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Title: FRUIT SIZE AND YIELD OF MANDARINS AS INFLUENCED BY SPRAY VOLUME AND SURFACTANT USE IN NAA THINNING

Author
item Stover, Eddie
item CILIENTO, SCOTT - UNIV - OF FLORIDA
item MYERS, MONTY - UNIV - OF FLORIDA
item JACKSON, JOHN JR - UNIV - OF FLORIDA
item STILL, MAX - UNIV - OF FLORIDA

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/16/2006
Publication Date: 10/1/2006
Citation: Stover, E.W., Ciliento, S., Myers, M., Jackson, J., Still, M. 2006. Fruit size and yield of mandarins as influenced by spray volume and surfactant use in naa thinning. HortScience. Volume 41: pages 1435-1439

Interpretive Summary: Some citrus cultivars are prone to alternate bearing, in which trees produce very heavy crops of small fruit in the “on” year and much smaller crops of larger fruit in the “off” year (Monselise and Goldschmidt, 1982). This often reduces cumulative fruit value, since many “on” year fruit may have much lower or even no market value, and production may be so low in the “off” year that net commercial production is compromised. Heavy “on” year production trees may also result in substantial tree stress, reducing growth or breaking limbs, which may adversely affect future production. Florida citrus producers have primarily relied on hedging, topping, and/or skirting to reduce cropping in orchards with excessive bearing (Stover et al., 2003), but increased production of larger fruit is often modest or negligible despite substantial cropload reductions (Hilgeman et al., 1964; Monselise and Zaphris, 1960; Morales and Davies, 2000; Stover et al. 2002a; Stover et al., 2003). In addition, pruning for cropload reduction is likely to delay canopy development in young orchards (Stover et al., 2002a). Use of 1-naphthaleneacetic acid (NAA) during early fruit development has been reported to reduce cropload and increase fruit size for various citrus cultivars in diverse growing regions (Brar et al., 1992; Farmahan, 1992; Gallasch, 1988; Greenberg et al., 1992; Hield et al., 1962; Iwahori, 1978; Ortolá et al., 1991; Sharma et al., 1993; Wheaton, 1981). Although NAA has been labeled for thinning of many Florida citrus varieties for more than twenty years it has received little commercial use. Published reports on citrus thinning typically involved high spray volumes (2300-4600 L.ha-1). At rates of 250-500 ppm NAA reported to thin Florida citrus, NAA sprays at these volumes would cost US$400-1600.ha-1 for NAA alone at 2004 prices. Since annual spray material costs for fresh Florida citrus currently average US$500.ha-1 (Stover et al., 2002b), high cost is likely to be a major limitation to more widespread NAA thinning of Florida citrus. We conducted six trials to determine whether lower spray volumes and/or inclusion of different surfactants would permit adequate thinning at much lower cost per hectare.

Technical Abstract: Six trials were conducted to determine whether lower spray volumes and/or inclusion of different surfactants would permit adequate thinning of mandarin hybrids (Citrus reticulata hybrids) at a much lower cost per hectare. Sprays were applied using a commercial airblast orchard sprayer during physiological drop, when fruitlets averaged 8-16 mm in diameter. Surfactant was always included at 0.05% v/v. NAA always reduced fruit per tree, increased fruit size, and decreased production of smallest size fruit. However, in only three experiments, contrast of all NAA treatments vs. controls indicated increased production of the largest (80-100 fruit per carton) and most valuable fruit. In four of five experiments, comparison of spray volumes of 600 (only examined in 3 of 4 experiments), 1200, or 2300 L.ha-1 demonstrated significant fruit size enhancement from all NAA applications. Most individual NAA treatments resulted in fewer fruit per tree, but there were no statistically significant differences between NAA treatments at different spray volumes. In only one of the four experiments, there was a marked linear relationship between spray volume and fruit per tree, yield, mean fruit size, and production of largest fruit sizes. The effects of surfactants (Activator™ a non-ionic, Silwet L-77, and LI-700) on NAA thinning were tested in both ‘Murcott’ and ‘Sunburst’. In comparisons between Silwet L-77 and Activator™ surfactant, one experiment with ‘Murcott’ showed greater fruit per tree and yield reduction from using Silwet, but with a smaller increase in production of largest fruit sizes, while in another ‘Murcott’ experiment, Silwet L-77 reduced numbers of smaller fruit size with no increase in production of larger fruit. Based on these findings, current recommendations for NAA thinning of Florida mandarins are use of spray volume of ~1100-1400 L.ha-1 on mature trees, with proportionally lower volume on smaller trees. These data appear to support use of a non-ionic surfactant rather than other tested surfactants in NAA thinning of Florida mandarins. Because experience with NAA thinning of Florida citrus is limited, it is only recommended where the disadvantages of overcropping are perceived to substantially outweigh the potential losses from overthinning.