Chemical thinning is practiced in most tree fruit-growing areas around the world. It is the easiest, most practical and cost-effective way to reduce cropload on heavily cropping trees (Wertheim, 1997; Williams and Edgerton, 1981). Reduction in the number of fruit to a more favorable leaf-to-fruit ratio allows high-quality fruit to be produced in the current year and improves bloom the next year. There is an ongoing search for new thinning chemicals that have different modes of action. This search has been prompted by the withdrawal of some fruit thinners and fueled by consumer-driven concerns for food safety, thus stimulating a search for environmentally friendly and naturally occurring compounds (Dennis, 2000). Furthermore, there are fewer thinners that are available for use on or are effective on pears than on apples.
Recently there has been increased interest in understanding the way(s) chemical thinners cause fruit abscission (Greene, 2010). It is reasoned that greater understanding of the modes of action will lead to more consistent and effective results (Fallahi and Greene, 2010). This would be accomplished primarily by allowing use or application at times and under environmental conditions that would naturally supplement the natural tendencies of the compound to cause abscission. Several possible modes of action have been proposed for the presently used thinners (Dennis, 2002). Therefore, matching the thinner, timing, concentration, and the cultivar and with the environmental conditions at the time of and immediately after application is likely to improve the consistency of the chosen thinner (Robinson and Lakso, 2011). A fairly substantial body of evidence has accumulated in recent years linking the carbohydrate status within a tree with thinning effectiveness and severity (Byers et al., 1991; Kondo and Takahashi, 1987; Lakso, 2011; Stopar et al., 2001). Cited modes of action of naphthaleneacetic acid (Stopar et al., 1997), benzyladenine (Yuan and Greene, 2000), and carbaryl (Knight, 1983; Williams and Batjer, 1964) suggested that they all or in part work through their influence on the carbohydrate status within the tree or spur.
Abscisic acid is a naturally occurring plant hormone that is involved in several physiological responses (Milborrow, 1984). Perhaps one of the most visible and important plant processes that it regulates is stomatal opening and closing. The ability of ABA to regulate stomatal movement led to the development of a proprietary product that, among other things, is meant to improve plant survival by lowering water loss by closing stomata when plants are exposed to stressful conditions that may lead to excessive water loss (Blanchard et al., 2007; Waterland et al., 2010). ABA has the potential to influence the carbohydrate status within a plant by closing stomates, thus reducing photosynthesis during the time the stomates are closed. It is for this reason that ABA was evaluated as a thinner on apples, and it was found to be quite an effective abscission-promoting compound (Greene et al., 2011).
Benzyladenine is the most recently registered thinner for use on apples (Greene, 2002). It was subsequently registered for use on pears. In general higher rates are used on pears than are generally recommended for use on apple (Vilardell et al., 2005). BA has been reported to thin several pear cultivars including Clara Frijs (Bertelsen, 2002), Bartlett (Curetti et al., 2011; Dussi and Sugar, 2011), Packham’s Triumph (Bound and Mitchell, 2002), and Conference (Maas et al., 2010; Vilardell et al., 2005). A mixture of thinning compounds is frequently recommended for use on apples because combinations are perceived to be a safer approach to thinning because lower rates of the individual thinners may be used and different thinner modes of action are involved (Greene, 2002; Schwallier, 1996; Williams and Edgerton, 1981). This strategy has been particularly effective when BA is combined with other thinners such as carbaryl on apples. Maas et al. (2010) reported that BA alone was a marginal thinner on ‘Conference’ pear but when combined with 1-naphthaleneacetic acid (NAA), significant thinning was achieved along with enhanced return bloom. The combination of ABA and BA as a thinning spray was used by Greene et al. (2011) on apples not only to improve thinner response, but also counteract the leaf yellowing and leaf abscission (Greene et al., 2011) effect caused by the ABA on some apple cultivars.
This investigation was undertaken for several reasons: 1) to determine if ABA might be a useful thinner on pears; 2) to determine the most effective time(s) of application; 3) to evaluate ABA and BA thinner combination; and 4) to determine if BA could counteract leaf yellowing and abscission that are frequently induced by ABA.
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