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Citrus Rootstock Improvement

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Citrus Rootstock Improvement -92. Citrus Rootstock Improvement w. P. Bitters By their very nature, rootstock experimentstake a long time. I feel most rootstock workerswould like a 20 year period to draw their conclusionsand maketheir recommendationsto the industry. A citrus tree takesa long time in coming into full bearingbut oncein full bearingmay havea long productivelife. A rootstock is a very important part of any orchard and is not changedovernight like a cultural practice,a fertilizer program,or an irrigation schedule.Th~refore, it is impor- tant to choosethe right rootstock. What rootstock to userequires the right answer. The principal problem confronting the rootstock worker is that the answershe obtainsfrom his plots today are the answersto questionsthe plot was designed to answer20 yearspreviously. Theseanswers may not fit today's needs,and under today's conditions the answerswe need tomorrow arestill 20 yearsaway. It is urgent, therefore,something be done to closethis gapfrom 20 yearsto 15, or even 10 or less. If reliableinformation upon wtlich dependablerecommendations can be madeto the industry can be obtained in a shorter interval than previously,then we owe this responsibilityto the industry. To someextent we needto anticipate ~at a few of theseproblems might be and havesome answers waiting. I amhappy to sayprogress is beingmade with Citrus rootstocksand evenmore progresslies ahead.This has come about through new ideas,new tools, new techniques,new equipment,and new facilities. It hascome about through better collabo- ration and cooperation with those in associated fields of interest suchas the nematologist,the plant pathologist,the nutri- tionist, the plant breeder,the plant physiologist,the antomist, and others. Eachcan and hascontributed something. In a sense,the rootstock researchermust be the key to the rootstock puzzle. It is up to him to bring togetherthe knowledge contributed separatelyor cooperatively by the other investigatorsand try to interpret meaningfullythe overall rootstock picture. For example,what good is gummosisresistant if the rootstock is suaptible to tristeza? What good is tristeza re- sistantif the stock is susceptibleto burrowing nematode?What good is resistanceto the burrowing nematodeif the tree is too susceptibleto frost? The nematologistmay not be interestedin cold hardinessand the plant pathologistmay not be in- terestedin fruit quality, but the rootstock man is. In a sense,the rootstock man must be the hub aroundwhich the total rootstock programrevolves. He must not be subservientto the other groups,and they must not be a servicegroup for him. He must either work cooperativelywith his associatesor borrow techniquesthey haveperfected and which are useful to his cause. Both may be to his advantageand to the advantageof rootstock progress. Progress is beingmade at Riversideas well aselsewhere, I'm sure,and I'd like to cite a few examplesto illustrate somenew horizonswhich I think havebeen attained. One of the most important stepsis the availability of cleanbud lines for rootstock test purposes.Many a rootstock trial hasbeen a sulVivaltest to disease,rather than a matter of measuringtrue rootstock increments. Cleanbud lines may arise from severalsources. They may arisefrom nucellarlines in thosevarieties in which this is possible,and they of coursein due time can be recontaminatedby mechanicalmeans, by propagation,or by insectvectors. It is important to know at all times if and what virusesare present In many instances,nucellar lines are not availableand virus-freeold lines are a necessity.These may be ascertainedby virus indexingprocedures which havebeen perfected by the plant pathologists. At Riversidewe havea Citrus Variety ImprovementCommittee, which consistsbasically of plant pathologists,horticulturists, and plant breeders.The solepurpose of this committee is to provide to the industry and researchpersonnel thru a budwoodcertification programvirus- free budwood which is true to type. It is not only important to provide it initially, but it is equally important that it continues to be maintainedas such through budwoodfoundation blocks and a budwoodcertification programwhich verifies the freedom from recognizedviruses. Certified rootstocksmust be recognizedas well. The effectivenessof this programis basedon the use of indicator plants,plants which, when inoculatedto a particularvirus to which it is extremelysensitive, express key symptoms of the diseasein a very short period of time. I cite, for example,the useof the 'West Indian' lime asan indicator plant for tris- tela, the 'Etrog' citron asan indicator plant for exocortis,and Citrus excelsaas an indicator plant for tatter-leaf. As horticul- turists, we can assistthe plant pathologistby