Horticultural Considerationsand Summation

A. H. Krezdorn

Introduction

Contemporary citrus rootstock research is heavily oriented to virus and virus-like diseaseproblems and, to a somewhat lesser extent, to resistanceto nematodes and Phytophthora spp. Lectures presented in this course have reflected the concern of both researchersand growers with pest control through the use of appropriate rootstocks.

The current state of science in relation to systemic disorders is obviously in its infancy. The complexity of these problems hasbecome evident as researchhas advanced. We continue to identify most of thesedisorders by symptomexpression rather d1an on the basis of definite chemical bodies that constitute virus or other particles. We speak rather vaguely of severe and mild strains of viruses, and we still occasionally find a diseaseonce thought to be a virus is not.

Evenso, much hasbeen learned about virus and virus-likediseases and we are becomingsurer of how to dealwith them. There is a beginningof researchrelated to cross-protectionor immunization. We are learningnot only to "ive with" systemicdisease problemsbut to harnessthem by infecting treeswith selectedviruses to control tree size.

The problem of nematodeshas also becomemore complexwith the discoverythat nematodescan changeslightly and cause damageto plants hitherto undamagedby them. As in the caseof viruses,we are alsolearning to acceptand control them. Use of Milam rootstock asa biologicalbuffer to preventthe spreadof Radopholussimilis, the spreading-declinenematode, is an ex- cellent exampleof an imaginativecontrol measure.

Even that old citrus nemesis,Phytophthora spp. has become more complex than was once thought, as new strains are found. The proper method for evaluatingrootstocks for resistanceis still beingdebated but we are becomingsufficiently knowledgeable to acceptthe fact many rootstockswhich are much lessresistant to Phytophthora spp. than sour orangeand trifoliate orangeare sufficiently resistantto serveas rootstockson the well-drainedsands of central Florida. Someof thesesame rootstocks would not be satisfactoryunder soil conditionsfavoring this disease.

Florida growersshould considerthe fact they are almostalone in not taking full advantageof the rootstock'sresistance to foot-rot caused by Phytophthora spp. Lecturers have repeatedly emphasized budding "high" to avoid foot-rot problems. Florida nurseriesstill produceand growersplant virtually all of its treeswith bud unionsat or nearthe ground line - a costly mistake!

Rootstocks,also affect yield, fruit sizeand quality, resistanceto cold and drought, and tree size. Thesefactors aswell as re- sistanceto pestsmust be consideredin selectingrootstocks.

Yield

Yield of fruit, whether measuredin units of weight, boxesof fruit, or poundsof solublesolids, is the most important factor in profitable fruit production. Yield is alsothe most difficult tree responseto measurebecause of the tremendousvariation in yield from tree to tree and from year to year. This variation may be severalfold eventhough environmentalconditions are similar and plant material is geneticallythe same.

Moreover,a givenrootstock experiment, even when well designedand evaluatedover a period of many years,describes only a limited set of conditions. Thus, a reviewof all availableresearch and observationsof tree performanceunder groveconditions are neededto developvalid conclusions.Nevertheless, there are somegeneralizations that can be made.

Citrus treeson rough lemon, 'Palestine'sweet lime and rangpurlime rather consistentlyoutyield treeson other commonly used rootstocks over a wide range of soils; however, their advantage is particularly notM:eableon coarse, sandy soils where moisture reten tion is low. The advantage of these stocks is also less when yield is calculated on the basis of pounds of solids per unit of land. .163.

Treeson cleopatramandarin and sweetorange often yield poorly but this responseis not consistent,varying with tree age, soil, virus statusand scionvariety. Sweetorange on cleopatramandarin generally yields poorly the first 15 yearsof age. Yields of 'Valencia' sweetorange on cleopatraare particularly low. 'Hamlin' and 'Pineapple'sweet oranges and grapefruit yield reason- ably well on cleopatramandarin but not aswell ason rootstockssuch as roughlemon. 'Temple' yields well on cleopatra,as do properly pollinated 'Orlando' and 'Minneola' tangelosand 'Robinson' tangerines.Parthenocarpically produced (seedless) tange- los yield more poorly on cleopatrathan on the lemon and sweetlime rootstocksunless girdled or sprayedwith gibberellicacid. Treeson cleopatrayield better on rich soils and when clay is closeto the surf~e of sandysoils.

