To achieve this one needs a healthy, productive, long-lived tree. It is increasingly evident that one of the most basic factors in achieving this end is the use of the best available root stock. The initial cost of a good tree is no more than a bad tree; but it sure makes a difference over the life of the orchard. differ, it is not surprising that there is, and has been, a wide range of diversity in citrus root stocks used throughout the world. Fawcett pointed out the resistance of the sour orange to gummosis or foot rot after the disease had decimated trees on sweet orange root stocks in the Azores about 1834. Also see Klotz , and Klotz and Calavan , or the section on disease later on in this monograph. This discovery led to sour orange root stock being the most widely used in most of the Mediterranean areas of Europe and Africa, in South America and the United States, and to a lesser extent in Australia, Central America, Mexico, the Caribbean, Iran, Iraq, Jordan, etc. The occurrence and spread of tristeza , a virus disease which is insect transmitted and affects principally various scions on sour orange root stock, precluded its general establishment in South Africa, and later rapidly eliminated plantings in Brazil, Argentina and other countries of South America, in California and Florida, and more recently in Spain and Israel.
Sour orange may still be the world’s leading root stock because of existing plantings as yet unaffected by tristeza, especially in certain of the Mediterranean countries,vertical farming supplies western and northern South America, Central America, Mexico, and the Caribbean. However, the use of sour orange as a root stock is rapidly decreasing because of the imminent threat of tristeza to all citrus areas. The discovery of mal secco disease in Sicily about 1923 , to which the sour orange root stock is particularly susceptible, has discouraged somewhat its use for lemon plantings in that area. The disease is caused by a fungus and root stock resistance would be desirable. The future of sour orange in California, where mal secco does not occur, appears to be limited to certain types of Lisbon lemon, because of phloem necrosis with Eureka types and sour orange necrosis . The sour orange never became established as a root stock in Japan, China, and Taiwan, which grow predominantly mandarin types of citrus, because growers for decades were aware the two were not congenial—perhaps, again, due to the presence of tristeza disease. In the United States during the early citrus developments in both Florida and California, sweet orange seedlings were commonly used. In the period after 1870, the expansion of commercial orange growing in Florida included the top working of scattered areas of sour orange seedlings growing wild in that state. The success of these top-worked groves in the often imperfectly drained but rich “hammock” lands of eastern Florida focused attention to the success of sour orange as a root stock for sweet orange varieties. When gummosis became prevalent in sweet orange seedling orchards, the sour orange was generally adopted as a root stock.
Later experience in Florida indicated that the sour orange stock was not satisfactory on the deep, well drained, very sandy soils of the “ridge area” in the central part of the state, nor in the shallow oolitic limestone soils of the southeastern coastal section. Thus, it came to be that the Rough lemon became the dominant stock until the 1970’s. At that time it became clear that a disease known as “blight,” or “young tree decline ” was particularly devastating on Rough lemon stock . In California, budded trees were first employed instead of seedling trees after the introduction of the Washington navel in 1873 and the Chapman Valencia in 1876. Sweet orange was commonly used as the root stock because at that time there were thousands of sweet orange seedlings in the Riverside area and probably very few sour oranges. After 1900, the sour orange and the sweet orange were used almost equally, with the predominant use of the sour orange in the heavier soils and the more saline and calcareous soils of the desert and adobe soils elsewhere. After 1920 the tendency was somewhat away from the sour orange, especially for lemons and oranges, but not for grapefruit. Thus, more sweet orange root stock was probably used in California than elsewhere. There was very little use in Florida and in South Africa. The identification of the virus nature of tristeza by Fawcett and Wallace almost completely eliminated the further use of sour orange in California. The threat of tristeza around the world to trees on sour orange root stock will result in complete replacement of this root stock in the very near future. About 1952, the trifoliate orange and a hybrid, the Troyer citrange, were introduced to the California growers , and the use of sweet orange has since been on a marked decline. In California, trees on sweet orange stock would not generally be grown by the nurserymen today, except on special order. There may still be some interest in South Africa, but it cannot be used in areas where gummosis is a serious hazard. In spite of the fact that there is no problem with tristeza, sweet orange is now almost completely replaced by other root stocks.
