Seeds produced from these crosses were germinated at the University Lindcove Field Station in Tulare County and grown for approximately one year. In 1982, the seedlings were tested by isozyme procedures and the difficult-to-identify nucellars discarded. The hybrid seedlings are being field grown at the Lindcove Field Station and, when they come into fruiting will be tested by Dr. M. L. Roose of the Department of Botany and Plant Sciences for their disease resistance and ultimate horticultural characteristics as root stocks. New possibilities may also arise from recombinations of species, or genera, from graft hybrids or somatic hybrids. Grapefruit develops an excellent root system and although deeply rooted, spreads more widely than that of sweet or sour orange . It is apparently best adapted to growth in heavy or loamy soils and is poorly adapted to light sandy soils. Most Citrus varieties on grapefruit stocks show an overgrowth of the stock at the bud union similar to that of shaddock. In some cases, this is like a shoulder extending out from the bud union, in others it is smooth at the union and flares outward and downward as it approaches the ground line, sometimes referred to as an ‘elephant’s foot’ . The union is, however, cylindrical and shows no tendency for fluting or ridging. The bark is usually slightly thicker than the scion. With some selections, there is a slight bud union overgrowth with lemon scions . At the Citrus Research Center, Riverside, 1927 root stock plantings of trees on grapefruit root developed into normal-sized trees and, at 34 years of age,arandano cultivo were comparable in size to adjacent trees on sweet orange stock and larger than on sour stock with all scion varieties . In spite of their large size, such trees were characterized by surprisingly low yields, which in some cases was only 70 percent of their sweet orange counterpart.

The tendency for low yields has also been noted in Florida by Hume and Ziegler and Wolfe . Fruit size tended to be larger than on sweet orange stock and comparable to that on sour orange or Rough lemon stocks . The larger fruit size might possibly be associated with the fewer numbers of fruit per tree. Fruit quality was good, equal to that on sweet orange stock and frequently slightly superior . Grapefruit root stocks are probably slightly more resistance to Phytophthora than trees on sweet orange or Rough lemon, but are not as resistant as sour orange . That is one of the reasons California growers used it, although Wutscher indicates susceptibility to foot rot was one of the reasons it was discontinued in Florida. California lemon growers also used it because they felt it was more resistant to foot rot than sweet orange or Rough lemon, and that lemon trees budded on it were not as susceptible to shell bark as those on some orange and Rough lemon. Also, phloem necrosis of the lemon scion was less than on sour. They show no resistance to the citrus nematode or the burrowing nematode . Numerous cultivars tested for their resistance to tristeza were variable in their response, some varieties being almost as susceptible as the sour orange, others less, but none were tolerant . Under field conditions, trees declined from natural infection by tristeza at a much slower rate than those on sour orange, erroneously causing some growers in California to assume the combinations were tolerant to the disease. Under California conditions, the stock does not generally stem-pit from tristeza, or only mildly so. In experimental trials conducted by the Citrus Experiment Station, Riverside, the ‘Duncan’ variety did not perform very well. This was the original ‘Duncan’ variety introduced from Florida in the early 1900’s. Recently, all seedy varieties of grapefruit in Florida were lumped as ‘Duncans’. Unfortunately, the original ‘Duncan’ source in the Citrus Variety Collection at Riverside was eliminated and a seedling of it, of doubtful authenticity, substituted.

The original ‘Duncan’ may now be lost. Of all the varieties tested at Riverside, the C.E.S. No. 343 gave the best performance, but even it did not compare with the sweet orange or the sour orange . The C.E.S. No. 343 never gained commercial acceptance. In the eastern Los Angeles basin the ‘Jochimsen’ and the ‘Hall’ were grown. In Ventura County, the ‘Camulos’ was principally grown, and in Santa Barbara County, it was the ‘Stow’. Trees budded on grapefruit stock are less cold resistant in California than those on sour or sweet orange. Although both orange and lemon trees were budded commercially on grapefruit stock, but primarily lemons, results were not outstanding and the stock was never very popular. Because of tristeza, orange trees are no longer propagated on this stock in the U.S. Lemon trees on grapefruit stock can only be purchased through special orders to California nurserymen. Grapefruit cuttings, like shaddock marcots, have performed about equal to the budded trees.The root system, like those of the Valencia and the navel, was less penetrating than those of the seedlings, with a strong shallow lateral system. It is conceivable that in some areas commercial grapefruit orchards under the right conditions could be propagated from cuttings or grown as seedlings and eliminate any bud union problems. In areas where freezes are a major hazard, as in Texas, this could eliminate the practice of mounding to regenerate trees frozen to the bud union or groundline. Several hybrids of the grapefruit have been tried as root stocks and show some interesting results. Among these are the ‘Sampson’ tangelo, ‘Williams’ tangelo, ‘Siamelo’ and the citrumelo. The tangelo will be discussed with the mandarins, and citrumelos with the trifoliate hybrids. The Poorman’s orange is neither an orange nor a grapefruit, although it is used commercially as a grapefruit substitute. Sometimes it is referred to as ‘Morrison’s’ grapefruit or ‘New Zealand’ grapefruit . It is monoembryonic and passes along a genetic scaly bark to its progeny. Like the grapefruit, when budded to Valencia orange it was susceptible to tristeza . The Israeli Poorman is not a Poorman’s orange but something else, perhaps a seedling selection from Poorman .