supplyingnew plant materialto be tested. With thesetechniques, it is possible to determinein monthsinformation which would take yearsto obtain under field conditions. We usethese techniques to fol. low virus progressionin our rootstock blocks in the field, althoughvirus-free bud lines may havebeen used initially. Other pathologicalproblems will be referredto later. Greatprogress has also beenmade through increasesin the genebank of material availablefor rootstock testing. This has resultedfrom increasedimportations and exchangeof geneticmaterial, increased interest in the Citrusrelatives, and increased efforts by the plant breeders.During the 1940's,citrus variety collectionshad somewhatstagnated. The inroadsof tristeza, .93. if it did nothing else,stimulated interest in rootstock and variety collectionsand really awakenedinterest in this field. Since d1e1940's, there hasbeen a tremendousexchange of citrus materialbetween many of the citrus producingcountries of the world, and varietiesand speciesthat were onceextremely localized are now worldwide in their distribution. The researcher can try not only selectionsfrom his own nativecountry but from many other countriesas well. Citrus relativesare relativelyunexplored as rootstocks. Within the subfamilyAurantioideae there are 33 genera,of which Citros is one. This means32 other generaand their speciesare potential rootstocks. The successof the trifoliate orange,Pon- ciros trito/iata, and one of its hybrids, the 'Troyer' citrange,are well established.Precursory investigations indicate severalof the relativesmay alsohave a potential rootstock value. I cannot agreewith someof the conclusionsdrawn by other investiga- tors about non-congenialityof someof thesecombinations. Perhapssome of the reactionswere basedon diseasedmaterial or under improperecological conditions. The fact that citrus may not do well with 1 speciesof a genusdoes not meanit won't do well with another. Interestingresults have been obtained with specieswithin someof the generaof Severinia,A ta/antia, Citropsis,Hesperethusa, Microcitrus, C/ymeniaand P/eiospermium, to mention a few. It should also be pointed out that Citros hybridizesreadily with someof theseother genera,as for examplethe hybrids, the citranges,the limequats,the faust- rimes,etc. At Riverside,we havenot succeededin creatingnew bigenerichybrids other than with Poncirus,although we havetried, but arehopefully looking forward to followingsuch crosses and testing their progeny. The citrus breederhas also become aware of the importanceof breedingcitrus for other purposesthan edible fruits. Here- tofore, if existing hybrids had no commercialvalue and servedno useful purpose,they were as a last resort,tried as a rootstock. I am happy to say that the plant breedersat the Citrus ResearchCenter at Riversideand the USDA Dateand Citrus Field Sta- tion at Indio havemade crosses with specific rootstock purposesin mind. At Indio, accentis primarily on rootstockstolerant to salinity and to Phytophthora. At Riverside,nematode resistance has been a primary factor. In someinstances they have taken the best rootstock performanceof somespecies and crossedwith the best rootstock performanceof other speciesin hopesof improvingthe beneficialcharacters of both in the hybrids. Examplesof suchcrosses are Poncirus trifoliata crossed vlftthrough lemon, rangpurlime, cleopatramandarin, Citrus macrophylla,and others. Someof the first generationseedlings havecome into bearingand progenyplants are in variousstages of testing. Progresshas also been made in the selectionof Phytophthora resistantstocks. Underfield conditions, it may take years for an epidemicto develop. Underlaboratory conditions,considerable information may be developedin a rather shQrtperiod of time. Two techniquesdeveloped by pathologistshave proved very useful. Oneconsists of developinga heavyaqueous sporesuspension of the Phytophthora speciesin question. Immersingthe roots of test speciesin the sporesuspension for a givenperiod of time and estimatingthe percentageof rotten feederroots providesa fairly good index of susceptibility. Some- times the techniqueis modified to a soil type pest hole, but againthe degreeof survivalis determined. The secondtechnique consistsof insertingmycelium of the test organismunder the bark of a host plant. The surfaceof the bark may be surfacester- ilized, a plug of bark removedwith a cork borer, and a small bit of mycelium insertedunderneath. The organismis permitted to incubatefor a r/lvenperiod of about 60 days. The lesionarea to which it hasspread can then be calibratedand the suscepti- bility determinedin this way. Both tests aresomewhat indicative, in a short period of time, of the relativetolerance of the host plant The soil pest hole may be too severe,and othersargue that the bark itself may contain the resistancefactor
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