w. P. Bitters

World Citrus RootstockSituation
w. P. Bitters
Introduction
A numberof different citrus rootstocksareusedin the variouscitrus-producingareasof the world. The performanceof
eachhasbeenselectedasbestadaptedto the areain which it is used. This variation in usageis understandablein view of the
differencesin soil types,environment,water relationships,nutrition, the miscellaneous
diseasecomplexesinvolvedand other
factors. Rootstocksonceestablishedare not very susceptibleto change;however,occasionallysomethinghappenswhich requiresa change. Rootstocksdo change.Sometimesthe changeis slow and gradual,and at other times it is suddenand almost
catastrophic. The slow, evolutionarychangeswe are not worried about asthe industry hastime to adjustto them with little
consequence.The suddenor catastrophicchangesoccur right now, and unlessthe industry is prepared,havedire consequences.
Many
of the rootstock problemsfacedby citrus growersthe world overarethe sameregardlessof the country or areain
which they live. However, in a few cases,what has been a catastrophe in one area seemsto be of little economic importance in
another. For example, tristeza almost overnight wiped out citrus planted on sour orange rootstock in Africa, South Americl
and California; yet it seemsto be of little import.nce in some of the Mediterranean countries. However, it is probable thlt this
disease,and many others, will become more widespread and disastrous in areaswhere they are now minor in nature.
The ravagesof tristezain Spainat the presenttime area good example. It hasalsobeentrue in the pastandwill undoubted.
Iy be true in the future that organismscausingdiseasecan mutate to more virttlent forms,or new diseases
appearwhich necessi.
tate finding or developingnew rootstockswhich will be better suitedto meet newthreats,whateverthey may be.
A reviewof the variouscitrus areasand their current rootstock situation is thus very much in order.
Spain
Spainis the world's largestexporter of fresh citrus fruit and is exceededonly by Japanand the United Statesin citrus production. Becauseof the prevalenceof Phytophthora, nearlyall Spanishcitrus hasbeenplanted on sour orangestock. However,
tristezais rapidly spreadingin Spain,and presentestimatesarethat asmany asa million to a million and a half treesmay now
be infectedparticularly in the areaaroundAlcira and the Rio Jucar river. Much of the mandarinacreagein recentyearshas
beenplantedon 'Cleopatra',and experimentalplantingsare beingmadeon 'Troyer' and 'Carillo' citrange,sweetorangeand
trifoliate. However,most of the soilsalongthe Mediterraneancoastof Spainare too calcareousfor the successfulgrowingof
trifoliate. However,it may proveusefulin the interior of the GuadalquivirValley betweenSevillaand Cordobawhere new
plantingsarebeingestablished.
Almost all scionwood in Spainis infectedwith exocortis,and a budwoodvirus indexingprogramis beingestablished.Success
of this programwill be necessarybeforesomeof the newerrootstockscan be used. Psorosisand xyloporosisarealsowidespread.
However,the most prevalentdiseaseisPhytophthora, and in someareasearth ridgesarethrown up in a circle aroundthe trunk
to keepirrigation water awayfrom the foot of the tree.
Recently,becauseof the threat of tristeza,the Ministry of Agriculture haslicensednurseriesand prohibited the sellingof
treesbuddedon sour orange. However,this doesnot preventthe private growerfrom raisinghis own sour rootstock. Many of
the growersstill prefer sour and do not believethat tristezais a problem.
Italy
Italy is the world's fifth largest producerof citrus, the secondlargestgrowerof lemonsand the major supplierof citrus in
the EuropeanCommonMarket CitrusIrels Ire locatedin southernItaly and Sicily.
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All varietiesof citrus in Italy aregrown on sour orange. Becauseof the fear of what tristezacould do to the orangeand
mandarinindustry, researchworkershavebeenstimulatedto searchfor and test new rootstocks.
Tristezahasnot becomean economicthreat in Italy, but it hasbeenfound in backyardplantingsof 'Meyer' lemon and
Satsuma.Eradicationof thesetreeshashopefully removedor delayedthe threat. An indexingprogramfor exocortisend
psorosisis now underway,and I virus-screening
progrlm hasbeenestablished.
Researchis taking placeat Acirealeto developnucellarlines of local orangevarieties. Whensufficient virus-freebudwood
becomes available, 'Troyer' citrange may become a recommended rootstock for oranges. Other trials are being carried on to
develop lemon varieties and rootstocks more resistant to mal secco. 'Monachello' has been recommended becauseof its resistance to this disease,but bud union overgrowth limits tree life when used on sour rootstock.
Tests with Citrus volkameriana have shown advantagesin vigor and resistance
to mal seccooversour,and it is being recommended as a rootstock for lemons.
The generalpracticein Italy is to bud at a height of about 1 meter, or 40 inches. This height is usedbecauseof an extreme
fear of gummosis,which practically wiped out the citrus industry beforethe turn of the century, when most treeswereprop.
gatedon their own roots from cuttings.
Corsica
Corsica, with very limited citrus acreage,has a fine rootstock program underway, perhaps the best in the Mediterranean area.
It has fairly extensive rootstock trials with most of the world's existing or potential rootstocks and has complimented it with
the introduction of numerous nucellar lines. Their problem is to find a replacement stock for sour orange, one with both tristeza and Phytophthora tolerance. Cold resistance is a rootstock assetwhich cannot be overlooked for Corsica.
Israel
Israel, with slightly over 100,000acres,is the fourth largestcitrus-producingcountry in the Mediterraneanarea. 'Palestine'
sweet lime and sour orange are the principal rootstocks. Soil types range from sandy loams along the coast and in the northern
Negevto heavy clay in the Jezreel Valley. In some areas,light, alluvial soil overlays heavier soil.
It is commonpracticein light soilsto bud on sweetlime for early production and to inarchwith sour, usuallybeforethe
treesare 10 yearsof age. This is done becausetreesleft on sweetlime invariablydeclinefrom xyloporosis. In heavysoils,trees
may be plantedon sour from the beginning.
