Vegetative growth control in apples is one of the most basic and necessary activities that an orchardist is required to do. Frequently, vegetative growth is excessive and if not appropriately controlled, it could influence many aspects of fruit production, including flower bud formation, fruit set, fruit quality, physiological disorders, pest management, and postharvest fruit life.
Various methods of growth control have been practiced in the past (Greene, 2003; Miller, 1988). The widespread use of dwarfing rootstocks has been a major help in regulating vegetative growth. However, rootstock is only a partial solution to control vigorous vegetative growth because aberrant weather, frost, biennial bearing tendencies, lack of crop resulting from excessive thinning, and planting errors cause situations in which additional forms of growth control are needed.
Daminozide was used successfully for many years to control growth, but its registration was withdrawn for use on apples in 1989. Ethephon is an ethylene-liberating growth retardant that is currently available, but it may have limited use on bearing apple trees because the rate required for effective growth control can also lead to substantial thinning (Greene, 2003).
Prohexadione-calcium (ProCa) is a relatively new growth retardant that was labeled for use on apples in the United States in 1999. A number of reports have confirmed that it is an effective growth retardant (Basak, 2004; Byers et al., 2004; Greene, 1999; Medjdoub et al., 2005; Miller, 2002; Rademacher and Kober, 2003; Unrath, 1999). To achieve maximum growth control, it must be applied as soon as sufficient foliage has emerged to allow for foliar penetration (Byers and Yoder, 1999; Miller, 2002; Rademacher and Kober, 2003; Unrath, 1999). Generally, the timing of the first application is at petal fall (PF) (Rademacher and Kober, 2003). ProCa is rapidly inactivated within the tree (Rademacher et al., 2004). There is general agreement among those reporting satisfactory growth control that one or more follow-up applications of ProCa are necessary to achieve season-long growth control, especially when lower rates are used (Miller, 2002).
During the development and testing of ProCa, it was discovered that it was effective at reducing the shoot blight phase of fire blight (Yoder et al., 1999). Effective control generally required application at the highest rate of ProCa recommended on the label. The time of initial application for fire blight control was at PF, similar to that for vegetative growth control. A reduction in shoot growth appears to be a prerequisite for effective fire blight control with ProCa (Norelli and Miller, 2004). Closely associated with this growth reduction is a buildup of pathogen resistance, which is attributed to a novel flavonoid, leuteoferol (Rademacher, 2004).
Usually there are both positive and negative effects where a plant growth regulator is applied to regulate a physiological response on a crop. Adjustments are most often made in concentration applied, the time of application, or use that often minimize the undesirable effects while preserving the positive effects (Greene, 2002a). ProCa is no exception. Although the greatest degree of growth control is frequently observed when high rates are used (Medjdoub et al., 2005; Miller, 2002), these rates may result in increased fruit set and result in fruit that appear to be less susceptible to normal thinner application (Miller, 2002). Because effective and appropriate crop load management is a key to profitable apple production, it will be important to develop strategies for using ProCa that minimize increased fruit set and allow for effective and predictable chemical thinner action.
The purpose of this investigation was to confirm the effect of ProCa on fruit set and to examine several chemical thinning strategies that may be used to effectively and appropriately thin ProCa-treated apple trees.
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