It is nucellar and does not show the flakey bark of Poorman. It has given fairly promising results in root stock trials in Israel , but its tristeza tolerance is no longer in doubt and it proved susceptible in trials at South Coast Field Station in l97__ . The Natsudaidai of Japan, or Japanese summer orange , is not an orange, or a grapefruit, although it too is commercially used as a grapefruit substitute in Japan where it is the second-most important variety. However, it is used to a very limited extent in Japan as a root stock for the Satsuma. The trees on it are vigorous and do not compare with the more productive trees on trifoliate orange,growing blueberries in containers but the fruit quality is inferior. In California at South Coast Field Station, orange trees budded upon it are tolerant to tristeza, show no stem pitting and are moderately vigorous. However, they are shy bearers . The pummelos or shaddocks, C. grandis [L.] Osbeck , are often confused with the grapefruit, particularly some of the smaller fruited varieties. The pummelos are referred to in some areas as pumplemousse, zabon, buntan, bankan, etc. They differ from most other potential root stocks in one major character, however, in that all varieties of pummelo are monoembryonic and produce no nucellar seedlings. In spite of this, most of their progeny are remarkably uniform in the nursery row and require a minimum of rogueing. When budded to major scion varieties, the scions are remarkably vigorous and uniform in size, even up to an age of 34 years , with surprising uniform performance at the Citrus Experiment Station, Riverside . Some of the better eating commercial varieties like the ‘Alemoen’, ‘Thong Dee’, ‘Kao Phuang’ and ‘Kao Panne’ have been among the most uniform. From the author’s experience, variability is not the major factor in their failure to obtain commercial root stock acceptance . Pummelo fruits, if not parthenocarpic, generally produce an abundance of large seeds, although under certain conditions they may be nearly seedless. Many are self-incompatible and require pollenizers to set seed . The seeds germinate readily and produce vigorous seedlings which are somewhat bushy in character, like the grapefruit, and require considerable training prior to budding. Numerous pummelo varieties have been successfully budded with most of the major commercial citrus varieties in California experiments and observed in various plantings for periods of from 13 to 34 years of age. The reported failure of Satsuma mandarin on pummelo cited by Webber was undoubtedly due to the presence of the tristeza virus in the Satsuma scion. All varieties of pummelos budded with orange scions and inoculated with tristeza by the author , were extremely susceptible to the disease. The reaction of pummelo to tristeza is very similar to sour orange in this respect with many of the young inoculated trees collapsing and dying within a few years. Trees affected with tristeza do not show stem pitting in the stock. This then , is the greatest drawback to the use of pummelo as a root stock, not its variability. In California, especially in root stock trials at the Citrus Experiment Station, oranges, grapefruit and lemons budded on pummelo stocks make large sized trees about equal to those on sweet orange stock. Like grapefruit stocks, yields tend to be somewhat low in proportion to the size of the trees. Like grapefruit, fruit sizes tend to be larger than on stocks like sweet orange and Cleopatra mandarin and more comparable to that on Rough lemon stock . Fruit quality is comparable to that on sweet orange or sour orange stock . The bud union of trees budded on pummelo is good, and like grapefruit stock, shows an overgrowth of the stock.

This may be either a slight shoulder effect or a tendency to slope outward and downward from the union in a slightly pyramidal shape . There is no distinctive character to the bark, except that it tends to be considerably thicker than the scion bark, much thicker than grapefruit. In some instances, the bark may be very thick and extremely rough, like that of an oak tree . Lemon bud union overgrowth does not appear to be a serious problem with the stock in California in the limited plantings observed. The root system of the pummelos is a strong, deeply penetrating tap root system with strong laterals. There are many fibrous roots, with the feeder roots tending to be coarser in texture than feeder roots of most other species . It shows no tolerance to the citrus nematode or the burrowing nematode . Pummelos, however, do have a high resistance to Phytophthora . No problems with gummosis were experienced in any of the author’s plantings. In Thailand, where pummelos are extensively grown, they are propagated principally as marcots and grown on soil beds a few feet above the water table and are relatively short-lived. It is probable that few other stocks would tolerate these conditions as long as the pummelo. In the Philippines, trees grafted on pummelo roots reportedly showed more mottle-leaf disease than on other root stocks observed . In California root stock trials, it was not observed that zinc deficiency was any more of a problem than on the other root stocks. In one lemon root stock trial in Ventura, California, where boron and salt were both somewhat of a problem, the Alemoen shaddock did not show the boron tolerance of C. macrophylla or C. pennivesiculata, but of 40 root stocks, gave essentially the best total performance after 25 years. In Thailand, it is grown under conditions where brackish water would be a problem for many other stocks, but in Thailand salinity, not boron, is the problem. Some varieties of pummelos are not very vigorous when grafted on other root stocks, nor as cuttings, at Riverside. Whether this is due to some unrecognized virus disease, or is of a genetic nature, is not known. Certain hybrids of the pummelo at Riverside have been extremely vigorous, as for example sibs of shaddock X St. Michael sweet orange. One of these hybrids provided fairly interesting root stock results, but proved to be very susceptible to tristeza . The pummelos therefore, have never gained commercial acceptance as root stocks and probably never will in a tristeza area.