Tristezanow occursin Israelicitrusorchards.So far the plan hasbeento eradicateaffectedtreesasthey appearand some
indexingis beingdonefor symptomlesscarriers. This plan would delaythe onslaughtof tristezabut is not prKtical for controlling it They will haveto switch to tolerant rootstocks. Somework hasbeendone on nucellarlines but they are not in
generaluseasthey havenot beenas fruitful and aremore prone to alternatebearing.
An extensive
researchprogramis beingcarriedon in Israelin all phasesof citrus culture, rootstocksincluded. It is felt that
more information is neededon the nutritional requirementsof different rootstock-scioncombinations,and researchon this is
beingexpanded. Other researchis directedtoward predictingthe compatibility and performanceof rootstock-scioncombinationsfrom juvenile charKteristicsor chemicalanalysisof tissue. If this shouldprovesuccessful,it would help eliminatethe
longwait now necessary
to evaluatematuretrees. A closewatch is beingkept on diseaseproblemswhich could becomea
threat to the industry.
Greece
In 1963, Greece had approximately85,000 acresof citrus, of which about 48,000 were oranges,19,000were lemonsand
18,000weremandarins.A greatdeal of Greece'scitrus production is used for homeconsumption,and that which is exported
goesto eastern European markets and does not compete with the western European economic community. The major orange
.3variety plantedin Greecesince1955 hasbeenthe 'Washington'navel. Many of the older, commonorangeplantingshave
beentopworked to navelvarieties.
The predominant rootstock in Greece for all citrus is sour orange. Attempts have been made in the case of lemons to use
an interstock of sweet orange in the hope that this will give resistanceto mal secco. Such attempts have been unsuccessful.
As in the rest of the Mediterranean area, psorosis and exocortis are also present. Since all plantings are on sour, tristeza could
rapidly become a major problem. No major rootstock trials are underway. Frost can be a problem and water is somewhat
limited and of poor quality.
Lebanon
Lebanon hasbetween25 and 30 thousandacresof citrus. Areassuitablefor citrus are limited, and little expansionis expected. As in Israel, the principal rootstocks are sweet lime and sour orange. Important diseasesare Phytophthora, xyloporosis and psorosis. No rootstock trials are underway.
Algeria
Citrus plantings in Algeria increased rapidly after World War II and reached 84,000 acres by 1960. However, during the
civil strife of the 1960's, few plantings were made and some groves were abandoned so that citrus acreagedeclined. During
the past few years, however, citrus acreagehas again increased. Algerian soils are mostly heavy loams, although there is a
anall coastal lemon area north of Algiers which has light, sandy loam soil. In some areas, the water table is only a few feet
from the surface, and drainage ditches and tiling are used. Many commercial varieties have been introduced into Algeria.
These include the 'Hamlin', 'Washington' navel, 'Cadenera', 'Thompson' navel, 'Shamouti' and many Portuguese and Spanish
varieties. 'Marsh' grapefruit has been introduced as have the 'Eureka' and 'Berna' lemons. There are smaller plantings of
'Lisbon', 'Meyer' and Italian varieties of lemons.
Sour orangeis the predominantrootstock. However,'Clementine',which is the secondlargestcitrus crop in Algeria,is
often plantedon trifoliate orangein heaviersoilswherehigh water tablesexist. Phytophthoraand foot rot are presentin
most Algeriangroves,and treesare buddedabout 2 feet abovegroundlevel. No rootstock trials areevident.
Morocco
In Morocco,citrus acreagehasgrown at an increasingrate during the pastfew years,with production mainly concentrated
on oranges.Varietiesaresimilar to thosein Algeria. The most important mandarinis the 'Clementine'. Grapefruit plantings
areprimarily 'Marsh',and lemonsarethe 'Eureka' type. Most Moroccancitrus soilsare light, well-drainedloams. Salinity is
a problem in someareas. Diseases
arepsorosis,Phytophthora, Rio Grandegummosis,stubborn disease,xyloporosisand exocortis. Tristezahasbeenidentified but hasnot reachedeconomicsignificance.Presentcitrus acreageis estimatedto be 180,
000 acres.
In 1969, a MoroccanAgricultural ServiceSociety,which is an advisorybureaucontainingresearchand extensionpersonnel,
wasestablished.Purposeof the societyis to carry out researchwork which will benefit Moroccancitrus culture. The principal rootstock is sour orange;however,new rootstocksare beingintroducedand tried.
Egypt
Egyptian citrus acreageis now estimated to be between 85 and 90 thousand acres. The principal orange varieties grown are
local selections called 'Beladi'.
Egyptian citrus soilsarefertile and consistof deep,alluvial silt and clay loams. Grovesareirrigated by permanentbasinsor
sprinklersystems.The principal rootstock usedis sour orange,althoughsweetlime is usedin lighter soils. Diseases
include
Phytophthora,foot rot, psorosisand xyloporosis. It might be expectedthat tristezaalsoexists,sinceit is presentin other areas
4
in the Mediterraneanbasin. However,tristezahasnot becomean economicproblem. Thereare no major rootstock trials
underway.
Cyprus
Major varietiesin Cyprusarethe 'Shamouti' and 'Valencia'orange,'Marsh'grapefruitand the local sour lemon which re~mbles the Lisbon. The 'Clementine'mandarin,'Temple' orangeand somevarietiesof satsumahavealsobeenintroduced.
Soilsaregenerallylight in texture in citrus areas,and the major rootstocksarethe sour orangeand sweetlime. There is a
major problemwith Phytophthora. Wateris scarceand of rather poor quality. A few new rootstock types haverecently
beenintroduced.
Tunisia
While citrushasbeengrown in Tunisiafor a long time, it hasonly beenwithin the pastfew yearsthat a sizeablecommercial industry hasbecomeestablished.Eventhis is small by presentday standards.
When budded, trees are usually on sour orange; however, most of the native groves are seedling trees which are densely
planted,200to 400treesperacre,smallin sizeandheavilypruned.
