Mechanical Blossom Thinning of North San Joaquin Valley Processing Peaches

Most processing peach (Prunus persica) varieties must be thinned in order to obtain acceptable fruit size.  Hand thinning is the most costly cultural operation in the production of early harvested varieties.   In 2009, a tractor-mounted, string blossom thinner (Darwin PT 250, Fruit Tec, Deggenhausertal, Germany) was evaluated in two, high density, commercial processing peach orchards in California’s North San Joaquin Valley, where trees had been trained to a two-scaffold “V” system.  Use of the mechanical blossom thinner reduced hand thinning requirements of green fruit by 22 - 27%, increased fruit size and marketable yield and increased net per acre income by $1251 - $1383.  In 2010, the string thinner was evaluated in three orchards where trees were trained to a more traditional open center “vase” shape (134 – 173 trees per acre).  In addition, blossom thinning on the sides and tops of trees was compared to blossom thinning the sides only.  In these orchards, follow up hand thinning requirements were reduced by 0 – 29% and fruit diameter was increased by 3.8 – 4.4 mm at harvest.  In one out of three orchards, saleable yield was increased significantly (P < 0.05) by blossom thinning, while yield was significantly reduced in another orchard when blossoms were thinned on the tree tops and sides.  Change in net per acre income ranged from an increase of $874 to a loss of $328.  Although trees trained in the traditional vase shape proved more difficult to mechanically blossom thin than high density “V” trained trees, follow up hand thinning costs can be reduced and yields increased if performed carefully.  Care must be taken during follow up hand thinning to avoid excessive fruit removal and reduced yields.  In an additional experiment, blossom removal at pink bud was equivalent to thinning at full bloom.  Flowers were more difficult to remove at petal fall but similar levels of blossom thinning could be achieved by increasing the rotational speed of the thinner.  This study suggested that there is at least a three week window from pink bud through petal fall in which the mechanical blossom thinner can be used effectively.