexceptions, are not transmittedthrough seeds.Thus, a cultivar, suchas 'Valencia'sweet orange, grown as a nucellarseedling will be true-to-type (a true 'Valencia') and free of viruses. This budwood is usableon root- stocksthat aresensitive to budwood-transmittedviruses, such as exocortis and xyloporosisthat may be containedin the parent 'Valencia' tree.

Unfortunately some nucellarseedlings vary slightly from the parent so they must be testedfor a number of yearsto be certain they are at least the equal of the mother parent Trees that have been propagated by budding for many generations are called old-line trees. Trees propagated from materi~ that has recently been developed from nucellar seedling soun:es are called nucellar trees or nucellar varieties. These tenns are vague and there is not a set period after which a nucellar variety becomes an old-line variety.

Juvenility

Citrus seedlingsexhibit a pronouncedjuvenility; ie, seedlingspass through a juvenile stagecharacterized by up right growth, extremethorniness, and absenceof flowering. The seedlingsgradually grow out of this juvenility, with the newer wood becomingless thorny and more productive of . The basalportions of seedlingsremain juvenile throughout the life of the tree. Floweringoccurs only on the later growth flushes. Thus, budwoodfrom nucellarseedlings should be obtainedonly from the older flowering and fruiting portions of the trees. This juvenility greatly lengthensthe time to pro- ducenucellar varieties of sweetoran~s and grapefruit and new hybrids.

The length of the juvenile period varieswith the speciesapproximately as follows:

SweetOrange . 6. 15years Grapefruit . 6.15 years Mandarins - 5 years Lemons . 2. 3 years Limes(Key) - 1 - 2 years Trifoliate Orange - 3 . 4 years

Growth conditions that slow growth, prolong the juvenile period. Conditionspromoting growth reducesthe juvenile period.

Nematodes

Nematodes aresmall worm or worm-like animalsthat arecommonly found in the soil. They differ from insectsand mites in many important ways.

The 2 most damaging nematodesof citrus are:

1. Tylenchulus semipenetrans (citrus nematode) - is of worldwide importance. The citrus nematode is immobile and it is transportedprimarily dlrough infestednursery stock. The trifoliate orangeand someof its hybrids offer protection againstthis pest but all other Citrus is susceptibleto it.

2. Radopholussimilis (burrowing nematode)- is a tropical region nematodethat hasbeen very damagingin Florida, causingthe spreadingdecline disease. The life cycle of this nematodeand the coarsesands of Florida havemade its elimination possiblein certain sites. 'Milam' lemon, 'Estes'rough lemon and 'Ridge Pineapple'afford someprotection from this nematode.

There are several significant terms related to nematodes:

(1) Immune. is that form of resistancewhere the nematodeneither penetratesnor feedsupon the root tissuesof the plant.

(2) Resistance.indicates a situation in which plant growth is not retarded,even though invadedby the nematode,and the nematodepopulation graduallydiminishes to a low level or completely disappears. .175-

(3) Tolerance. denotesa situation in which the nematode population is retained at a high level but, even so, the growth is retarded no more than 20 percent.

The terms immune,resistant and tolerant cannot be transferredto virus and other diseases.They are arbitrary terms usedto describethe reactionsof plants to nematodes.

(4) Biotype - denotesa slight biologicalvariant suchas often occursin living matter. For example,some citrus nema- todes haveevolved, either through mutations or the sexualprocess, that damagetrifoliate orangebut which cannot be distinguishedin any other way from other citrus nematodesthat do not damagethis species.By the sametoken, slight changesin the rootstock can take placethat enablethem to tolerate a pest other rootstocksof the species won't tolerate. The 'Estes' rough lemon, for example,is tolerant of the burrowing nematodebut indistinguishable in any other mannerfrom the common Florida rough lemon. Biotypescan occur in all living material, not just rootstocksand nematodes.

(5) ChemicalBarrier - denotesa relatively plant-freezone surroundinga spreadingdecline area that is periodically fumigatedto preventspread of both roots and nematodesinto adjacentnematode.free areas.