Trees on sweet orange arevery droughty. Yields of all cultivarson sweetorange rootstock are good to excellent on heavier soils that are retentive of moisture but very poor on coarse sands unless careful attention is given to irrigation.

Treeson sour orangegenerally yield moderatelywell, but 'Valencia' on sour orangeon coarsesand yields little if any better than treeson cleopatra.

Trifoliateorange yields rad1erpoorly on coarsesands because trees are stunted and fruit is small. The low yield on trifoliate orangediffers from that on cleopatrain d1atlarge trees of cleopatraproduce few fruit while treeson trifoliate tend to be dwarf- ed and fruit sizeis small. The interaction of variety with trifoliate rootstock on deepsands is inadequatelyknown.

Trees on trifoliateorange produce standard-size trees with goodyields and largefruit on rich soilsthat are retentiveof mois- ture.

Trees on 'Carrizo' citrange,a stock rapidly risingin popularity in Florida and of considerablesignificance in California,yield Mil on the basis of research results and observations of commercial plantings to date but their performance on deep sands over a long period of years is unknown.

There are other trifoliate orangehybrids with promise.'Swingle' citrumel8 hasyielded extremelywell in Texastrials but it has been inadequately tested on coarse sands in Florida.

Internal Fruit Quality

Influenceof rootstock on internal fruit quality (juice quality) is rather constant. Rootstockscan be separatedinto about 2 or 3 groupson the basisof the solublesolids (an estimateof sugar)and acid content of the juice.

Lemon and lime rootstocksgenerally result in d1elowest soluble solids and acid contentsby a wide margin,with rough lemon characteristically producing d1e poorest quality. Trees on rangpur lime have somewhat better quality than trees on rough lemon oot still rank well below d1oseproducing high quality fruit.

Treeson sour orangehave long beennoted for producingfruit of high internal quality; however,cleopatra mandarin, sweet orange,trifoliate orange,'Carrizo' citrangeand similar stocksproduce fruit of quality equalto that producedon sour orange.

Some cultivarsproduce fruit of excellentquality for the fresh fruit market evenon stockssuch as rough lemon. 'Valencia' and 'Pineapple' sweet oranges and 'Temple' are notable examples. Grapefruit produces reasonable quality fruit on rough lemon and sweet lime stocks, which are widely used for grapefruit on coarse sands; however, stocks such as sour orange result in super- ior quality.

Mandarins(tangerines) and tangelos,with the exceptionof satsumas,are generallyof sufficiently high quality to tolerate the reduction in solidsresulting from the useof lemon rootstock. In fact, the low acidity of the fruit often resultsin early legalma- turity and shipment 'Temple' in Florida, for example,matures fruit of excellentquality on rough lemon that is mature in Decem- ber, while treeson cleopatrado not reachmaturity until Januaryor February.

On the other hand,the flesh of many mandarinstends to becomedry or "ricy" early, and rough lemon contributesto this problem. Fruit from young 'Temple' treeson rough lemon may actually becometoo dry to ship beforelegal maturity is reached. -164-

Totalsoluble solids of me juice and juice content are, of course,important when sellingfruit for processingon the basisof poundsof solublesolids. The high levelsof total solublesolids in combinationsof sweetorange on sour orangerootstock results in more poundsof solidsper box of fruit but the greaternumber of boxesof fruit per tree or per unit areaof sweetorange on rough lemon rootstock resultsin much largeryields of solublesolids per unit areawhen rough lemon is the rootstock. 'Hamlin' sweetorange on rough lemon,for example,produces the poorestquality of fruit and the lowest poundsof solidsper box but mis combinationproduces the largestamount of total solublesolids per unit areawhen comparedto mat producedby other rootstockstraditionally usedin Florida. This may not be true for someof the newerstocks such asrangpur lime and 'Carrizo' citrange,for which mere are insufficient data from commercialplantings.

The date fruit reacheslegal maturity, which is largelydetermined by the content of total solublesolids, acidity, total soluble solids/acidratio and juice content. variesaccording to wf1ichrootstock is used. Fruit of high quality, suchas 'Valencia' and 'Pine- ~ple' oranges,and 'Temple' generallyreach legalmaturity earlieston rootstockssuch as rough lemon,which reducesboth the total solublesolids and acid contents. Legalmaturity of thesecultivars is limited by the high acid content, which is reducedrel- atively more than is the total solublesolids content On the other hand, low quality cultivars,such as'Hamlin' sweetorange and 'Orlando' tangelooften reachlegal maturity first on rootstockssuch as sour orange. Fruit from low-quality cultivarsare limited by failureto meetthe minimumlevel of total solublesolids required, not becauseof excessiveacidity. Juice content is rarely a limiting fKtor in legalmaturity of varietiesexcept pink and red grapefruit on any rootstock.