The climate in Tunisiais arid with low humidity and greatextremesin temperature. Irrigation water is limited and salinity is a problem. Periodsof drought often occur. Tunisiansoilsare light textured and covercropsaregrown in someareas
to preventwind erosion. It is reportedthat few virus diseases
occur in seedlinggroves. However,both Phytophthora and
Armillaria arepresentand psorosisis presentin vegetativelypropagatedorchards. No real rootstock trials exist.
Turkey
In Turkey, the citrus industry is being modernized, and today there are over 100,000 acres of citrus in this country.
Orangesare the most important, followed by mandarins, lemons, and grapefruit. Nearly all budded citrus in Turkey is
grown on sour orange rootstock, with the exception of satsuma mandarin which is grown on trifoliate. Major diseasesare
Phytophthora on all citrus and mal secto on lemons. Psorosis,stubborn, xyloporosis, and exocortis are present. Tristezalike symptoms have been observed, indicating the virus may be present and a threat to Turkish orange plantings. No rootstock plantings are in evidence.
Middle East
In Iran, orangesaregrown on their own roots or arebuddedon sour orange,rough lemon,or sweetlime. Sour limesand
lemonsare grown entirely from seedlings.A similar situation existsin Iraq, and nematodeshavebeenmentionedasa serious
problem. The Ministry of Agriculture hasimported largequantitiesof 'Troyer' citrangeseedfor trial. No nucellarbudlines
areavailable.
India
India, which is secondonly to Chinain beingthe oldestcitrus-growingareain the world, hasprovidedthe industry with
severalimportant commercialrootstocks. Amongtheseare rough lemon,sour orange,'Rangpur' lime (which is really an
Kid mandarin)and mitha the sweetlemon. Gajanimma,alsocalledCitrusmoi and Citruspennivesiculata,'Kharna Khatta'
(Citruskharna)and nasnaran,Citrusamblycarpaarealsorootstocksof lesserimportance.
There
are about 250,000 acresof citrus undercultivation in India, most of which is in small plantingsand much of which
is double-cropped or more.
Citrusdiseases
affecting rootst~ks in India includePhytophthora,tristeza,psorosis,xyloporosisand citrus canker. A
seriousthre.t is citrus diebackwhich is a complexproblemmaybe due to a mycoplasmsuchasgreeningbut which is terribly
complicatedby nematodes,fungi and nutritional problems.
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Roughlemon or Citrusjambhiri is the most widely usedrootstock for orangesand mandarinsin India. 'Rangpur' lime is
alsousedasa rootstock, especiallyin BombayStatewhereit is usedwith the 'Mosambi'orange. C. Kharnais alsousedextensivelyexceptunder blood orangeswith which it is incompatible. 'Malta red' is a1soincompatibleon roughlemon stock.
A vigorousrootstock of somelocal importanceis 'Gajanimma'or C.pennivesiculata.It hasalsoattractedsomeattention in
California,but is very susceptibleto tristeza,stem-pittingseverely. It is alsoextremelysusceptibleto cold. The 'Kharna
Khatta' is very susceptibleto gummosis.The roughlemon is susceptibleto gummosisto about the samedegreeor perhaps
a little less than sweet orange or grapefruit. Rough lemon is also susceptible to Armillaria or oak root fungus. It is tolerant
to tristeza,cachexiaand exocortis. It is susceptibleto nematodes.Roughlemonwill grow fairly well on all soils but is particularly adaptedfor usein light sandyand sandyloam soils. It is averageasto salt tolerancebut will not do well in poorly
drainedsoil asit will not stand"waterlogging"for any length of time. It is more drought resistantthan either sour or sweet
orange. As a replantin former citrus soils its growth hasbeenaverageto good. Fruit quality is poor.
Many rootstock trials have been conductedin India. Their resultsaresomewhatinconclusiveand rough lemon remains
the rootstock of note. There are many forms of the rough lemon and variable performance in part can be attributed to the
genetic differences between these cultivars. Of all the varieties coming out of India the nasnaran or C. amblycarpa has shown
the most interest and best performance in California. More will be said about the 'Rangpur' lime later.
Pakistan
About 92 per cent of the 40,000 acresof citrus grown in Pakistanis in west Pakistan. Rootstocks,varietiesand disease
problemsaresimilar to thosein India. Fairly extensiverootstock trials havebeenconductedin Pakistanbut in generalthey
were of the samenatureasthoseconductedin India and the conclusionsaresimilar.
China
Certainly
majorcitrusdiseases
suchasPhytophthora,
tristeza,psorosis,exocortisand xyloporosisarepresentin China.
In the past, Citrussunkihasbeenthe principal rootstock in China. C. sunkiis mentioned
in the literature asbeinga dwarf-
ing stock. However,in trials in CaliforniawhereC.sunkihasbeengrown on rich, loam soils,it hasnot provedto be dwarfing. It is, therefore,probablethat the cultural conditionsunderwhich citrus is grown in Chinaare responsiblefor the
dwarfingeffectof sunki. In southeasternAsia,sunki is close-plantedon ridgesonly a few feet abovethe water table. Principal varieties grown on C. sunki arethe 'Swatow' orange,alsoknown asthe 'Ponkan', and anothermandarinorange,the
'Tankan'.
Citrussunkiis a sour mandarinin Swingle'sclassification.
In China,sunki is reportedto be slow growingand similar to ponki, which is similar to the 'Cleopatra'mandarin.
Sunki is tolerant to tristezaand, if like 'Cleopatra',tolerant of exocortisand susceptibleto xyloporosisandPhytophthora.
Sunki is short-lived, about 10-12 years, in the Orient, and fruit size and quality are averagefrom trees grown on it.
The Satsumamandarinis alsogrown, usuallyon trifoliate rootstock (Poncirustrifo/iata).
While a largepart of the southernplantingsin mainlandChinaarenearthe seain heavysoils,inland citrus-growingareas,
which areusuallyin river valleys,havesandy,alluvial and loam soils.
In addition to nativecitrus, a numberof westernvarietiessuchas'Washington'navel,'Valencia'and 'Shamouti' orange
havebeenintroduced,aswell as'Marsh'grapefruit,'Eureka' and 'lisbon' lemon.
Although buddingand grafting arecommon,propagationby marcottingand cuttings is alsoused. No rootstock trials
are known.
