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HORTSCIENCE 45(1):11–15. 2010. ual rootstock characteristics. A rootstock primarily provides a reduction in juvenility (time to bearing) and tree vigor when com- A Career Perspective on pared with seedling trees; thus, citrus trees propagated with a rootstock combined with Rootstocks, Their Development, and a pathogen-free scion bring a much improved degree of uniformity and consistency to an Commercialization orchard. Secondarily, rootstocks have many individual characteristics that contribute in William S. Castle1 positive ways to the performance of a citrus University of Florida, IFAS, Citrus Research and Education Center, 700 tree (Table 1). They influence various horti- Experiment Station Road, Lake Alfred, FL 33850 cultural traits and provide tolerance to pests and diseases and certain soil and site condi- ‘‘No problem in citrus culture is wor- interest in citrus rootstocks and changing the tions that contribute significantly to orchard thier of painstaking research than the direction of citriculture; first, the damaging profitability. Also important are rootstock one having to do with rootstocks. The effects of Phytophthora foot rot when own- nursery traits such as the degree of nucellar whole gamut of citrus fruit production rooted sweet trees were common. embryony that is related to the ease, expense, is affected by the relation of rootstock Many of those trees were actually seedlings and consistency of propagation. These rea- to scion and the adaptability of differ- with their own inherent disadvantages. The sons combined explain why propagation of ent combinations to the environment. second disease problem was citrus tristeza citrus trees with rootstocks has long been Something is known but much remains virus, which devastated trees of sweet orange preferred over the use of scion cuttings taken to be found out.’’ and other varieties on the highly susceptible from mature trees. —H.H. Hume (1957) sour orange, the most popular rootstock in The existence of choices many parts of the world before the spread of has historically meant that: 1) sour orange Professor Hume was right: rootstocks are the virus. As these problems spread and were could be replaced, thus averting future di- critical and much is still to be learned. encountered by citrus growers worldwide, sasters from large-scale tree loss resulting Rootstocks and scions are the foundation of budding, i.e., using a rootstock, became the from infection; 2) citrus many tree fruit industries of the world. accepted practice. Grafting or budding is orchards could be grown on marginal soils Together, those components establish profit- actually a centuries old horticultural tech- and locations because rootstocks suitable for ability, but it can be argued that the rootstock nique but one not widely practiced in citri- high pH soils, salinity problems, or colder is the critical component; otherwise, scions culture until the late 1800s or the early 1900s. areas were available; 3) particular objectives would be grown on their own roots every- It was not until the 1940s, however, that the could be met by selecting not the best overall where. There is no precedent for the failure of cause of tree decline on sour orange came to choice, but by selecting a rootstock for one a citrus industry because of an inadequate be associated with a graft-transmissible en- specific purpose like large fruit size, for scion variety, but serious problems have tity later shown to be a virus. example, the use of rough rootstock occurred because of a less than satisfactory The ‘‘modern’’ era of citrus rootstock use for navel oranges or for rootstock. Therefore, after 35 years in the and subsequent history could be described ‘’; 4) rootstocks with complementary field conducting rootstock and scion evalua- as beginning in the early part of the 20th tolerances to pests and diseases could be tion research and working with our Univer- century in response to the diseases mentioned selected to reduce the risk of tree loss; and sity of Florida citrus plant improvement (Webber, 1967). Field trials with usually 5) the harvest season could be extended team, here is an historical perspective on a small number of rootstocks were initiated because of the effect of rootstock on juice citrus rootstocks along with some observa- and eventually followed with ones of larger Brix/acid ratio. tions and reflections on the human or social scale (Batchelor and Rounds, 1948; Gardner Today, breeding citrus rootstocks con- side of research and grower cooperation. et al., 1967; Moreira and Salibe, 1969). Those tinues with emphasis on many characteris- It is well established from decades of early trials also revealed the importance of tics, but especially on creating rootstocks that experience that citrus rootstocks bring many other unknown virus and viroid diseases reduce tree size beyond what is currently advantages and profitability to commercial that affected the rootstocks being evaluated. possible among commercial rootstocks. Size- enterprise. In some instances, citrus root- However, they were