There is interest in developing new chemical thinners that are safe and naturally occurring. Existing thinners can cause phytotoxicity, pose hazards to beneficial and pollinating insects, and some are under review by regulatory agencies for possible withdrawal from legal use (Greene, 2002; McArtney, 2011).
Two frequently used post-bloom chemical thinners for apple, the synthetic cytokinin 6-benzyladenine and the synthetic auxin 1-napthaleneacetic acid, produce some level of thinning over a range of timing from petal fall up to 15- to 16-mm fruit diameter (Batjer et al., 1968; Byers, 2003; Donohoe, 1968; Greene, 2002). Sometimes it may be necessary to apply thinners later than is optimal for these compounds as a result of unfavorable weather during the optimal application window, uncertainty of the strength of initial fruit set, and/or because earlier attempts to thin have failed to adequately adjust the cropload.
Currently there are two registered compounds that are effective in the late thinning window (17- to 25-mm fruit diameter): (2-chloroethyl)-phosphonic acid (ethephon) and 1-napthyl(N)methyl carbamate (carbaryl). The ethylene-generating compound ethephon is a potent thinner that is therefore considered a risk to overthin (Greene, 2002), especially with ethephon-sensitive cultivars such as ‘Golden Delicious’ and ‘Rome’ (Byers, 2003). Carbaryl is considered a mild thinner and is most often used in a tank mix to boost the activity of other thinning chemistries. Sensitivity to chemical thinners ends when fruits reach 25 mm in diameter (Byers, 2003; Greene, 2002). It would be desirable to identify compounds with moderately strong thinning activity and a relatively low risk of excessive thinning for eventual registration and use as late thinners.
ACC is the precursor to ethylene (Adams and Yang, 1979). Several characteristics of ACC seem favorable for potential development as a plant bioregulator. ACC is a naturally occurring amino acid. The conversion of S-adenosyl methionine to ACC is the rate-limiting step in the ethylene biosynthesis pathway, and ACC is readily translocated in plants (Bradford and Yang, 1980). ACC is readily converted to ethylene by a wide variety of plant tissues (Cameron et al., 1979). Recent evidence suggests that foliar application of ACC has activity as an apple thinner (McArtney, 2011).The objective of these studies was to evaluate the efficacy of several concentrations of ACC for thinning apple at the standard growth stage for chemical thinning timing, 10-mm fruit diameter, and a late thinning growth stage of 20-mm fruit diameter.
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