.6.
Thailand
In southeastern
Asiamarcottage(aeriallayering)is the commonmethod of propagation. This is true in Thailandwhere95
per cent of the citrus, whether pummelosor mandarins,is grown this way. One of the reasonsmarcottingis so popular is that
it avoidsany unfavorablereactionoften experiencedwhen the scionwood is virus-infectedand susceptiblerootstocksare used.
However,in recentyearsThailand,morethan any other country in southeastAsia,hasbeenexperimentingwith modern
me'dlodsof production which includesrootstock trials aswell asfertilization trials and the introduction of modernpesticides.
A considerablenumberof pummelosaregrown in Thailand,principally on their own roots. Pummeloshavea high resistance
to Phytophthoraand aregrown on soil bedsa few feet abovethe water table underconditionswherebrackishwater is a problem. It is questionablewhetherfew other stockswould toleratethis type of environmentaslong asthe pummelodoes.
In northern Thailandthere tendsto be drought at certain seasons.All of Thailandis characterizedby rather heavycover
crops.
Pummelo hasneverbeen successfulasa rootstock in trials in Californiabecauseof its susceptibilityto tristeza. Yieldsalso
tended to be light in relation to tree size.
Rootstocksin Japan
Japan
is the world's largestproducerof mandarin-typecitrus. Satsumais widely grown,and the 2 majorvarietiesarethe
'Wase', an early type, and the 'Owari', a late type. A summer orange, called 'Natsudaidai', is also grown. Ninety per cent of
the satsumasare on P. trifoliata. The remainder are on 'YUlU'. In many cases,'YUlU' is used as an inarching stock for satsuma
on trifoliate after the trees are about 10 years of age and begin to decline.
While tristeza,psorosis,exocortisand other diseases
are recognized,they do not seemto be a seriousproblem. A virusindexing program for scion budwood sources is administered on a national basis in Japan.
Trifoliate orange,the number1 rootstock in Japan,is alsothe world's third most widely usedrootstock. Trifoliate is an
excellentstock for mandarins,particularly on heaviersoils. It is resistantto Phytophthora andArmillaria and tolerant of
tristezaand cachexia(xyloporosis). Trifoliate is, however,susceptibleto exocortit. It is tolerant of nematodesand does
well in heavy,poorly drainedsoils. It is very resistantto cold and makesgood growth asa replant in old citrus soils but has
a low toleranceto salt and calcareoussoils. Fruit quality of orangesand mandarinsgrown on trifoliate is excellent. Trifoliate
is not compatiblewith lemonsand is a poor stock for grapefruit.
'Yuzu' is usedextensivelyin Japanasa rootstockfor orangesand asa secondarystock for satsuma.'Yuzu' hasbeenused
in Japanfor centuriesand wastheir principal stock prior to the acceptanceof trifoliate. Treeson trifoliate aremore precociousin bearingand initially grow more rapidly than treeson 'Yuzu'. However,in Japantreeson trifoliate stock sometimes
beginto declineafter 10 to 20 years,and it is the practiceto inarchthesetreeswith 'Yuzu' assoon asgrowth seemsto be
retarded. The decliningtreesrecoverand becomelong-livedand productive.
'Yuzu' doesbeston deep,rich soils,sinceit producesan extensivetaproot. Gummosisresistanceof 'Yuzu' is comparable
to sour orangeor better. It is not affectedby citrus cankerand showsconsiderableresistanceto oak root fungus. It grows
relativelywell asa replant. In California,its replant growth is comparableto sour, but it doesnot show resistanceto the
citrus nematodeasdoestrifoliate. Treeson 'Yuzu' are nearlyasresistantto frost astreeson trifoliate orange. Since'Yuzu'
doeswell in Japanwheretristezais widespread,one may assumethat it is tolerant of this disease.
Indicationsin California arethat 'Yuzu' would probably be a good stock to try in calcareoussoils,sinceit picks up more
iron and lesscalcium. On the other hand,it picked up more boron and sodiumthan other rootstocksand shouldnot be used
undersalineconditions.
1.
In general,
Japanseemsto havevery little in the way of rootstock problems. They haveescapedthe ravagesof tristeza,
primarily becauseof their widespreaduseof tolerant rootstocks. Poor resultshavebeenattainedwith 'Vamamikan','Gir.
imikan' and 'Tachibana'.
Taiwan
Taiwan
is anotherimportant citrus-producingcountry in Asia. Sincecitrus is a valuableexport commodity, the industry
has been expanding and will probably continue to do so since production efficiency is increasing.
'Ponkan' and 'Tankan' makeup 83 per cent of the commercialcitrus production. C. sunki, which we havealreadydiscussed,is the principal rootstockfor thesevarieties. Satsumamandaril1saregrown,with trifoliate orangebeingthe favored
stock. Somepummelosaregrown,and thesearepropagatedby marcottingand are planted on their own roots.
Phytophthorais 1 of the principal diseases,although all of the common virus diseasesare present. Likubin, or yellow
shoot is also prevalent. A nursery-tree certification program is operated by the Taiwan Department of Agriculture. Mother
trees are selected for yield performance and are virus indexed as pedigreed sources of propagating material. Nurseries are
registered and inspected. Little rootstock work is conducted.
Philippines
In general,citrus plantingsin the Philippinesaresmall,and manytreesaregrown on their own roots. However,there
hasbeensomeinterestin trying new rootstocks,and theseare principally the rootstocksthat havedevelopedand tried
elsewherein the world. At the presenttime, many of the treesthat are buddedin the Philippinesare on calamondin(Citrus
mitis).
Major citrus diseases
arePhytophthora,foot rot, psorosis,xyloporosis(cachexia),tristeza,citrus cankerand leaf mottle
yellows. Phytophthora has caused the loss of many trees during recent years and has spurred the use of rootstocks. Because
of Phytophthora, the height of budding has been increased to 12 to 18 inches above ground level. Cover crops also increase
the incidenceof Phytophthora.