Until recently, Rough lemon was probably the world’s second leading root stock behind sour orange. Rough lemon has long been used in India, where it is commonly known as “jambhiri” and other names. In South Africa it was in use before 1900 due to the failure of sour orange stock because of tristeza. It is frequently referred to there and in Australia as the “citronelle.” Rough lemon had been extensively used in Florida for many years in the sandy “ridge” section and along the southeast coast. However, in recent years the incidence of “young tree decline” or “blight” on this stock has almost eliminated its use in Florida . It was never widely used in California except in the sandy soils of Coachella Valley,mobile vertical grow racks and most plantings since about 1950 in the sandy soils around Yuma, Arizona are predominantly on this stock. Recent decline of trees on Rough lemon stock in the Yuma area are a matter of great concern. The decline appears to be principally from the severe incidence of gummosis, which was probably aggravated by their cultural practices and irrigation methods. Citrus volkameriana, Pasq. used in Italy and Sicily since the mid-l900’s as a mal secco resistant stock for lemons, is one of the many Rough lemon cultigens. It would best be called the “Volkamer” lemon and not “volkameriana.” This cultivar is not recognized as a valid species by Swingle , Tanaka , and Hodgson , or anyone else. Tests show that it has exactly the same isozyme pattern as the type specimen of Rough lemon, which suggests that it is not a hybrid. Taxonomically the variety should be referred to as C. limon under the Swingle system and C. jambhiri under the Tanaka system. The grapefruit , Citrus paradisi, has not been used extensively as a root stock. Although tried to some extent in Florida, it did not gain favor, and this is also true in California. In California perhaps its best performance was with lemon tops, but its low and variable yields, its erratic performance, and its sensitivity to tristeza with orange scions have all but eliminated its use as a root stock anywhere. To some extent it may still have some limited use in some of the citrus growing areas of the Caribbean. The pummelo, or shaddock has never been used commercially as a root stock, although it may have been tried experimentally in a few areas—the Philippines, Indonesia and parts of the Malayan archipelago. The fact that the species is monoembryonic and produces only hybrid seedlings indicates variability would have been the most serious problem.
As a root stock the species is extremely susceptible to tristeza, which would further remove any consideration. The Cuban shaddock is an obvious hybrid. It differs from the regular shaddocks in that it is highly polyembryonic and that as a root stock it is tolerant to tristeza. In the early 1900’s it saw limited use as a root stock in Cuba, and hence the name. It was incorporated into the 1927 root stock trials at the Citrus Experiment Station, Riverside, but its performance was only mediocre and did not result in any commercial plantings. An ornamental citrus nurseryman in California does use it as a dwarfing stock for some of the Citrus varieties he grows. The trifoliate orange, Poncirus trifoliata, has been used for decades in Japan as a root stock for Satsuma mandarin and other mandarin and pummelo types. Japan grows over 400,000 acres of citrus on this stock, making it the principal user of trifoliate orange. The People’s Republic of China also has extensive plantings, principally mandarin, but some oranges and miscellaneous. In Taiwan it is also very popular. These countries predominantly use the “small-leaved” strains of the trifoliate orange as root stocks. This root stock has been used sparingly in Australia, Argentina, South Africa, Spain and Turkey, but is now seeing extended usage. This root stock’s cold hardiness, precociousness, and disease resistance have been its biggest assets. When interest was high in the United States about 1900 regarding establishing a Satsuma mandarin industry along the coast of the Gulf states, the trifoliate orange was also introduced and tried as a root stock. The introduction of citrus canker and the freeze of 1916-1917 essentially eliminated all orchards and the use of the stock, except for a limited area south of New Orleans, Louisiana. In California, old orchards on this stock were a curiosity and most were seriously affected by exocortis. Large commercial plantings did not start until around 1950 after the seriousness of tristeza was recognized, accompanied by the release of nucellar strains of the major varieties which were free of exocortis. In some areas of Japan, for some reason, it has been a relatively short-lived stock, necessitating inarching with Yuzu at an early age. Since about 1980 in California there has been interest in the Flying Dragon strain as a true dwarfing root stock and high density plantings. Also in Australia, considerable interest involves the use of selected trifoliate orange strains inoculated with a graft transmissible agent to attain controlled dwarfing or “viroidal dwarfing” as it may be called. All these aspects of the trifoliate orange will be discussed in detail at more appropriate sections of the text. The mandarins have not been widely used as root stocks. In the People’s Republic of China, the Ponki and Sunki have been principally used, although the Ponki is not used today. Taiwan also uses the Sunki . The Cleopatra mandarin has been used to a slight extent in Florida, particularly on the heavier soils, but it did not gain favor in Texas or Arizona. In California it was used to a limited extent after 1950, but that use has declined, although a few plantings may still be made on saline or alkaline soils. The Empress mandarin was used sparingly as a root stock in South Africa. Ponkan types have been occasionally used to a very limited degree elsewhere. Interest has persisted in the Philippines in the use of mandarin stocks, and current interest for nearly the past 50 years is centered on the Calamandarin. The taxonomic status of the variety is not known. The heen-naran and the nas-naran have occasionally been used in India. The limes have been frequently tried as root stocks, but few have endured. The East Indian lime has been used to some extent in Iran and Egypt and in a few other areas, but mostly with little success or commercial importance. In Israel, the Palestine sweet lime was the most commonly used stock for over a half century. Because of xyloporosis, it was shortlived and was inarched with sour orange at an early age. Both it and the sour orange are susceptible to tristeza and the recent occurrence of this disease has forced Israel in pursuit of new root stocks.