Obviously.factors other than rootstocksinfluence quality and maturity but rootstocksexert a major influence in this respect

F/UitSize

Rootstocksplay a large role in determining fruit size,which in turn is importantin determiningprice on the freshfruit mar. ketoSome cultivars can tolerate a reductionin fruit sizebut otherscannot.

Trees on rough lemon, 'Palestine'sweet lime, rangpurlime and Citrusmacrophylla are noted for largefruit sizes. 'Dancy' tangerinesare producedalmost exclusively on rough lemon in Florida to take advantageof the largefruit sizesproduced on this rootstock. Sweetlime is usedin someareas of the world partly becauseof largefruit sizesproduced in it and this rootstock war- rantsserious consideration for grapefruit in Florida. 'Orlando' tangelosare beingbudded on 'Palestine'sweet lime to someex- tent in Florida both to increaseparthenocarpic yields and fruit size.

Cleopatra mandarin,on the other hand, is notoriousfor small fruit sizesresulting from its use. 'Dancy' tangerineon cleopat- ra would be a poor combination. 'Hamlin' orangetrees on cleopatrawere frequently topworked to other cultivarsthat perform. ed better on cleopatra,before the developmentof frozen orangeconcentrate. Grapefruit, particularly 'Redblush' on cleopatra suffersa substantialreduction in fruit suitablefor fresh fruit due to the small sizesproduced. On the other hand, 'Temple' fruits and sizeswell on cleopatra, 'Minneola' and 'Orlando' tangelosalso size well on cleopatra. 'Robinson' and 'Murcott' sizefairly well on cleopatrabut not aswell aswould be liked.

Fruit sizesfrom treeson rootstockssuch as sour orange,sweet orange, trifoliate orangeand 'Carrizo' citrangelie betweenthe largesizes produced on rough lemon and the smallsizes on cleopatra. 'Carrizo' approachesrough lemon in the size of fruit pro- ducedby treesbudded on it and sour orangelies closerto cleopatra. Trifoliate orangeproduces very small fruit on coarse,droughty sandsbut largefruit sizeson rich soils. Usingtrifoliate asa rootstock for 'Dancy' would be highly questionableeven on rich soils but nucellarred grapefruit and exocortis-freeold-line grapefruithave produced fruit of satisfactorysizes even on sandysoils.

External Fruit Quality

The influence of rootstock on suchfactors as rind color, texture and thicknessis lesswell known, and lesspronounced and consistentthan the differencein internal quality. For example,the total solublesolids content of citrus fruit grown on sour orangewill be higher than that producedon rough lemon almostregardless of the conditionsunder which they are produced wtlile peel color is so closely relatedto temperatureand exposureto sunlight that climate may maskany small differencesdue to rootstock.

Generally speaking, fruit produced on stockssuch as rough lemon and sweetlime, which impart greatvigor of growth, have thick, coarse-textured rinds and less intense orange color. Rootstocks, which tend to produce fruit of high internal quality, such assour orange, cleopatramandarin and trifoliate orange,and induceless growth vigor than the lemon and lime rootstocks,have .165-

thinner, smootherrinds and slightly better peel color. Cleopatramandarin is particularly noted for producingfruit with smooth, thin rinds. Suchfruit is beautiful but it increasesthe tendencyof sweetoranges, and sometangerine cultivars to split Splitting and creasingare important problems,in Florida at least and a significantcause of low yields in somegroves. There are few if any caseswhere a thinner rind would not improvegrapefruit Thick rinds are quite desirablein kumquatswhere the edible peel is the primary part used. Thinner rinds are particularly desirablein hot, desertregions where the fruit rinds tend to be thicker than in humidregions.

Resistance to Orouth

Influenceof rootstock on d1eability of d1etree to withstand drought hasnot beenas intensivelyor aswidely evaluatedas other rootstock influences. Roughlemon and 'Palestine'sweet lime are acceptedas best adapted to coarsedrouthy sandswhile sweetorange has been considered very unsatisfactory.