Australia
In 1966 it wasreportedthat there were 76,000 acresof citrus in Australia,and that acreagewasincreasingat an annual
rate of 4 per cent. Australiancitrus consistsof about 50 per cent 'Valencia'oranges,30 per cent navels,10 per cent lemons
and limesand 10 per cent grapefruit,mandarinsand other minor citrus.
Theclimatein whichcitrusis grown in Australiarangesfrom humid tropical to arid subtropicalwith the greaterpart of
the acreagebeingin the latter lone. In general,soilsin the citrus areasarecalcareous,alkaline,high in naturalsalinity and
rangefrom sandto sandyloam in texture.
In the past,rough lemon hasbeenthe main rootstock becauseof its vigor and drought resistance.However,other rootstocksare now under trial and are beingusedin commercialpractice. Theseincludesweetorange,'Carillo' citrangeand
trifoliate. Rootstocksunder trial in experimentalplantingsinclude 'Cleopatra'and 'Emperor' mandarins,'Sampson'and
'Orlando' tangelosand 'Troyer' citrange.
Phytophthora root rot is present and its spread is mainly blamed on faulty irrigation practices. Virus diseasessuch as
exocortis and psorosis occur but are avoided by a scion selection program. Tristeza virus is also present in Australia, but
lack of susceptible rootstocks
make~ it of little economic importance.
New Zealand
Total citrus acreagein New Zealand is relatively small, being about 2,000 acres. However, citrus plantings have increased
by over 50 per cent during the past 5 years, primarily becauseof the introduction and use of trifoliate as a rootstock.
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Becauseof unfavorableclimatic conditionsand the useof rough lemon asrootstocks,solublesolidsin orangestendedto
be low and acid high, givinga poor quality fruit. However,the useof trifoliate asa rootstock causeda markedimprovement
in the solublesolidscontent and improvedfruit quality. As a result,this fruit hasfound a market and there hasbeenan upsurgeof orangeplantings.
SouthernandCentralAfrica
In the Republic of South Africa, almost all buddedcitrus treesare on rough lemon becauseof the devastatinginfluenceof
tristeza. Recentlya numberof test plantingshavebeenmadewith new rootstockswhich includesweetorange,'Empress'mandarin, 'Troyer' citrange,and trifoliate orange.Thereis recentinterestin a hybrid of 'Troyer' citrangeand 'Cleopatra'mandarin
4t639. In addition to tristeza,exocortisand xyloporosisarewidely distributed in most old-linecitrus varieties. So far, they have
beenof minor economicimportancebecauseof the toleranceof most scion-rootstockcombinations.
Psorosishasbeenalmostcompletelyeliminatedin South Africa becauseof an efficient eradicationprogramcarried out by
the South African Departmentof Agriculture. This programwasstartedin 1927,and at the presenttime hardly any psorosisinfectedcitrus treesare to be found.
In southernRhodesia,rough lemon is againthe primary rootstock, althoughexperimentalplantingshavebeenmadewith
trifoliate orangeand 'Troyer' citrange. Trifoliate orangemay proveto be of value,particularly in view of the heaviersoilsand
extensiverainfall which occur in many citrus-growingareas.
Citrusis grownfor local consumptionin many of the newly createdAfrican countries. In almostall cases,the principal
rootstock is rough lemon. Ghanasupporteda lime industry, the treesof which werealmostentirely on their own roots. In
1941,tristezavirus decimatedthe industry, and thesetreeshavegraduallybeenreplacedwith healthy scionmaterial on rough
lemon rootstock. This wasalsotrue of the Gold Coast(Ghana).Phytophthoraand tristezahavebeenthe 2 limiting diseases
in most instancesin lime culture.
South America
The wide-scalecommercialdevelopmentof citrus which beganat the turn of the century in South Americawas basedon
citrus buddedon selectedrootstocks. Sincethen, the citrus industry of South Americahasexperiencedseveralsetbacks,most
of which havebeenrelatedto rootstock problems.
In 1900, almostall plantingsmadein South Americawere on sweetorangerootstock. However,lossesfrom Phytophthora
were immense,and by 1915 sour orangewasrecommendedand beingplanted asthe bestrootstock for citrus. For the next
quarter of a century, sourwasalmostuniversallyused. The onslaughtof tristezain the mid-1930'scausedthe lossof more
than 20 million treesin Argentina,Brazil,Paraguay,and Uruguay. While badly hurt, the citrus industry wasnot wiped out
Newplantingswereestablishedon rootstockstolerant to the tristezavirus. In Brazil, the 'Rangpur' lime hasbecomethe major stock. Sweetlime and trifoliate orangearealsoused,particularly in Argentinaand Uruguay,respectively.
In Peru, sour rough lemon and 'Cleopatra' mandarin are being used in the drier coastal areaswhere sandy soils prevail, and
'Rangpur' lime and 'Troyer' are used in the more humid tropic regions.
Major rootstock researchstudieswere startedin tf1elate 1920'swhen trials were installedsimultaneouslyin Brazil, Argentina, and Surinam. Sincethen over 100 rootstock trials havebeenestablishedin theseand other South Americancountries,
principally Brazil.
In usingtristeza-resistantrootstocks,it hasbecomevery important that virus-freebudwoodbe used,and constantindexing
programsarenecessary.For example,tristeza-tolerant,'Rangpur'and other mandarinlimesare intolerant of exocortis and
xyloporosis(cachexia).Trifoliate and most of its hybrids aresusceptibleto exocortis. Most tangerinesand tangelosaresusceptibleto xyloporosis. Thus,there hasbeenan increasingtrend toward the useof nucellarlines of budwood.
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Original trials in the searchfor tristeza-resistantrootstocksweremainly interestedin diseaseresistance
with little regard
to fruit quality. Now that tolerant stocks have been found, attention has been turned to resistant stocks with improved fruit
quality
.
In SouthAmerica,a greaterdiversity of tristeza-resistantrootstocksarein usethan in perhapsany other part of the world.
Thisis primarilybecausesuchrootstocksarechosenfor 1fteirbestperformanceunder a variety of climatic andecologicalcon.
ditions.