conducted under con- controlling rootstocks are important for stocks are the sole determining element that siderable constraints, in retrospect, given higher density orchards, and rootstocks that allows citrus to be grown in particular cir- the genetic and breeding tools available in induce scion precocity are needed for signif- cumstances. Yet, despite their popularity and 2009. icant cropping to begin early. Of course, new necessity, every rootstock has one or more Formal breeding programs have emerged rootstocks must also lead to excellent yields undesirable traits that preclude its universal and use new technologies such as somatic of high-quality fruit and possess other key use. Those limitations have sometimes been hybridization and molecular markers that are tolerances. discovered during research and development; altering and expanding the range of possibil- Distinctiveness of citrus rootstocks versus at other times, they have not been revealed ities for genetic manipulation leading to other tree fruit rootstocks. The long list of until after significant commercial use. improved rootstocks (Kahn, 2007). As a re- citrus tree performance factors attributable to Until the mid-1800s, citrus was grown sult, there is a new era of citrus rootstock the rootstock (Table 1) is difficult to match by primarily as seedlings. Commercial opera- research and development underway that is any other tree fruit. Yield, for example, can tions were limited in scope and size as linked to different ways of thinking about be affected by the choice of rootstock in compared with today and certainly were not rootstocks and their role in future citrus apple, pear, or peach, but in those instances, operated with the same level of technical orchards. It is conceivable that genetic ad- fruit quality and yield efficiency are usually knowledge or experience. It is generally vances might eventually eliminate the need maintained by canopy management rather known that two grand disease events were for rootstocks as rootstock traits are incorpo- than rootstock choice (Castle, 1995). Root- apparently responsible for creating the initial rated into the scion variety. stocks for deciduous tree fruit crops are usually selected for tree size control and their Importance of Citrus Rootstocks tolerance to various pests or diseases. An- Received for publication 11 Sept. 2009. Accepted other distinction is the nature of the citrus for publication 18 Oct. 2009. The importance of a citrus rootstock rests fruit itself and its morphology and physiol- 1To whom reprint requests should be addressed; on the subtle distinction between the general ogy. The citrus fruit consists of small sacs e-mail bcastle@ufl.edu. reasons why rootstocks are used and individ- filled with juice. The composition of the juice

HORTSCIENCE VOL. 45(1) JANUARY 2010 11 Table 1. Rootstock-related citrus tree attributes.z Attribute Remarks on attributes as a rootstock selection criterion 1 Yield A major consideration 2 Precocity A special factor as interest in higher-density plantings increases 3 Yield efficiency Can be important, but not usually a selection criterion 4 Fruit quality A major factor for fresh fruit growers 5 Fruit size Important enough to be considered separately from quality 6 Juice quality Brix and acid are affected; a major factor for growers of both fresh and processing fruit 7 Tree growth Usually considered in terms of vigor and eventual tree size, a criterion that is increasing in importance 8 Compatibility Scion-rootstock vegetative compatibility is often important 9 Ease of propagation Largely a matter of seed production and degree of nucellar embryony 10 Shoot flushing A new criterion related to Huanglongbing and spread by psyllids 11 Mineral nutrition Not usually a selection criterion, but there are rootstock effects 12 Salinity In some instances, an important selection criterion 13 Clay soil High content or sol horizons can affect a rootstock decision 14 High soil pH A very important factor with -based rootstocks 15 Wet soil (flooding) Not usually a selection factor but can be important 16 Drought Modern irrigation methods usually preclude this as a selection factor 17 Cold (freezes) Often considered in regions threatened by chronic cold 18 Citrus blight At one time, a major consideration, but less so now in Florida 19 Phytophthora rots Still an important factor in rootstock decisions 20 P. palmivora/root weevil complex A problem specific to Florida involving a particular species of Phytophthora fungus and Diaprepes root weevil 21 Root weevils A troublesome problem at times in some areas 22 Burrowing nematode A problem in Florida for which specific rootstocks are used 23 Citrus nematode A more universal problem with specific rootstock options 24 Tristeza virus A serious threat with some rootstock options 25 Exocortis and xyloporosis viroids Not generally a threat today with clean budwood zThe list is not intended to be comprehensive but to show the most important attributes representative of the world’s citrus industries. determines flavor and is affected by plant hybrids are being produced that may change