Apple and peach trees normally produce significantly more fruit than the tree can carry to a marketable size crop (Dennis, 2000; Wertheim, 2000). Hand thinning at 35 to 60 d after full bloom is the standard practice to reduce crop load and increase fruit size at harvest. Hand thinning is expensive and, with recent labor shortages, is more problematic for growers to complete in a timely manner. Chemical thinners have long been available and used in apple production (Byers, 2003). Thinning at bloom time results in larger fruit at harvest than achieved with the later hand thinning (Byers and Lyons, 1985). Removing 50% of peach flowers increased average size of the remaining fruit and increased overall crop value (Myers et al., 2002). Chemical thinning of peach flowers reduced the estimated cost of hand thinning up to $310 U.S. per ha (Southwick et al., 1995). A number of materials (Byers, 1999; Fallahi, 1997; Moran et al., 2000; Southwick et al., 1995; Wilkens et al., 2004) and mechanical techniques (Baugher et al., 1991; Glenn et al., 1994; Schupp et al., 2008) have been studied for bloom thinning, but none has proven completely successful and registration and/or acceptance by the peach-growing industry has been slow and difficult.
Numerous chemical flower thinners of apple and peach have been tested, including DNOC (Na 4,6-dinitro-ortho-cresylate; Elgetol), ethephon (2-chloroethylphosphonic acid), pelargonic acid, monocarbamidedihydrogen sulphate (Wilthin), ammonium thiosulfate (ATS), and urea (Byers, 1999; Costa and Vizzotto, 2000; Fallahi, 1997; Wertheim, 2000). Thinning apple and pear flowers with DNOC was widely used to consistently desiccate stigma tissues, but it was removed from the market in the United States in 1989 (Dennis, 2000). Most of the other chemicals have been less satisfactory as a result of inconsistent flower thinning or foliar phytotoxicity. However, fruit thinning of apples and peaches by chemical thinners may be associated with inhibition of photosynthesis (Pn) rather than directly inducing fruit abscission (Byers, 2003; Dennis, 2000). Byers et al. (1991) demonstrated and Byers (2003) described the effects of artificial shade and the subsequent fruit abscission in apple when photosynthate reserves are depleted. Partial defoliation has also been shown to affect whole tree Pn (Ferree and Palmer, 1982) and fruit set (Llewelyn, 1968) in apple. Dormant spray oils have been shown to reduce Pn in apple leaves (Ferree and Hall, 1975) and enhance the thinning activity of mild chemical thinners (Byers and Carbaugh, 1991). McArtney et al. (2006) reported that liquid lime sulfur (LS) and fish oil applied at bloom time suppressed Pn and likely contributed to the thinning response of these materials.
Currently, buds, blossoms, and fruit can be removed by hand or with chemicals, but the number of chemical thinners available is limited, especially in peach, and those available are not always effective (Costa et al., 2004). There is a need for new chemical thinners, especially “environmentally friendly thinners,” to replace the costly, labor-intensive hand thinning that is currently used by growers (Dennis, 2000).
Several chemicals, including sodium chloride, LS, acetic acid, and soybean oil, have been found to have potential flower-thinning application for organic apple orchards (Moran et al., 2000; Stopar, 2008). Tworkoski (2002) reported that an essential oil was effective as a contact herbicide and that such plant-derived oils may be acceptable for organic management systems. He found that low rates could cause burn-back of sensitive plant tissue without killing the plant. The a.i. in several essential oils is eugenol [2-methoxy-4-(2-propenyl)phenol] (Fig. 1). This report examines the efficacy of eugenol and a eugenol-based contact herbicide, Matran 2EC (50% clove oil; EcoSMART Technologies, Inc., Franklin, TN) or Matratec AG (50% clove oil; ClawEl Specialty Products, Pleasant Plains, IL), as blossom thinners in apple and peach. Rates, application techniques, timing, and frequency of application were modified each year to address pressing questions raised by previous trials with this novel, newly patented thinning agent, eugenol. In selected years, thinning effects of eugenol were compared with hand defoliation and with known chemical thinners. The fertilizer, ATS, which has demonstrated bloom thinning response and is being used by some commercial growers, was included in selected tests. Several tests also included LS, 6-benzyladenine (6-BA), carbaryl, or the commercially available bloom thinner, Wilthin (Entek Corp., Brea, CA). The objective was to determine the effectiveness of different concentrations and time of application of an essential oil, represented by eugenol, on blossom thinning, yield, and fruit size distribution.
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