'Rangpur'lime, which we havesaid beforeis really an acid mandarin,is usedextensivelyin South Americaand very little
elsewherein the world.
In trials in California,with 'Valencia'scion,it wasthe most prolific bearerof 25 rootstocksin the test. However,fruit
quality waslow, beingonly slightly better than fruit grown on roughlemon. Fruit sizewasgood. 'Rangpur' is susceptible
to exocortisandxyloporosis,a little lesssusceptibleto Phytophthora than sweetorange,and resistantto tristeza. Its salt
toleranceis moderate,and it is similar to sweetin cold hardiness.It hashad no observedcompatibility problemswith oranges
and mandarinsbut should not be usedwith lemons. All in all, it hasbeena good backupstock for usein South America where
tristezahasbecomeendemic.
British Honduras- Jamaica- Trinidad
In British Honduras, Jamaica, and Trinidad, as in many tropical areasof high rainfall, foot rot caused by one or more species
of Phytophthora is the major diseaseproblem connected with rootstocks. Sour orange is the principal rootstock, although some
sweet orange is used. In some of the heavier Jamaican soils, a local shaddock known as "Hog Shaddock" servesas a rootstock.
Mexico
Sour orangeis the major stock usedin Mexico. However,sweetis alsousedand in someareasnumerousseedlingtreesare
grown on their own roots. Seedlingorchardsare numerousin the state of Veracruzwherecitrus is interplantedwith coffee.
Productionfrom suchorchardsis low, rangingfrom 60 to 125 boxesper acre.
Some of the best cultural practices are said to occur in the state of Nuevo Leon. New plantings there are mainly 'Valencia'
orange budded on sour. Again, foot rot and other diseasescaused by Phytophthora are serious, particularly in the areas of
high rainfall and humidity. The 'Cleopatra' mandarin and the 'Troyer' citrange were recently introduced and as yet there is
no report on their performance.
United States
In the United States,the main citrus producingstatesare Florida, California,Arizona,and Texas.
Many of the early orangegrovesin Californiawereseedlingtreeson their own roots, and a few of theseorehardsremained
until World War II. However, many of the early California orehards were planted on sweet orange. When Phytophthora became a problem, sour orange became a dominant stock and remained so until the advent of tristeza.
Most of the diseases
which attack rootstocksof citrus arepresentin California. For example,during the wet yearsof 1938,
1941,and 1943, Klotz estimatedthat almosta million treesin Californiawere infectedwith brown rot gummosisor foot rot.
Othertypesof gummosis
arealsopresent.
Psorosisand exocortisalsoinfect Californiaorchards,but xyloporosisis not a problem.
Sweetorange growswell on fairly heavy soils, but is best adapted for growth on rich, sandy loams. It does not do well on
extremely sandy soils or extremely heavy or calcareous soils. On heavy, poorly drained soils it may show iron chlorosis symp.
toms, as it also does on calcareous soils. It does not develop a well-differentiated taproot as does sour, and is moderately
shallow-rooted. It does develop an abundant system of lateral roots which generally are deeper than those of rough lemon.
.10-
Scionsbudded on sweetorangeproducelarge,vigoroustreeswith all commercialvarieties. The treesare largerthan when
similar scions are budded on sour orange. Yields on sweet orange are good --the highest with all scion varieties except when
navel orangesare budded on sour or 'Morton' citrange which outyields sweet Trees budded on sweet orange are not as precocious in bearing as trees on trifoliate orange or Citrus macrophylla, but trees on sweet are long-lived and bear well into advanced age of 50 or 60 years. Fruits from trees on sweet are thin-skinned, juicy, and of high quality, hotd up well on the tree,
and also store well. Per cent juice, solub1esolids, and citric acid are essentially identical to those obtained on sour orange and
are better than from fruits grown on rough lemon. However, fruit quality is not as good as when scions are grown on trifoliate orange or 'Savage' citrange. Granulation of fruit from trees on sweet orange is not a serious problem. Fruit sizes from
trees on sweet tend to run somewhat smaller than average,although they are larger than when 'Cleopatra' mandarin is a rootstock but smaller than when rough lemon and sour orange is used.
Trees on sweetorangeareextremelysusceptibleto gummosis.This is true evenwhen the treesare buddedhigh, planted
high, and the best cultural practjces are used. This is the greatest disadvantagethat sweet orange has. It seemsremarkable,
considering the susceptibility of sweet orange to Phytophthora, that so many seedling orchards survived in the early history
of California and Florida. Sweet orange is the most susceptible commercial stock to this disease,being somewhat worse than
roughlemon and not nearlyastolerant as sour orange,C. macrophylla,or 'Troyer' citrange.
Sweetorangeperformsvery poorly asa replantin old citrussoil, eventhough the soil hasbeenfumigatedand goodcultural
practicesareused. Sweetorangestock is especiallyprone to collar rot and haspoor root regenerationcapacityonceinfected
Nth Phytophthora. Thereis somevariation in susceptibilityto Phytophthora
betweenvariouscultivars,and 'Indian River'
Meet seemsmore resistantthan others. However,this may be of little practicalimportance. A sweetorangerootstock resis18nt to Phytophthora
wouldbe a greatassetto the citrus industry.
Sweetorange rootstocksare resistantto sour orangescaband to mal seccowhich are not problemsin California. Psorosis
or scalybark is more seriouson sweetorangethan it is on the resistant sour orangestock. lemon treeson sweetdo not expressshell bark symptomswith the sameseveritythat treesdo on sour orangeor roughlemon. Sweetorangeis not affected
by exocortis,cachexia(xyloporosis),or woody gall. Reactionof sweetorangeto tristezainoculationshavegenerallybeen
negative,so that most cultivars of sweetaretolerant of tristeza. Sweetorangestock is very susceptible,however,to citrus
nematode,althoughcertainselectionslike the 'Pineapple'and 'SanguineGrosseRonde'haveshown resistanceto the burrowing nematodewhich doesnot exist in Californiabut is presentin Florida.
Sweet orange is not an answerto the world's rootstock problems,but it-hasso many good qualitiesthat its germplasm
should be perpetuatedin other rootstockssuchasin hybridswith trifoliate orangewhich haveresultedin citrangessuchas
'Troyer' and 'Carillo'.
