In California, they are larger than on Cleopatra mandarin, but smaller than those grown on Rough lemon, sour orange, or Troyer citrange. Seedlings of sweet orange are resistant to verrucosis according to Klotz . Again, this is not a problem in California, but in nurseries or grown trees it can be a problem in areas with more humid climates. Sweet orange is also resistant to mal secco , but this disease is not known to exist in the United States. It is a severe problem in some Mediterranean countries. Psorosis, or scaly bark, can be more serious on sweet orange than the resistant sour orange , since if both the scion and the stock are affected, the rate and degree of decline of the trees may be greater. Lemon trees on sweet orange do not express shell bark with the severity that they do on sour orange or Rough lemon. The sweet orange does not appear to be affected by exocortis, cachexia, xyloporosis, or woody gall . Trees on sweet orange root stock in Florida are not threatened by blight according to Lawrence and Bridges . The reaction of sweet orange root stocks to tristeza inoculations has generally been negative . However, in my Baldwin Park experiments in California,blueberry container there was some light stunting as a result of the inoculations. A seedling cultivar of ‘Bessie’ sweet reacted severely enough to be classed as susceptible.
The mother tree source did not. Trees on ‘Koethen’ sweet were also somewhat stunted and borderline in their reaction. In Brazil, trees on ‘Caipira’ sweet orange are also adversely affected. Sweet orange is susceptible to the citrus nematode . In fact, it is one of the most susceptible root stocks. Certain selections of sweet orange like the ‘Ridge Pineapple’ and the ‘Sanguine Grosse Ronde’ have shown some resistance to the burrowing nematode . Not only are sweet orange trees very susceptible to nematodes, they are very susceptible to gummosis . This is true even though the trees are budded high, planted high, and the best cultural practices are used. This is probably the greatest disadvantage that sweet orange has. It is remarkable, considering the susceptibility of sweet orange, that so many seedling orchards survived for a half a century or more, in the early history of California and Florida plantings. Surviving affected trees often bear scars of the infection by this disease. Sweet orange trees are as susceptible as trees on Rough lemon, often worse, but not nearly as tolerant as sour orange, Alemow, or Troyer citrange. It is interesting that at the Citrus Experiment Station, Riverside, and in some of my other root stock plots, that although the sweet oranges in the original plantings were remarkably free of gummosis, that in replant situations, even though the soil was fumigated and good cultural practices used, it is almost impossible to successfully re-establish trees on sweet orange root . It should be pointed out, however, that the soil fumigation is generally for eliminating the citrus nematode. The fumigant rate to kill nematodes is lower than the dosage to act as a fungicide. Because of roots the higher dosages are generally not used.
Sweet orange has very poor root generation capacity and once infected, usually succumbs to the attack. Klotz and Fawcett reported that the ‘Indian River’ variety, an ambiguous name, from Florida was somewhat more resistant to gummosis than other sweet orange varieties tested in the variety collection at Riverside, or in existing root stock trials also at the Citrus Experiment Station. However, the tests were not replicated and the gummosis differences were small. The difference was not enough to attract any grower interest and consequently, it was never used as a commercial root stock. The gummosis resistance of ‘Precoce de Valence’ reported by Olson, et al. in Texas would also warrant further investigation. When the author came to Riverside in the mid-l940’s, there were still many seedling orange orchards in Riverside and adjacent areas. The first sweet oranges were planted in Riverside in 1871. One of these, the ‘Koethen,’ was probably planted prior to 1880, and was selected by H. B. Frost in the early 1900’s, and later incorporated into the 1927 root stock trials by H. J. Webber and reported on elsewhere in this text. The ‘Koethen’ turned out to be one of the better performing sweet orange cultivars and is still grown commercially, especially in Southern California. Seedling orange trees were first planted in San Bernardino County in 1857. One of these, the old Cram tree, at East Highlands is shown on page 36 of Volume I of The Citrus Industry, with Dr. H. J. Webber standing beside it. It was widely used in the San Bernardino area. Nearby was the ‘Hinekby’ orchard at Bryn Mawr. It also was used commercially.