Trees on sweet orange wilt during dry periods in advance of the occurrence of wilt of trees on other rootstocks and moisture stress is reflected by lower yields.

Treeson trifoliate orangealso show early wilting of leaveson coarsesands when the treesare relatively young but still bear heavycrops, the moisturestress being reflected in smallfruit sizes.

Causesof differencesin resistanceto drouth are incompletelyknown. Differencesin depth and density of rooting must play an important role in drouth resistance.Field studiesof the depth of rooting of variousrootstock generallybear out the greater depth of rooting of 'Palestine'sweet lime and rough lemon ascompared to cleopatramandarin, sweet orange, sour orange,tri'o- liate orange,and various citranges; however, the correlation of rooting patternswith drouth resistanceis not perfect.

Hardk1essto Cold

Research,and observationsof commercialplantings following freezeshave shown conclusively cold hardinessof treesis in- fluenced by rootstocks. The differences are not great but they are very meaningful. The difference of 2 or 3 degrees F of ccld hardiness often means the difference in damage or no damage and even death or survival.

There is little infonnation asto how rootstocksinfl~nces cold hardinessin the cultivar buddedon it but their influence on the dormancyof the tree is at leastpart of the reason. Many cultural practicesand climatic conditions aswell asrootstock influ. encedonnancy. Thus, the responseof treesto freezingtemperatures varies to someextent with eachfreeze. Onecan only gen- eralizeon the basisof all of the researchresults availablq and the history of tree perfonnanceon variousrootstocks following freezesover a spanof many years.

Treeson sour orange,cleopatra mandarin and trifoliate orangeattain their maximum cold hardiness.Trees on tr1foliate orange, however,are morevariable in this respect. There is evidence,although not ascomplete as would be liked, that trees on trifol;ate orangetend to go into dormancylater in the winter then do those on sour orangeand cleopatramandarin and are thereby not as tolerant of cold asare those of sour orangeand cleopatramandarin during early winter.

Treeson rough lemon and 'Palestine'sweet lime are 1f1emost sensitiveto cold, while those on sweetorange and 'Carrizo' cit- rangeare intermediate. Treeson sweetorange, however. approach the cold hardinessof those on sour orange. The general!ation hasbeen made that there is a relationshipbetween the clld hardinessof the rootstock and its hardinessto cold, with the mO$t cold hardy rootstocksimporting the most cold hardiness;to the top. This generalizationis not sufficiently consistentto be useful.

Tree.Size

There is a definite influence of rootstock on tree growth and the cumulativeeffect over the spanof severalyears may resul1: in an appreciable difference in tree size. Trees on rough lemon and 'Palestine' sweet lime are larger than those on other stocks on the deep sandy soils of central Florida. Trees on cleopatra mandarin, sour orange, sweet orange, and 'Carrizo' citrange are Inter- mediate in size while those on trifoliate orange and 'Rusk' citranQe are much smaller. .166.

Soil type greatly influences,however, me performanceof somerootstocks in this respect. As examples,citrus treeswithout exocortison trifoliate orangegrow as largeor largerthan treeson sour orangeif the soil is fertile and retentiveof moisture.Trees on cleopatramandarin grow much better when clay is closerto the surfacethan doesrough lemon,which is better when clay is closerto the surfacethan doesrough lemon,which is better adaptedto deep,sandy soils. Thus, one must define the soil condi- tions in describingthe performanceof a rootstock in relation to tree size.

The influenceof a rootstock on tree sizeis alsogreatly influencedby the virus content of the tree. Many of the resultsof past rootstock experimentsare cloudedby the fact the budwood containedcertain viruses. There are severalstrains of eachvi. rus and thesestrains vary in the severityof the symptomsthey cause.

The entire body of infonnation involving rootstock must be understoodin order to interpret and draw valid conclusionsfrom the tree responsesobserved.

There hasbeen much interest in developingdwarf citrus treesthat arecomparable to dwarf appletrees. Eventhough the vigor of growth of variouscultivars is relatedto rootstock, there is no way asyet to satisfactorilyproduce dwarf citrus. The useof cer- tain strainsof exocortisvirus to dwarf treeson trifoliate orangeis perhapsthe most promisingmeans. There hasbeen considera~e researchto demonstratethe feasibility of this procedureand plants are intentionally innoculatedwith exocortisin Australia and with xyloporosisin Israelto dwarf trees. As yet, however,completely satisfactory production of dwarf citrus treeswill not be possiblefor sometime to come.