in Florida for processed oranges. Rough water relations. Rootstocks are linked to plant the situation in the new era. lemon also has a history as a rootstock for water relations and consequently have a role Phases I and II occurred in the early era fresh fruit in South Africa where it produced in the taste of the fruit (Albrigo, 1977). The and Phase III in the modern era. In the history good-sized navel oranges and California apple fruit has a different morphology and its of rootstocks, Phase I is the world story of where it had some use as a rootstock for flavor and texture are not so directly affected sour orange (1) rootstock, the only rootstock ; likewise, Volkamer lemon attracted by the rootstock. Furthermore, apples are that might be considered as universal because similar interest, but not usually on the scale of a climacteric fruit; citrus is nonclimacteric. of its widespread use. That phase was fol- . Cleopatra and other mandarins That difference is related to the virtual lowed by two others, the first one involving have often been chosen as the rootstock for absence of a rootstock effect on quality in rootstocks like Cleopatra mandarin (2), rough mandarin and mandarin-hybrid scion varie- fruits such as apple and emphasizes the lemon (3), Volkamer lemon (4), and sweet ties. Their use with those scions has remained importance of citrus rootstocks. orange (5) as choices of interest until re- essentially constant; however, commercial placements were found for sour orange. The experience consistently revealed their slow- History of Citrus Rootstocks third phase began when the and ness to bear and small fruit size limitations rootstocks were discovered. They that preclude their use with other scions. There are at least three reasons for de- were shown initially to have enough similar- Phase III begins with trifoliate orange (6), fining an early era (1900 to 1970) and ities to sour orange to be quickly accepted a rootstock with a long, successful, and modern era (1970 forward) of citrus root- commercially in Florida and California and traditional history as the major rootstock in stocks: 1) commercial enterprises of today’s of sufficient promise to be included in root- Japan and China. Trifoliate orange also has scale did not emerge until toward the end of stock evaluation trials elsewhere. a significant place in northeast Argentina and the early era. As a result, the value of All of sour orange’s excellent attributes Uruguay for fresh fruit and to a limited extent a rootstock, even the critical need for the are well known along with its major limiting in Australia and California for the same right rootstock, was known but was not fully factor, susceptibility to citrus tristeza virus. purpose. In some parts of the world, trifoliate exploited because rootstock science and That virus was responsible for virtually orange did not become an important root- commercial experience were relatively un- eliminating or forcing drastic changes to the stock until sometime in the modern era. It had developed; 2) international trade in citrus citrus industries in places like Australia, a reputation in its early history of being fruit and products was established on a larger Argentina, Brazil, the United States, the erratic. It was subsequently learned that scale than during previous decades partly Mediterranean region, and South Africa. In exocortis viroid was the reason for that because new rootstocks allowed citrus to be other parts of the world, like Mexico where reputation. The commercial prospects for grown in large plantations with fewer risks the virus or vectors are not present at a dam- trifoliate orange were also improved when than before; and 3) budwood protection pro- aging level, sour orange remains a part of the it became apparent that there were selections grams were established toward the end of the rootstock portfolio, but its future is threat- categorized primarily by flower size that early era and advanced in the modern era. ened. The spread of the efficient vector, the affected budded tree size. Among those is Those programs offered new choices because brown citrus aphid, is a major factor in that the selection Flying Dragon that produces the rootstocks with virus and viroid susceptibil- threat. There are a few areas remaining where most size-controlled trees of any rootstock. ities could then be used with clean budwood. sour orange continues as the rootstock for For that reason, it has attracted interest There are essentially 11 rootstocks (see lemons, the only common commercial scion among rootstock breeders and for commer- following numbered rootstocks) that formed that can survive tristeza when grown on sour cial use, largely in Argentina and in Costa the world’s portfolio during three historical orange rootstock. Sour orange remains the Rica where substantial numbers of lemons phases. Most of those rootstocks have been major rootstock in Italy. and sweet orange trees, respectively, are known for decades and are the mainstay of Phase II rootstocks achieved different being planted on this rootstock. the global citrus industry today. Few addi- levels of commercial importance. Rough and citrumelos, hybrids of tions have been made since then, but new lemon was at one time the major rootstock trifoliate orange with sweet orange and

