ARS | Publication request: Blossom thinning in apple and peach with an essential oil

Submitted to: Annual Cumberland Shenandoah Fruit Workers Conference
Publication Type: Proceedings
Publication Acceptance Date: March 15, 2007
Publication Date: April 1, 2007
Citation: Miller, S.S., Tworkoski, T. 2007. Blossom thinning in apple and peach with an essential oil. Annual Cumberland Shenandoah Fruit Workers Conference.

Technical Abstract: The essential oil, eugenol, and a eugenol based herbicide (Matran EC) were applied to apple and peach trees during bloom to evaluate the thinning effect of these materials. Several additional bloom thinners including ammonium thiosulfate (ATS), liquid lime sulfur, and sulfcarbamide were included in some of the trials as materials that have demonstrated bloom thinning qualities. Tests were conducted in 2005 and 2006 on mature bearing apple and peach trees growing at the Appalachian Fruit Research Station. At 6 percent to 8 percent (v/v), the essential oil caused extensive phytotoxicity to flower and leaf tissue. Phytotoxicity produced by the essential oil treatment was dose dependent and transitory. Generally visible evidence was lacking within 3 to 4 weeks following spray treatment. At rates of 2 percent to 6 percent the essential oil reduced the yield of apple and peach and increased fruit weight at harvest. The essential oil treatment reduced crop load and improved fruit size in peach generally equal to that achieved by hand thinning at 40-60 days after full bloom. The tests over two years indicated that apple may be more responsive to an essential oil as a bloom thinner than peach. The essential oil reduced crop load far below the hand-thinned level and below a commercially acceptable level at the 8 percent rate, and even in some tests at the 6 percent rate. It is suggested that the greater thinning effect of the essential oil on apple may be the result of leaf phytotoxicity and reduced photosynthetic rates rather than a direct effect on the flower reproductive tissues. Additional studies are needed to improve efficacy of the essential oils as bloom thinners, especially in peach.