Selecting Rootstocks

The rootstock selectedfor a commercialplanting is obviouslyof utmost importancebecause it hassuch a largeinfluence on the performanceof the tree. Moreover,selection of a rootstock is a final decisionthat cannot be changedwithout greatdlfficu!ty There are raresituations in which the rootstock is replacedby planting seedlingsaround the baseof the tree and then inarchlng them into the trunk. Treesare sometimes scion-rooted by girdling the trunk just abovethe bud union and mounding overthe girdled areawith soil. Roots developfrom the trunk and form a new root system. Replacingthe rootstock both by inarching and scion rooting areunusual cases designed to overcomedecline problems.

There is a tendencyin selectinga rootstock to oversimplify its responseto variousenvironmental conditions. For example, there is the tendencyto categorizerootstocks as either susceptibleor resistantto diseasessuch as foot rot and either hardy or sensitiveto cold, disregardingthe broad spectrumof responsesthat exist.

There is also a tendencyto placetoo much emphasis on resultsobtained from laboratory or greenhousetesting. Suchtech- niques are valuable in screening large numbers of selections in rootstock improvement programs but they are not a satisfactory substitute for field t,rials and widespread observations of tree perfom1ance in commercial plantings.

The most reasonableway to approachthe selectionof rootstocksis to first determinethe environmentin which the rootstock is to be used. Oneshould detenninethe depth of the soil to restrictivelayers such as a water table or an impermeableclay layer. Fertility of a soil and its capacityfor retainingmoisture, the needto protect againstdiseases and other pestsare all important Thp purposefor which the fruit is to be usedmay bearon the rootstock selection. For example,fruit sizeand quality may be of C1n- siderablymore importanceto thosewho producefor the fresh or gift fruit market than thosewho are producingfor process:ng.

There is often 11imiting factor that must be taken into accountabove all othersand thereby limits the choicesavailable. For example, any rootstock selected for use in the northern fringes of the Florida citrus area must impart maximum cold hardiness to the tree becauseof the occurrence of damaging freezes precludes the use of such rootstocks as rough lemon. There are areas In which a disease,such as tristeza, is so severethat those rootstocks not having the highest level of resistanceto the diseasemust be discarded. Brazil is an example of such an area.

Some cultivarsproduce fruit of suchsmall sizethat the useof rootstockssuch as cleopatra mandarin which would further accentuate the size problem cannot be tolerated. -161-

After me elimination of obviouspoor choices,there is often not much from which to choose. Evenso, the groweris men facedwim me problem of potential mreats. For example,the mreat of tristezahas been looming over the Florida industry for cwer2 decadesInd yet dlmlge from mis di.ase his beenmodest. Sour orlnge rootstock, In unthinkableselection in a tristeza lrea, hasbeen planted in Ilrge numbersdespite this mreat. Becauseof the mreat of young-treedecline on rough lemon, there hasbeen I fur1t1ersurge in me useof sour orangeand it is me leadingrootstock beingplanted today. Whetherhistory will con. tinue to bearout me wisdom of mosewho continued to u. sour orange,only time will tell. The financial condition and me personalphilosophy regardingrisks of the growerare involvedat mis point.

Thereare some MlO would hedgeagainst the possibility of certain threatsby usinga seriesof rootstocks,assuming that not all will prove to be unsatisfactory. On the other hand,such a procedurealmost guarantees that 1 or more of the rootstockswill proveto be undesirableor, at least,much lesssatisfactory than the others. Hereagain, one's personal philosophy on risks and economicsis the decidingfactor. Many prefer to makethe most intelligent selectionpossible and chancebeing wrong.

There aresome who appearattrM:ted to new rootstocks,even though theserootstocks have been tested superficially. In such a case, one should weigh the possible advantagesagainst the known performance of an established rootstock. Using a rootstock simply becauseit is new has no merit but those willing to take risks for great advantageshave often made fortunes.

In summation,one should not overemphasizesmall advantagesor take into accountgeneral advantages of a rootstock that havelittle relevanceto . givensituation. After one eliminatesall of the obviouslypoor choicesfor a carefully studiedsituation, there are generallyonly a few choicesleft. Choosingrootstocks on the basisof their unknown potential or on the blSis of I potential threat to a standardrootstock involvesthe personaleconomic condition and philosophy of the individual.