12 HORTSCIENCE VOL. 45(1) JANUARY 2010 , respectively, represent the next the cold hardiness attributes of trifoliate However, between 1907 and the 1940s, many generation of rootstocks and they changed orange to scion cultivars, not to develop citrus operations had not matured beyond the world by becoming the dominant root- a new rootstock. It was tested as accession a localized fresh fruit business; two world stock types in many of the world’s largest CPB 4475. The rootstock was widely studied wars had occurred and the means and market citrus industries. It is astounding that in in Florida, but it was the research of an- for large-scale fresh and juice fruit operations the 1904 USDA Yearbook of Agriculture, other USDA scientist in Texas, Dr. Heinz were yet to arrive. The world was reasonably Willits citrange was described as one of 40 Wutscher, that attracted the most attention content with the available rootstocks except citranges that all came as seed from a single to Swingle citrumelo (Wutscher, 1979). The for the need for ones with tristeza and Phy- fruit. Willits was described as the second official release of Swingle citrumelo in 1974 tophthora tolerance. The imperatives and op- citrange showing potential value, but it was was followed by a largely unprecedented portunities that are in place today are much Morton citrange that was initially thought rootstock shift to this rootstock in Florida as more demanding on citrus businesses than to be the most promising based on research a replacement for Carrizo citrange. Swingle those of 1940s. Also, citrumelos were a new in California. Eventually, Carrizo (7) and citrumelo is still the most popular general-use and relatively underevaluated class of root- Troyer (8) emerged as the top commercial rootstock in Florida; however, in addition to stocks. In the 1940s, there was considerable rootstocks. Their history shows, however, the well-known intolerance of trifoliate or- interest in tristeza-tolerant rootstocks and that they originated in 1909 in California ange and its hybrids to calcareous soil, other Swingle citrumelo was tested in the United from the same hybrid seedling (Savage and exceptions have been noted. It is not suitable States, Brazil, and Argentina where it was Gardner, 1965). It was their subsequent dis- for use in certain soil situations related to soil shown to be tolerant. The human and social tribution to Florida, Alabama, and Texas plus horizons high in clay or low in organic matter elements in this story are mostly ones of a little intrigue involving fire and a Russian content (Bauer et al., 2005). Its poor perfor- timing, circumstances, and interest. delegation that resulted in the plants being mance in such soils was also reported in Recent rootstock research in Florida. The separately named in the 1930s. The question Texas, but not fully recognized until the arrival of canker followed by Huanglongbing persists as to whether they are the same plant. experiences in Florida. (HLB) has almost eliminated rootstocks as Modern molecular tools indicate that they The historical shift away from sour or- a common topic of discussion or concern. have the same genotype and field comparisons ange in Brazil was different. Why? Because some growers feel that unless support that contention (McCarty et al., 1974). (11) was the preferred choice when sour control is gained over HLB, the Florida citrus Carrizo citrange first became a commer- orange was no longer a viable option. How- industry may not survive, and they are cial rootstock in Florida in the 1970s where it ever, tree decline became a serious matter satisfied with the currently available root- essentially replaced rough lemon and even- until it was learned that exocortis viroid was stocks. Thus, financial resources have been tually was the rootstock for more than 40% to the cause. The problem was eliminated with directed to the HLB problem and the creation 50% of annual nursery propagations (Castle, the advent of clean budwood sources. Rang- of tolerant plants that may or may not be 1987; Wutscher, 1979). It remains popular, pur lime continues as the main rootstock in rootstock-related. Citrus diseases can have especially for Valencia sweet orange, but is Brazil because trees are productive and a stimulating or depressing effect on root- no longer the number one choice because of drought-tolerant. stock research. problems with citrus blight. Carrizo citrange During the course of Phases I to III, a Rootstock research in South Africa. Al- gained dominance in Spain as well, but its number of rootstocks had a minor commer- though not a large industry by international sensitivity to calcareous soils has limited it cial history. More recently, other promising standards (less than 2% of world production), potential use. Carrizo and Troyer citranges citranges like C-35 have been evaluated. C- South Africa has one of the most significant have been widely evaluated