In the tropical climatic zone, the vigor observed in citrus fruit trees represents a handicap for orchard development and limits yield increase of cultivated surface area. As a result, in New Caledonia, growers often plant at very low densities (150 to 200 plants/ha) leading to poor sanitary conditions linked to the sizes of the trees to manage; pruning and harvesting operations for such trees result in high crop costs and low incomes. Such constraints become particularly troublesome when site topography hinders mechanical harvesting techniques. Various procedures to tree size control based on the use of pathogenic agents (Broadbent et al., 1986; Golomb, 1988), and specific horticultural or cultural techniques (Golomb, 1988; Krezdorn, 1978; Piner, 1988), have been tested. Pathogenic agents can induce numerous drawbacks and are not easily introduced to areas where they are absent or could present the risk of mutation toward more virulent forms. On the other hand, propagation and cultural techniques that are unreliable and/or expensive have not been yet exploited by nurserymen. The most researched method has been the use of rootstocks, which has also shown to be reliable and simple. In the deciduous fruit industries of the world, particularly apple, size-controlling rootstocks have been very effective in relation with the availability of suitable plant material (Webster, 1997, 2004). With citrus, only a few rootstocks have shown any potential for consistent size control and the best one has been Flying Dragon trifoliate orange (Bitters et al., 1979) in association with good yield and fruit quality (Roose, 1986).
The term “dwarfing” can only be applied to rootstocks that reduce tree volume by at least 75%, thus limiting tree height to 2.5 m at adult age (Bitters et al., 1979). FD would be one such dwarfing rootstock. Originating from Japan, it was used to produce ornamental potted plants and introduced in the United States by Walter S. Swingle in 1915; it remained for long as a botanical curiosity. Thereafter, economic considerations led researchers to identify dwarfing rootstock, which would make it possible to increase planting densities and, consequently, yields. Like Poncirus trifoliata, of which it is a mutant (Castle, 1992), FD has tolerance traits toward citrus tristeza virus, resistance traits toward Phytophthora spp. and citrus nematodes as well as a good level of tolerance for cultivation in heavy soils. Its use requires the selection of scions, which are free of exocortis, a degenerative viroid disease to which Poncirus and their hybrids are very sensitive.
Taking into account this overall set of characteristics, the objective of this trial was to determine the potential of FD dwarfing rootstock under New Caledonian climatic conditions at the Pocquereux Fruit Research Station with citrus cultivars actually planted for local fresh market (orange, mandarin, grapefruit) and for export to Australia and New Zealand (Tahiti lime). New Caledonian global citrus production has reached 3000 t in 2009 (DAVAR, 2010). Expected results would be assigned to extension services to improve the profitability of the New Caledonian citrus industry.
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