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Chemical thinning and summer PGRS for consistent return cropping of 'Honeycrisp' apples

Robinson, T., Lopez, S., Iungerman, K.
Acta horticulturae 2010 no.884 pp. 635-642
apples, carbaryl, ethephon, naphthaleneacetic acid, summer, trees
A field study was conducted at Geneva, NY from 2005-2007 with ‘Honeycrisp’/M.9 apple trees where a range of croploads was imposed on 3rd-5th year trees by hand thinning at 10 mm fruit size to determine the effect of crop load on return bloom. A relatively low crop load (4 fruits/cm2 trunk cross-sectional area (TCA) was required to ensure a strong return bloom the next year with young trees but with older trees higher crop loads of 6 fruits/cm2 TCA gave good return bloom. ‘Honeycrisp’ trees had a very high return set and could set a commercial crop with as low as 20-40% return bloom. A series of field chemical thinning studies was conducted from 2004-2007 where bloom, and post petal fall chemical treatments were evaluated for their effects on return bloom of ‘Honeycrisp’/M.9 apple trees. Very early (bloom or petal fall) aggressive chemical thinning gave the best return bloom while later thinning resulted in poor return bloom. The trial in 2007 showed that aggressive petal fall thinning (10 mg•L-1 naphtaleneacetic acid (NAA) + 600 mg•L-1 carbaryl (Sevin XLR) gave the best return bloom of 14 treatments. Later thinning sprays at the 10-12 mm stage or hand thinning later in June or July were not effective at improving return bloom. A third set of trials evaluated the influence of summer sprays of 5 mg•L-1 NAA or 150 mg•L-1 ethephon (Ethrel) on return bloom of ‘Honeycrisp’/M.9 trees. In one year summer NAA or ethephon treatments improved return bloom but in two other years there was no improvement of return bloom. With ‘Honeycrisp’, the use of Ethephon in July led to increased pre-harvest drop and advanced maturity.