in places other 35 was first used on a commercial scale in the fresh fruit export businesses with exacting stan- than in Florida and California and have Dominican Republic and proved to be very dards that can be traced back to the rootstocks slowly been adopted in some countries as satisfactory on their volcanic soils and in the used. Also, as one of the citrus industries badly a replacement for sour orange. presence of Diaprepes weevils. A Cleopatra affected by tristeza, a rootstock improvement Troyer citrange became popular in Cal- mandarin · trifoliate orange hybrid (x639) was program has been underway for many years. ifornia as early as the 1950s and was domi- produced in South Africa. Other mandarin- That program languished for awhile and was nant for 20 years thereafter (Ferguson et al., trifoliate hybrids such as US 897 and US 812 not rejuvenated until the 1980s. Why? Per- 1990). Before its use, sweet orange and sour and have been produced and appear to be very haps, again, because of personal, institutional, orange were the common rootstocks along promising along with those of the breeding and industry priorities that, although realistic, with more limited use of rough lemon and programs in the United States, Argentina, may have gone unchallenged given the long- Cleopatra mandarin. However, in the 1940s, Brazil, and Spain. term commitment needed to develop new citrus tristeza virus was ravaging California rootstocks. citrus trees on sour orange; thus, Troyer Social Aspects of Citrus Rootstocks Tradition and circumstances. The intro- became the leading rootstock. Troyer also duction of new rootstocks can face a number proved satisfactory for Lisbon-type lemons, Breeding citrus rootstocks, conducting their of powerful obstacles such as tradition and but not Eureka types because of compatibility evaluation followed by some form of release, the normal industry condition. ‘‘Tradition’’ issues with trifoliate orange and its hybrids. and then their adoption by growers are all can exert a strong influence. Oftentimes, the Troyer citrange is a common rootstock for the human endeavors, which mean that the out- balance of good versus negative rootstock popular Limoneira 8A lemon scion in Argen- comes are subject to the personalities of people attributes is positive and the rootstock con- tina, the world leader in lemon production. In and their reactions to circumstances. There- tinues to have commercial acceptance. Until California and other industries, Citrus mac- fore, there can be little doubt that the history of that balance changes in some negative way as rophylla (9) was discovered as a suitable citrus rootstocks has a human or social com- perceived by growers, introducing a replace- rootstock for lemon. ponent. That component has affected or per- ment rootstock has not been easy. Carrizo Phase III included Swingle citrumelo haps even determined the course of citrus citrange is a good example. Florida growers (10), an important rootstock that basically history and so should be considered important have had some problems with Carrizo cit- followed the citranges and is still being in any discussion of citrus rootstocks. Here are range, but those problems have only reduced evaluated and adopted. It has an interesting a few examples of the alignment of rootstock interest in the rootstock, not replaced it. In history that is often invoked as the main history, people, and events: California currently, Carrizo citrange is in- evidence of the time, commitment, and ser- Why did it take 67 years from hybridization creasing in popularity and replacing Troyer. endipitous nature of the life of a rootstock to the release of Swingle citrumelo, and why That changeover is difficult to understand from the point of creation to commercial did extensive field trials not begin until the given the very similar performance usually acceptance. Dr. Walter Swingle, USDA, mid-1940s? The answer to that question does experienced among trees on either rootstock. made the hybrid in Florida in 1907 to transmit not involve any deliberate social interference. In other circumstances, one rootstock trait

HORTSCIENCE VOL. 45(1) JANUARY 2010 13 can be identified and eliminate a rootstock, rootstock in Florida? The answer is because will do with his or her management and e.g., sour orange and tristeza; however, the of risk and its perception. ‘‘Risk’’ is the single conditions. sour orange story does not end there. Sour factor that integrates all human components orange was the traditional rootstock in Flor- in the citrus rootstock business from research The Future ida partly because it grows well in a broad to commercialization. Swingle citrumelo range of soil types. That particular tradition may not produce the most income, but it Although citrus has been cultivated in (grower experience) is a powerful factor has fewer perceived risks than other root- some form for centuries, most the world’s because it explains why some growers dis- stocks and possesses certain attributes that citrus industries as we know them today are satisfied with other choices are returning to are viewed as relatively high value like 100 years old or less. Yet history shows that sour orange. tolerance to many pest and disease problems during that period, new commercial root- Sometimes there simply is not a place in leading to long-term survivability, acceptable stocks resulted from only two mechanisms: a citrus industry for a new rootstock or yield, and good juice quality. 