(6) BiologicalBarrier. is appliedto a zone of immune or highly resistantplants surroundinga nematodeinfested area. Nematodespread is preventedby the inability of the nematodeto live and producein the plants separatingthe in. festedzone from a nematode.freezone. Citrus on 'Milam' rootstockshave been used experimentally as barrier plants.

Virusand Virus-Like Diseases

Virus Oi.wases

Virus diseasesare lesscompletely understoodthan fungusand bacterialdiseases. The latter 2 types of organismshave beenknown for many yearsand can be identified on the basisof their sexualforms and the morphologicalfeatures of their vegetativestructures as seen under the standardlight microscope.Viruses, on the other hand, havebeen recognized only recently. They arewithout sex and their particle sizeis so smallthe electronmicroscope is generallynecessary for their study. Virusesreproduce in host cells by alteringthe nucleicacid metabolismof the infectedcell. Viruses,unlike fungi and bacteria,generally cannot be grown on artificial media.

There is no completelysatisfactory definition of a virus. Virusesgenerally consist of ribonucleicacid (RNA) or deoxyribonucleicacid (DNA) surroundedby a thin protein coat. This definition fits the citrus tristezavirus but not the exocortis virus, which lacks the thin protein coat. Citrus virologists have not yet resolved problems of classification and nomenclature. Such names as tristeza, exocortis, xvloporosis and psorosis identify diseasesymptoms, eac~ of which could be caused by one or more closely related viruses.

Transmissionof someviruses may be achievedby mechanicalmeans (knives, pruning shears,etc.), budsand grafts, insects,nematodes, fungi, bacteria,parasitic plants and through seeds.In citrus, however,transmission through seedsis rareand mechanicaltransmission is limited to exocortis and certain minor viruses.Tristeza is the only major insect-trans- mitted virus. All major citrus virusescan be transmittedthrough the useof infected budwood. Psorosisand xyloporosis .. aretransmitted only through infected budwood and exocortisis transmittedlargely in this manner. Someviruses cause damage regardless of the rootstock-scionvariety combinationwhile othersare rootstock-scion, rootstock or scion-dependent

Some useful terms or definitions relatedto virusesare:

1. RegisteredBudwood - is budwood registeredfree of one or more viruses. -176-

, 2. Indexing. is determiningthe presenceor absenceof a virus in a plant that may carry, but not expressor only slowly eKpress.asymptom responseto the virus, by infecting an index plant (one which expressesthe symptom relatively rapidly following infection). For example,exocortis is carried in sweetorange on rough lemon without symptom expression.Cer- tain selectionsof citrus buddedwith exocortis infected materialsoon expressthe presenceof the virus with distorted or epinasticvegetative growth. The plant growsnormally when the virus is not present.

Virus-LikeDiseases

Stubbornor acorn diseaseand the greeningdisease of citrus were oncethought to be virus diseasesbecause of their modeof transmissionand becauseno fungal, bacterialor physiologicaldisorder could be found. Thesediseases are now diagnosedas mycoplasmaor mycoplasma-likein nature.

Mycoplasma are extremelysmall organismswid1out cell walls that cannot be grown on artificial media. Plantsin- jected with tetracyclenerecuperate at leastpartially. Plantswith stubborn and greeningrespond to tetracycleneand have beenfound to contain mycoplasma.Both diseasesare insect transmitted.

Mycoffhiza

Mycorrhiza are fungi living on and in the roots of plants in a symbiotic (mutually beneficial)relationship. The my' corrhizal fungi needthe carbohydratesproduced by the leavesof the plant while they (the fungi) take up nutrients from the soil and transfer them into the plants. In effect the mycorrhizal fungi serveas roots that are in someways more effectivethan citrus roots in taking up mineral elements,such asphosphorous.

The role of the mycorrhizal relationshiphas long beenestablished for certain forest treesbut it hasonly recently re- ceivedattention in citrus asa result of methyl bromidefumigation of citrus nurserysites. Methyl bromide kills the my- corrhizal fungi aswell asthe harmful organisms. Researchhas indicated the presenceof mycorrhizalfungi on all root- stocksexamined to date.

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