In California,almostall plantingsof orangesbeingmadetoday are on 'Troyer' or, in somecases,'Carilla'. There is some
interestin trifoliate orangein the heaviersoils of the SanJoaquinValley. Almost all lemonsbeingplantedare on C. macrophylla. However,there is someuseof rough lemonfor grapefruitand lemonsin the sandysoils in the desertareas.
The reasonfor the switch to 'Troyer' citrangeasa primary rootstock for orangesis its toleranceto tristeza,coupledwith
resistanceto Phytophthora. It is, however,susceptibleto exocortis,and it must be usedwith budwoodfree from this virus.
'Troyer' is susceptibleto Armillaria and to nematodes.Fruit quality and production are good with oranges,mandarins,
and grapefruit. Most lemons, with the exception of 'Lisbon', suffer from bud union difficulties when grown on 'Troyer'.
'Troyer'is aboveaverageasto cold hardiness,low in salt tolerance,and doesnot standup well underconditions of poor
soil drainage. Its replantgrowth in old citrus soilsis good,asis its longevity.
'Carillo' citrange,which camefrom the sameseriesof crossesthat gaveriseto 'Troyer', is very similar to 'Troyer', al.
though in sometestsit showsmore resistanceto citrus nematode. For this reason,it is now beingplantedin preferenceto
'Troyer' in someareasof California.
.11
Researchtrials on rootstocksindicatedthat C. macrophy//amight be a good rootstock for lemonsunder Californiaconditions,
and most of the lemons now being planted in California and Arizona are on this rootstock:
While macrophyllabudswell to all varietiesof citrus, its susceptibilityto tristezaand cachexiarule it out for usein most areas
exceptfor lemonsand limes.
Macrophylla is highly resistant to Phytophthora and tolerant of exocortis. It does well on all soil types from sandy to heavy
clay loams and has a high degree of resistance to calcareous and saline soils. It shows perhaps the highest boron tolerance of all
citrus rootstocks, and it is also capable of absorbing certain other micronutrients in slightly greater quantities than other citrus.
Hence, trees on macrophylla are deep green in color and seldom show leaf patterns that denote micronutrient deficiencies.
Young lemontreeson macrophyllaarevigorousand precociousin bearing. Oneproblemis that fruiting is so heavythat severe limb breakage occurs. Bud unions have been good with all scion varieties tested, and yields have been heavy with navels,
'Valencias', 'Oancy' tangerine, grapefruit, 'Eureka' and 'lisbon' lemons, and lime. Fruit from trees grown on macrophylla are
larger than average. However, fruit quality is below averageand with some varieties can even be classified as poor. 'Dancy',
for example, granulates badly when macrophylla is used as a rootstock. Solub1esolids and citric acid from fruit of trees grown
on macrophylla are low, being comparable to fruit grown on rough lemon. However, as mentioned before, recommendations
are for use only with lemons and limes.
Treevigor is goodwhile the treesareyoung, but older treesshow no more, and perhapsevenless,vigor than treesbudded
on rough lemon. lemon treeson macrophyllaare relativelyshort lived. 'Eureka' on this stock hasa productivelife of 20 to
25 years,and the life of 'lisbons' may be only slightly longer.
Macrophylla is very sensitive to cold, and trees on this rootstock do not fare as well as trees on rough lemon when subjected
to freezing temperatures.
Macrophyllais an interestingrootstock, and hybridswith trifoliate orangemay combinethe goodqualitiesof both. Testing
of suchhybrids is in progressin Californiarootstock trials.
Arizona
Citrusgrowingin Arizona is centeredin 3 districts. The lower ColoradoRiver Valley nearYuma,the Weliton-Mohawkarea
eastof Yumaand the Salt River Valley areanearPhoenix.
Arizona now hasover 50,000acresof citrus and rough lemon is and hasbeenthe primary rootstock usedfor oranges,lemons
and grapefruit in the Yuma and Weliton-Mohawk areas. Otherwise sour orange was used. Recently 'Troyer' and mocrophylla
have been tried for oranges and lemons respectively. Some 'Cleopatra' mandarin has also been used as a rootstock.
Diseaseswhich affect rootstocksinclude Phytophthora,psorosis,exocortis,tristezaand a severetype of gummosisknown as
Rio Grande gummosis. So far this diseasehas primarily attacked grapefruit trees. Rio Grande gummosis also exists in the desert
areasof California and has been a problem in recent years in some grapefruit orchards. Tristeza, while present, has caused no
seriousproblems.
Soilsin citrus growingareas of Arizona rangefrom sandand silt alongt~e ColoradoRiver areasnearYuma to sandy,clay
loamsoilsin the Phoenixdistrict. Salt accumulationand iron chlorosisareproblemsin someareas.Small rootstock trials
exixt atYuma and Phoenix.
Texas
The citrus industry of Texashasbeenbadly hurt severaltimes during the pastcentury by severefreezes.Eachtime, however,
the industry hasreboundedwith increasedplantingsof both orangesand grapefruit. The principal rootstock hasbeensour orangefor both grapefruitand oranges.Texasgrapefruitis of high quality and the fruit hasbeensold at a premium on the market.
-12.
Plantingssincethe 1962 freeze,which nearlydestroyedthe industry, havebeenabout two-thirds red ('Ruby') grapefruit
and about one-thirdof other varietiesof citrus. The orangesgrown areprimarily varietiesfrom Florida.
Salinityis frequently a probfemin Texasorchardsand considerableresearchhasbeendonetoward finding more salt tolerant rootstocks.
. Rio Grandegummosisand foot rot areseriousdiseaseproblemsin Texas. Psorosis,cachexia,and other virus diseases
are
alsopresent
Florida
Florida is now the largestcitrus-growing area in the wortd. It produces almost 2/3 of the world's grapefruit and about 25
per cent of the world's oranges. Also, most limes, tangelos and tangerines which are produced in the United States are grown
in Florida.