1) searching and selecting among existing technology. In Australia, the use of viroids plants; or 2) breeding new hybrids and to control tree size was a well-researched and Citrus Rootstock Category selecting among them. Of the 11 rootstocks developed technology, but it was never used. as a Selection Criterion identified here, most were existing species or The long-term integrity of Australian citrus selections and citrus rootstock breeding has orchards is such that there are few opportu- Long-term observation suggests that cit- relied heavily on one parent, trifoliate orange. nities resulting from tree losses, and so on, to rus rootstocks can be placed into three cate- Today, trifoliate orange remains a singularly replace orchards and introduce new root- gories: 1) replacement rootstocks, ones that important source of genetic traits to be stocks or technologies. provide a large improvement or order of captured in hybrids with new parental com- magnitude leap in attributes, e.g., a tristeza- binations, especially with various mandarins. Commercial Rootstock Selection tolerant sour orange. The improvement Very little has been or is simple and easy would result in the new rootstock being in the field of citrus rootstocks. Rather than The selection process is the ultimate accepted most quickly for commercial use; try to predict the near or far future, perhaps human experience in the realm of rootstocks. 2) alternative rootstocks, ones that are some- some of the following will be factors of some It is when good science plus tradition, expe- what better than existing rootstocks, but their consequence: rience, and circumstances come together as improvements are primarily incremental or in the most meaningful forces determining the a particular trait. An example is C-35 cit-  The potential to produce new citrus root- choices. It is a time when a new rootstock range, which has many of the same traits as stocks by either conventional means or in may begin its commercial history. It is a time any citrange, but it also has others, including combination with modern technologies is for the associated researchers to enjoy a mo- some control of tree size that has resulted in substantial and demonstrated. Therefore, ment of pride and satisfaction, and it should increasing interest in it as a commercial it is interesting to consider whether those be a time when researchers and growers rootstock; and 3) special purpose rootstocks. approaches will be the only means of reflect on their long-standing joint efforts to Kuharske citrange is an example. It is being creating new rootstocks in the future or if define rootstock needs and make commercial used in Florida because it is a citrange and searching and selection will compete with rootstock decisions. Nevertheless, I can attest has exceptionally good tolerance to burrow- modern breeding and then, what role to the fact that the process remains largely ing nematodes, a problem in some areas of social aspects will play. The social com- a mystery. From my perspective, a rootstock the citrus industry. These categories are ponent, in this instance, is sustained hu- never seems to be selected strictly on its important because placement in a particular man interest in developing new rootstocks. merits as revealed from research and some category as a result of grower perception and  There may be shifts in priorities. Yield and commercial experience. experience may determine the subsequent fruit and juice quality are likely to remain I can also attest to the fact that viewpoints history of a rootstock. key rootstocks attributes, but in the words on rootstock decisions can be quite different of Dr. Bill Bitters, the dean of rootstock and founded on different motivations. At Cooperative Research research, ‘‘Rootstocks once established times, rootstock choice is certainly as much are not very susceptible to change; how- a philosophical decision as a technical de- In Florida, we do not have the field ever, occasionally something happens cision. Researchers want to create and de- resources for an aggressive field research which requires a change.’’ Modern tools velop better rootstocks. Growers want to program. However, my colleagues and I are to identify genetic elements related to make money. I attempted to bridge that fortunate to have a substantial history of specific traits and to transfer them from potential gap by conducting a long-term field cooperative field study with growers. What one source to another, and quickly confirm trial with ‘Valencia’ orange on 12 rootstocks I have never understood is why every grower that the desired event has occurred, allow at two locations in Florida (Castle et al., is not a participant. We depend on grower creative concepts for rootstock improve- 2010). Yield and juice quality data were cooperation to conduct field trials. Each ment. As the knowledge base increases, collected for 14 years along with tree loss partner benefits from the relationship at little perhaps new rootstocks designed in re- to blight. Blighted trees were removed and to no cost to the grower. It is a classic win– sponse to particular concerns could more replaced annually with another tree on the win situation. readily be produced. ‘‘New’’ rootstocks original rootstock. All the data were used for The cooperative projects are highly vari- could also include well-established root- a cash flow analysis. We learned that despite able in scope and intent, so it is very easy to stocks that only need to have certain individual rootstock characteristics, yield and arrange something for any grower. Coopera- aspects modified or simply be rediscov- planting density had the greatest effect on the tor trials usually involve a commercial scion ered. Willits citrange did not fulfill its financial results. In fact, the yield of the trees variety so there is an income; no special initial expectation, but today, it may on Volkamer lemon was so large that al- cultural or management issues are intro- have serious merit as a size-controlling though 50% of the trees were lost to citrus duced, harvesting is by a commercial crew, rootstock. blight, those trees had the highest income. and data collection is mostly by the re-  Modern molecular tools have provided We also made adjustments to tree spacing searchers with no impact on the grove oper- insights into the genetic background of and recalculated cash flow to accommodate, ation. For the small farmer, a project can be citrus species and selections. As a result, e.g., the smaller size of trees on Swingle as simple as a few trees on a few rootstocks geneticists are now attempting to recreate citrumelo. Those trees had improved finan- placed almost anywhere for observation. and maybe improve on rootstocks essen- cial performance but still did not exceed The learning potential is considerable, but tially lost to the world portfolio. Sour Volkamer lemon or Carrizo citrange. Why the main advantage to the grower is to learn orange is the best example. Its progenitors then is Swingle citrumelo the most popular at minimal expense what new rootstocks appear to be mandarin and pummelo and

14 HORTSCIENCE VOL. 45(1) JANUARY 2010 there are several researchers today making Batchelor, L.D. and M.B. Rounds. 1948. Choice Gardner, F.E., D.J. Hutchison, G.E. Horanic, and hybrids with those parents. of rootstocks, p. 169–222. In: Batchelor, L.D. P.C. Hutchins. 1967. Growth and productivity  Rootstock evaluation will be based on and H.J. Webber (eds.). The citrus industry. of virus-infected trees on economic interpretation and require no 1st Ed., Vol. II. Univ. Calif. Press, Berkeley, twenty-five rootstocks. Proc. Fla. State Hort. more than 10 years. CA. Soc. 80:89–92. Bauer, M., W.S. Castle, B.J. Boman, and T.A. Kahn, I. (ed.). 2007. Citrus genetic, breeding and Obreza. 2005. Economic longevity of citrus biotechnology. CAB International, Cambridge, To close, here is a quote from the recently trees on Swingle citrumelo rootstock and their MA. published book, ‘‘Dangerous Games, The suitability for soils of the Indian River region. McCarty, C.D., W.P. Bitters, and D.A. Cole. 1974. Uses and Abuses of History,’’ written by Proc. Fla. State Hort. Soc. 118:24–27. Comparisons between Troyer and Carrizo cit- a historian on the subject of history: Castle, W.S. 1987. Citrus rootstocks, p. 361–399. range. Citrograph 59:294–310. In: Rom, R.C. and R.F. Carlson (eds.). Root- Moreira, S. and A.A. Salibe. 1969. The contribu- ‘‘If the study of history does nothing stocks for fruit crops. Wiley, Hoboken, NJ. tion of research for the progressive changes in more than teach us humility, skepti- Castle, W.S. 1995. Rootstocks as a fruit quality citrus rootstocks for South America. Proc. First cism and awareness of ourselves, then factor in citrus and deciduous tree crops. N. Z. Intern. Citrus Symposium. 1:351–357. it has done something useful’’ J. Crop Hort. Sci. 23:383–394. Savage, E.M. and F.E. Gardner. 1965. The origin —Margaret MacMillan (2009) Castle, W.S., J.C. Baldwin, R.P. Muraro, and R. and history of Troyer and Carrizo citranges. Littell. 2010. Performance of ‘Valencia’ sweet California Citrograph 40:255, 276–278. orange trees on 12 rootstocks at two locations Webber, H.J. 1967. History and development of the Literature Cited and an economic interpretation as a basis for citrus industry, p. 1–39. In: Reuther, W., H.J. rootstock selection. HortScience 45: (accepted) Webber, and L.D. Batchelor (eds.). The citrus Albrigo, L.G. 1977. Rootstocks affect ‘Valencia’ Ferguson, L., N. Sakovich, and M. Roose. 1990. industry. Vol.1. Univ. Calif., Oakland, CA. orange fruit quality and water balance. Proc. California citrus rootstocks. Univ. Calif. Ext. Wutscher, H.K. 1979. Citrus rootstocks. Hort. Rev. Intern. Soc. Citriculture 1:62–65. Publ. 21477. (Amer. Soc. Hort. Sci.) 1:237–269.

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