Scientific name: Citrus paradisi
Origen: Florida, United States
Other names: 4475 Citrumelo
Last update: August 2008
Swingle citrumelo is a hybrid obtained in Florida in 1907 by the scientist W. T. Swingle who polinized flowers of 'Duncan' grapefruit (Citrus paradisi) with pollen from trifoliate (Poncirus trifoliata). The initial objective was to transfer cold resistance from trifoliate to grapefruit. This hybrid was originally called CPB 4475 and its fruit did not have edible qualities. In the 40's, however, it was tested as a rootstock. Soon thereafter, it was introduced into Brasil with many other germplasms for tests of resistance to the Tristeza virus disease which was killing all trees budded on sour orange. Since then, CPB 4475 has performed well in many rootstock experiments in most citrus areas of the world as an alternative to trifoliate and its other hybrids. It was then given the name Swingle as a tribute to its creator.
The main characteristic of Swingle is to advantageously substitute trifoliate type rootstocks. Its resistance to foot rot (Phytophthora spp), citrus nematodes (Tylenchulus semipenetrans), and to low temperatures is similar to or better than that of other common trifoliate types. In addition, Swingle has shown more tolerance to citrus blight. Fruit quality of oranges on Swingle is excellent with high sugar levels, excellent flavor for natural consumption, and high sugar yields in the processing plants. In years of high productivity, trees on Swingle require heavier potassium fertilization in order for the fruit to attain sizes similar to those on Rangpur lime.
Tree growth on Swingle is more vigorous than on trifoliate and similar to that on common citranges. It is, however, less vigorous than that of trees on Rangpur lime, which results in lower initial costs of spraying and other cultural practices. However, as trees on Swingle survive longer due to disease resistance, they ultimately attain very large tree sizes. This can only be avoided through hedging and topping, specially if higher density planting was utilized. These additional costs, however, are compensated by the longevity of the plants and the minimal requirements for resets in the area.
The main utilization of Swingle as a rootstocks is due to its disease resistance, specially, to citrus sudden death. Areas of high foot rot, nematodes, or citrus blight incidence also can be advantageously replanted with Swingle. Swingle trees do not tolerate free water on the surface. However, as they are resistant to foot rot, they survive humid soil conditions much better than Rangpur lime.
One limitation to the use of Swingle is its incompatibility to several commercial citrus varieties. Pera, Murcott, and true lemon incompatibilities are well documented. Other commercial varieties like Hamlin, Bahia, Valencia, Natal, Folha Murcha e Ponkan have been propagated on Swingle for more than 15 years with no problems. Incompatibility can be overcome through the use of a interstock between the desired scion and Swingle. Pera, for instance, can be successfully budded on Hamlin shoots previously budded on Swingle.
Trees budded on Swingle are very susceptible to water stress, just like other trifoliate types. However, trees on Swingle have usually recovered well from drought periods producing extensive flushes and blooming, with good fruit set. Obviously, irrigated orchards are the ones that better benefit from rootstock diversification, making Swingle an excellent alternative to Rangpur lime.
Swingle is resistant to Tristeza and, as a trifoliate relative, probably susceptible to Exocortis.
In the nursery, Swingle citrumelo poses more difficulties in the production of trees than more vigorous rootsttocks like Rangpur or Cleopatra do, but less so than trifoliate rootstock. As a seed producing tree yields are very high and the occurrence of hybrids or off-types on the seedbed is not as great as for Rangpur. Swingle seedlings are susceptible to nutritional deficiencies specially during the dry and cold seasons. It is common for the apical young shoot to die back after vigorous growth but no fungi or other microorganisms have been isolated from symptomatic tissue. However, soon after the dieback a new shoot emerges laterally with great vigor and the plant develops normally. As opposed to Rangpur, budding during the winter may not be possible due to lack of slipping of the bark, or problems may arise from lack of bud growth after budding. Trees on Swingle typically require additional 30 days to be ready for shipment from the nursery when compared to Rangpur lime.