In Ventura County, the ‘Olivelands’ sweet from the Limoneira Company has also been widely used as a root stock in that area and its other seedling orange cultivars have also been used, and their performance has been good. The Limoneira Company has continued to use the ‘Olivelands’ sweet as its principal root stock for lemons. They state “we know sweet orange, its performance and limitations. We feel that with good cultural practices, inspection, and sanitary procedures we can control gummosis. There is nothing we can do about the phloem necrosis of the Eurekas on C. macrophylla, the sudden incompatibilities of Eureka on ‘Troyer’, on ‘Swingle’ citrumelo, or #1452 citrumelo, and the bud-union overgrowth and compression girdling on ‘Cleopatra’ mandarin. Much of the success of an orchard depends on its longevity and orchards which have to be replaced every decade or several decades cannot be considered successful.” There was always interest in a “good-performing” sweet orange. About 1950 the author introduced the ‘Kona’ from the Kona district of the island of Hawaii on the basis the trees were quite old and grew quite well in an area characterized by 250 cm of rainfall. Its performance in California districts was mediocre. In 1940, H. S. Fawcett introduced sweet orange seeds from the Santa Ana Mission in North Argentina. The author incorporated these in later root stock trials and the performance has been good. The author visited many sweet orange seedling orchards in Southern California,maceta cuadrada which persisted well into the 1950’s. The very striking thing in most of these orchards was the freedom of the trees from gummosis which certainly was not a factor in their ultimate removal. In an effort to develop cold hardy edible citrus to extend its commercial growing range, Swingle hybridized the navel orange with trifoliate orange, and the ‘Troyer’ and ‘Carrizo’ citranges are part of the result. The ‘Washington’ navel, aside from producingno or few seeds, has nothing to recommend it as a root stock. More recently, Cameron and Soost hybridized the ‘Ruby’ blood orange with a trifoliate orange from which resulted the ‘C-32’ and ‘C-35’ citranges. However, the ‘Ruby’ blood has never been used as a root stock and has nothing to recommend it. Dr. Cameron’s defense was that they thought the ‘Ruby’ produced more hybrids than some of the other sweet oranges. However, that seems a poor excuse, when a few more pollinations or the use of developing tissue culture techniques may have increased the number of zygotic embryos obtained. One wonders how much better ‘Troyer’, ‘Carrizo’, ‘C-32’, and ‘C-35’ citranges might have been if the sweet orange female parent would have been a more desirable and proven root stock type. If new citranges are to be developed for root stocks, then certainly the plant breeders should consider using the ‘Argentina’ sweet, or the ‘Olivelands’ sweet, as the female parent. One of the disadvantages of some of the commercial citranges is their exceptional vigor, which doesn’t fit in with today’s interest in close spacing. However, the ‘Fuya Menuda’ cultivar is a genetic dwarf. Hybridized with a dwarfing strain of trifoliate orange, it is possible that more dwarfing hybrid citranges might be obtained.
Positive identification or verification of the ‘Fuya Menuda’ is essential. Unfortunately the specimen in the Citrus Variety Collection at Riverside is not true to type and hopefully has not been distributed in seed or budwood requests. None of the sweet oranges tested have ever shown any resistance to the citrus nematodes. However, the ‘Ridge Pineapple’ and the ‘Sanguine Grosse Ronde’ show resistance to the burrowing nematode . Perhaps they should be hybridized with select strains of trifoliate orange to produce hybrids possibly resistant to both the citrus and burrowing nematode. Also, the California strain of ‘Carrizo’ citrange which shows resistance to the burrowing nematode might be hybridized with the above sweet orange cultivar to provide improved resistance to this destructive soil organism. The sour orange, C. aurantium, was once the world’s most widely used citrus root stock. It was intensively used in the United Sates , all of the Mediterranean area, including the Near East, all of South America, Cuba, Central America, and to a limited extent in Australia and New Zealand. It was never successful in the Orient, Java and South Africa due to a virus disease called tristeza . Because of the rapid spread of this disease around the world, the acreage on this stock has continued to dwindle. It is still an important root stock in the United States in Arizona, Texas and Florida. It is still used in the Mediterranean area, but the occurrence of tristeza in Spain and Israel threatens adjacent countries as well as the Near East, and other root stocks are rapidly being employed. The stock is still used in Cuba, the western half of South America , in Central America and a few other scattered areas. As a root stock the sour orange possesses many fine qualities and it has been a major disaster to the citrus areas which have been decimated by the tristeza virus. A sour orange root stock fully tolerant to the tristeza virus would still be a tremendous asset to the growers and the plant breeders. All of the standard sour orange varieties are very seedy, producing large plump seeds, which are moderately to highly nucellar, not being as high as sweet orange, Rough lemon, ‘Troyer’ citrange, ‘Cleopatra’ mandarin, etc. This is not always true of the socalled aberrant types like ‘Chinotto’ or putative hybrids like the ‘Nansho daidai’ which will be discussed later. A minimum of rogueing is therefore required in the citrus seed bed or nursery and container growing. The seeds are also more resistant to drying out than seeds of many other citrus species like Rough lemon and trifoliate orange. The seedlings are fairly vigorous growers, not as much so as Rough lemon, Alemow, or ‘Troyer’ citrange, but more so than sweet orange or grapefruit. The seedlings develop mainly a strong single trunk which requires less shaping in the nursery than sweet or grapefruit, but more than ‘Troyer’ citrange and Rough lemon . This growth habit has facilitated easy budding, and the seedlings bud well to all common scion varieties. The sour orange may be grown quite easily from cuttings, but they are more difficult to root than cuttings of Rough lemon and sweet orange. Due to its seediness and the uniformity of its seedlings, there is no need for the cuttings. Seedlings are perhaps more resistant than other root stocks commonly used with the exception of the trifoliate orange and some of its hybrids like ‘Troyer’, ‘Carrizo’, ‘C-32’, ‘C-35’, ‘Swingle’ citrumelo, etc. When frozen back, the seedlings readily sprout from the base of the trunk and a sprout can easily be trained to a new leader. Sour orange seedlings are very susceptible to citrus scab , which sometimes causes serious injury to nursery seedlings in humid climates . The bud unions of the common sour orange variety are nearly normal with scions of sweet orange, mandarin, and grapefruit varieties, but rather commonly show a strong scion overgrowth with most varieties of lemons , limes, and citrons. This lack of congeniality was called “miriñaque” with lemon in Spain . With most scion varieties there is a slight bulge at the union, with the stock tending to be smaller than the scion. Grapefruit scions tend to overgrow the stock to a greater degree, but this is slight compared to the reaction with lemons and citron . Certain strains of Lisbon lemons such as the ‘Keen,’ ‘Monroe,’ ‘Bradbury,’ etc. on sour orange show a more congenial reaction.