Blossom Thinning in Pome and Stone Fruit

Abstract

Early thinning of apples is important because of its impact on fruit size and next season's flower bud initiation. In the past, apple cultivars were often sprayed with the blossom thinner sodium dinitro-ortho-cresol(Elgetol) during full bloom, followed by a post-bloom application of a fruit thinner such as carbaryl with or without naphthalene acetic acid (NAA). Elgetol was removed from the market in 1989 because of the high cost of re-registration. Full-bloom sprays of sulfcarbamide (Wilthin), pelargonic acid (Thinex), and endothalic acid (Endothal), ammonium thiosulfate (ATS) or petal fall spray of carbaryl (Sevin XLR Plus) were developed as replacements for Elgetol. Hydrogen cyanamide (HC) and other chemicals have been used to eliminate or to reduce chilling requirements of peaches grown under the warm desert conditions. HC applied at “pink bloom” stage was observed to reduce the number of open blooms in `Florda Prince' peach; therefore, it was first used for blossom thinning in this cultivar in Arizona. Later, HC was also found to be an effective blossom thinner for plums in Idaho. HC has recently been found to effectively thin apple and peach blossoms. Armothin has also been an effective blossom thinner for peach in California.