You will seemuch of Florida citrus and its problemson the field tours so I will spendvery little time on the rootstock situation here,sincethere are otherspresentwho arecertainly more qualified to talk about local conditionsthan I.
The predominantrootstock in Florida is rough lemonwhich is believedto havereachedhereearly in the 19th century. On
deep,well-drained,sandysoilsit produceslargerand moreproductivetreesin a shorter period of time than any other rootstock.
Sour orangeis the secondmost important rootstock and is preferredfor low hammockand flatwood soils. Other rootstocks
cominginto useinclude'Cleopatra'mandarin,'Rangpur' lime, trifoliate orange,and 'Troyer' and 'Carillo' citrange.
Major citrusdiseases
includepsorosis,exocortis,brown rot gummosis,tristeza,Armillaria, Clitocyberoot rot aswell asothers.
-13-
ROOTSTOCKSHORTCOURSE
11:15 Discussion
Monday,September24
Questionsfor Dr. W.'. Bitters
question:
I would like to ask Dr. Bitters to giveus his experienceon lemonson roughlemon rootstock in California.
How much doesscalybark and psorosisreducethe overallperformance?
answer:
Depending upon the soil type and the areain which they aregrowing,suchtreesmay beginto declineat 10
or 12 years of age and may be out of production by the time they are 20 years of age. They are generally
very short-lived. Shell bark hasbeena very seriousproblem on them. Shell bark of treeson rough lemon is
perhaps severeas it is on any rootstock combination.
question:
The trees we have now are 6 years old. We are experiencing some severeshell bark and gummosis. Would
you say that they will grow to be 20 years old?
answer:
They could survivethis long with the shellbark but I don't know with the gummosis.
question:
What is the bestway of keepingthe greening diseaseout of Florida and California?
answer:
Dr.E.C.Calavan
answering:Wewill needcontinuedquarantineand a very carefulenforcementof regulations.
~e5tion:
I hearthat lemon treeson sour orangeroot are not affected by tristeza. wonder if you had observedthat
answer:
I think this is correct. Lemontreesare not affectedby tristezaon sour orangeroots. I think one of the principal reasonsis that the vector itself doesnot feed on the lemonfoliageat leastunder Californiaconditions
and the secondthing is that the lemonis not a very favorablehost for the build-up of the virus.
question:
You mentionedthat in Israelthat they eradicatetreesto the point of eliminatingtristeza. thought that was
a hopelesstask.
answer:
I think it may be a hopelesstask but I do think it is a delaying action until they can at least establish a new
rootstock program. We have faced a similar situation in the central valley of California where we had an eradication program for removal of trees showing symptoms or adjacent trees which have been indexed and discovered to be symptomless carriers. So far the spread has been held down in the central valley. I would expect
this program to continue.
question:
Vou mentionedthat lemon shouldnot be usedwith Rangpurlime. Is that right?
answer:
We have had a little difficulty with Rangpurlime and I would like to seea little moreexperiencewith them.
question:
On trifoliate orange,how critical is the calcium leveland what is the proper pH range?
answer:
I couldn't tell you what the proper pH rangeis. I think it is primarily a calciumproblem becausewe run illto
problemswhereM havecalcareoussoilswheretrifoliate orangejust becomesso hard that we cannot grow it.
Wemay haveto treat with iron chelateat leasttwice a year to keepthe treesgrowing. At one of our field stations wherethe pH is around 7.3 to 7.5 we havethis problem in the nurserybut oncewe get a scionvariety on
it we can continueto grow the treeseventhoughwe do havesomechlorosisshowingon them.
.14-
11:15 Discussion(cont)
September
24
question:
In Spainand Portugaland in that areathe pH is very high. waswonderingif that wasone of the reasons
why theyput the bud union so high.
anSMr:
Not asfar asI know.
que$tion:
How will trifoliata work on tropical conditions?
answer:
To the bestof my knowledge,trifoliata hasnot beentried successfullyunder tropical conditions. I have
somereselVations
asto whetherit will do satisfactorily
undertropicalconditions.
question:
Would you expecttrifoliata to be more cold sensitiveif it were grown under tropical conditions?
answer:
I think it would. I say this becauseeven in California where we have had warm falls and then experienced a
sudden cold spell in the fall, I have actually seenworse frost damage on trifoliata orange then I have on rough
lemon ill the same orchard at the same time. I suspect its hardin,ss is due to the fact that as the temperature
i$ lower it gets a chance to harden off and become more resistant to cold. If it doesn't have this factor it
doesn't have the resistance to cold.
question:
Is there any differencein lemon quality (I am referringto Californialemon growers)with lemon on macro-
phyllaasfar asfreshqualityis concerned
asopposed
to souror roughlemon?
answer:
Quality wouldn't be asgood but it is still acceptable.
question:
What is the relativetoleranceor resistanceof grapefruitto tristezaon sour orangeascomparedto sweetoranges
on sour oranges1
answer:
Grapefruit onsourorange
in Californiaisveryslowin showingsymptoms
of tristeza.Underfield conditions
your treesof sweetorangeand sour orangewould probably be showingseveresymptomsand many treeswould
probably be deadbeforethe grapefruittreeson sour beginto showsymptoms. Whenwe inoculatedgrapefruit
treeson sour orangein the field we got symptomsrathersoon. But underfield conditionsit takesquite a while
for symptoms to appearso they aremore resistant.This is alsotrue for mandarins.Mandarinson sour orange
aremore resistantto tristezathan sweetorangeon sour orange.
question:
Is stock-scionincompatibility a problem of any significanceand if so hasvery much researchbeendone?
answer:
It is a matter of significancebecausewhile we generallyhaven'tfound it on the older rootstocks,by this I
meanrootstockslike sweetand sour, but this is a problemwith the trifoliate orangegroupand in our recent
rootstock trials wherewe haveabout 200 new rootstocksundertest we find that about 10%of thesewill show
very severeincompatibilitieswhen they havetrifoliate parentagein them. Evenwith trifoliate orange,we have
found bud union creaseto be fairly common. I think it is somethingto considerparticularly in the trifoliate
Drangeor trifoliate hybrid group.