Seedling citrus cultivars have encountered problems and become unthrifty in many countries of the world due to root fungi, drought, nematodes or salinity. To overcome these problems, citrus is now almost universally propagated by budding onto rootstocks. This practice has led to a new set of problems due to virus and virus-like diseases which are common in old line trees and are transmitted by vegetative propagation, but are not transmitted through seed. For a healthy citrus industry, a considerable and continuing effort is necessary to provide virus-tested budwood to nurserymen and growers. Trees propagated vegetatively do have the advantages of higher production, greater uniformity of product, and of being less thorny and more manageable than seedlings.
Rootstocks available in Australia vary considerably in their resistance to root fungi, virus diseases, nematodes and salinity. They also differ in longevity, mature tree size, drought and cold tolerance and in their influence on production and fruit quality. Because the virus disease tristeza is widespread and is transmitted by the black citrus aphid, rootstock scion combinations need to be tolerant of this disease except in the Northern Territory, where the aphid vector is unknown.
On sites where citrus has been previously grown, there is a build-up of fungi and nematodes in the soil which may affect the development of young citrus planted in the same area. To avoid this retarded development, only rootstocks tolerant or immune to soil fungi and nematodes should be grown in such replant situations.
The major rootstocks used in Australia include trifoliata, sweet orange, citrange, citronelle and Cleopatra and Emperor mandarin.
They are all highly polyembryonic but do occasionally produce hybrid progeny. The performance of such hybrids as rootstocks is uncertain and whenever they are recognised, hybrid seedlings are discarded in the nursery.
Apart from the sweet oranges, which are seedy, mid-season cultivars, and Emperor mandarin, fruits of rootstock varieties are unpalatable. Citronelle is inferior to lemon but may be used as a lemon substitute. Fruits of trifoliata and its hybrids are very bitter, while fruits of Cleopatra mandarin are very acid and very seedy.
Trifoliata (Poncirus trifoliata (L) Raf.) This stock is most suitable for lime-free soils and can withstand cold and wet conditions. It has excellent root fungus and nematode tolerance but is susceptible to high chloride levels in soils or irrigation water. Fruit quality for trees on this rootstock is excellent with high juice and total soluble solids contents. It has a good capacity for supplying nitrogen, phosphorous and potassium to the tree but, particularly on alkaline soils, trees can become deficient in zinc, manganese, copper and iron. For storing Valencias on the tree, this stock has been less successful than others because of the early onset of regreening and loss of juice. If scionwood is infected with any but very mild strains of the viroid disease, exocortis, trees may be stunted and unthrifty. Lemons, other than virus-tested Lisbons, have similar problems and are not recommended on this stock.
Sweet orange is widely used on virgin, well-drained soils. It is more tolerant of chloride than trifoliata but is less tolerant of soil fungi, nematodes and water-logging. In replant situations, this rootstock has been unsatisfactory and is not recommended.
Citranage (C. sinensis x P. trifoliata) is a widely-used rootstock, the performance of which is intermediate between that of its parents. It has sufficient tolerance of soil fungi and nematodes to be suitable in replant situations. Like trifoliata, it is intolerant of exocortis and cannot be used for lemons other than exocortis-free Lisbons. There are several varieties of citrange in use. Troyer and Carrizo citranges, both of which originated from the same cross of Washington navel with P. trifoliata made in California in 1909 (Savage and Gardner 1965), have very similar field performance. The Rusk citrange which developed from a cross of Ruby blood orange and P. trifoliata made by Swingle in 1897, is considered a good rootstock for navel oranges but has relatively low seed numbers. The Benton citrange originated in Australia and is showing promise in recent trials with oranges and in replant situations with Eureka lemons.
Citronelle (C. jambhiri Lush.) This rootstock was widely used in early Australian development because of its ease of propagation, excellent nursery characteristics and drought tolerance. Trees on this rootstock are vigorous with high early yields, but are generally short-lived and become unthrifty after about 15 years except on very well-drained, sandy soils in arid areas where water management is very good. Scion fruit quality is generally poor, with low juice and total soluble solids content. It is not recommended for mandarins or Ellendale tangor.
Cleopatra mandarin has good tolerance of soil fungi, nematodes, salinity and lime and is used for heavier-textured soils than those considered ideal for citrus culture. On this rootstock, trees are slow-growing with low yields in earIy years, but are long-lived and out yield most other stocks after about 20 years. Scion fruits tend to be small but have excellent quality.
Emperor mandarin has similar performance to Cleopatra but is less tolerant of salinity and lime and tends to produce lower yields of smaller fruit. Like Cleopatra it is slow-growing in early years but is long-lived.
Rangpur lime (Citrus reticulata var. austera hyb. or Citrus limonia Osbeck) This has good tolerance of drought, salinty and lime, but is only moderately tolerant of soil fungi. It is susceptible to exocortis, and only virus-free budwood can be used. It is similar to citronelle in imparting to the scion vigorous early growth and producing high yields of low-quality fruit. It performs best with lemons and limes.
Sour orange. Because of its susceptibility to tristeza, this stock cannot be used with sweet-fruited scions but is satisfactory with lemons and limes. It has excellent soil fungi, nematode, cold and drought tolerance and imparts high quality to fruit.
Macrophylla (Citrus macrophylla Wester) This rootstock has shown promise for lemons but only if they are budded before the seedlings become infected with tristeza. It cannot be recommended for sweet-fruited scions.
Sacaton citrumelo (C. paradisi x P. trifoliata) This rootstock has shown promise for lemons, grapefruit and tangelos in India and Florida but had the disadvantage of seedling variability.
* Require scion free of exocortis to prevent stunting
DATE: November 1988
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