Citrus Scion and Rootstock, Topping Height, and Tree Spacing Affect Tree Size, Yield, Fruit Quality, and Economic Return

A factorial experiment begun in 1980 included 'Hamlin' and 'Valencia' sweet-orange scions (Citrus sinensis (L.) Osb.), and Milam lemon (C. jambhiri Lush) and Rusk citrange (C. sinensis x Poncirus trifoliata (L.) Raf.) rootstocks, tree topping heights of 3.7 and 5.5 m, between-row spacings of 4.5 and 6.0 m, and in-row spacings of 2.5 and 4.5 m. The spacing combinations provided tree densities of 370, 494, 667, and 889 trees ha. Yield increased with increasing tree density during the early years of production. For tree ages 9 to 13 years, however, there was no consistent relationship between yield and tree density. Rusk citrange, a rootstock of moderate vigor, produced smaller trees and better yield, fruit quality, and economic returns than Milam lemon, a vigorous rootstock. After filling their allocated space, yield and fruit quality of trees on Milam rootstock declined with increasing tree density at the lower topping height. Cumulative economic returns at year 13 were not related to tree density. Citrus and other tree fruits yield more during the early years of and then becomes less vigorous is desired. The range of dwarfing production when planted at higher densities (Cary, 1981; Jackson, rootstocks used by apple growers to manipulate tree size and 1985; Patil, 1987). However, the advantage of higher densities at fruiting characteristics is not currently available to citrus growers. tree maturity is less certain. For citrus, we proposed that yield at Although no satisfactory dwarfing rootstock for citrus is commer- maturity is independent of tree density over a range of densities cially available, citrus rootstocks do provide a modest range of tree (Wheaton et al., 1978). Production of modern mature apple or- vigor and final tree size (Castle et al., 1989). Another method of chards increased with tree density, however, possibly due to reducing vigor and tree size at maturity is the introduction of improved genetic material with a higher harvest index, rather than dwarfing citrus viroids into trees on susceptiblero otstocks (Hutton, from higher tree density per se (Jackson, 198.5).