Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2010
Publication Date: 5/28/2010
Citation: Miller, S.S., Tworkoski, T. 2010. Blossom thinning in apple and peach with an essential oil. HortScience. 45(8):1218-1225. Interpretive Summary: Without proper management, peach and apple trees will often produce excessive crop loads that require thinning. Removal of excess fruit buds, blossoms, and fruit is necessary to ensure that the remaining fruit will achieve marketable size and that trees will produce sufficient flowers the following year. Growers in the U.S can spend $1,730 per ha per year to hand-remove excess blossoms, and chemical thinning can significantly reduce the cost of hand thinning. However, few chemical thinning agents are available, and those available are not always effective. 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. This research demonstrated that eugenol [2-methoxy -4-(2-propenyl)phenol], which is a component of several generally regarded as safe essential oils [e.g. naturally-derived eugenol extracted from cinnamon (Cinnamomum zeylanicum) or clove (Syzgium aromaticum)] can be used as a blossom thinner in peach and apple trees (at 50-100 percent blossom) when applied at rates of 1.5 to about 6 percent. These findings could benefit apple and peach growers nationwide, especially organic apple and peach growers.
Technical Abstract: A series of experiments were conducted with apple (Malus xdomestica) and peach [Prunus persica (L.) Batsch] from 2003-2008 to evaluate the flower thinning efficacy of eugenol and a eugenol-based essential oil. Flower thinning effects by hand defoliation and alternative chemical agents were compared to eugenol in different years. Eugenol or the eugenol based contact herbicide Matran 2 EC (or Matratec AG) produced noticeable phytotoxicity to floral parts and exposed leaf tissue within 15 minutes to one hour after application, and injury was proportional to rate. At the highest rates (8 and 10 percent), eugenol resulted in complete burning of all exposed tissue except bark tissue, where there were no visible signs of injury. Within 3 to 4 weeks of application, phytotoxicity was difficult to observe even at the higher rates of eugenol. In companion experiments, hand defoliation of young leaves at bloom resulted in abscission of young fruitlets in apple but not in peach indicating that eugenol may cause thinning by multiple mechanisms. Ammonium thiosulfate (ATS) [49 L per ha or 6.0 percent (v/v)] provided thinning in peach and showed little or no phytotoxicity, but the response was inconsistent. ATS was also inconsistent in thinning apple. The thinning response from MCDS (monocarbamidedihydrogen sulphate, Wilthin) at 3.2 percent (v/v) was inconsistent in peach. At the rate used, Wilthin caused some phytotoxicity on peach. Applications of 1 percent to 2 percent eugenol appear promising, but good blossom coverage is critical for thinning. Furthermore, eugenol formulations need improvement to ensure uniform coverage for predictable thinning.