Testing Adjuvants for Chemical Thinning of : Pilot Study, 2007

Problem and Its Significance: Reducing thinning costs

The cling peach industry faces significant economic challenges, particularly due to production costs. Production of extra-early varieties, such as ‘Loadel’, typically result in $400 per acre loss for Sacramento Valley growers, based on a 2004 UC cost analysis study (Hasey et al., 2004). This net loss is due to a number of factors, particularly the cost of hand-thinning labor. Estimated cost per acre to hand-thin extra-early varieties is $613, which is 31% of all cultural costs. Thinning costs could be greatly reduced by chemical thinning. In recent years a great deal of our research centered on the use of a surfactant, N-Ter (previously labeled as Entry and Armothin), used in combination with ammonium thiosulfate (ATS) at bloom. Although this combination thinned bloom, the results were inconsistent and we generally found extensive shoot and bud death. It is critical to find an alternative for clingstone peach growers, that may also benefit other of the growers.

Plans and Procedures:

Thinning response was compared in a UC orchard on freestone (‘O’Henry’) on a limited basis to observe both thinning response and potential for phytotoxicity with applications made by hand-held sprayer to shoots, three replicate shoots per treatment in a complete randomized block design among several . Treatments (Table 1) included an untreated control, 0.1% ATS control, ATS in combination with several test adjuvants, ATS + Activator 90 (tested in 2005 and 2006) and a 10% solution of horticultural vinegar and a yucca extract. Applications were made ‘to drip’ at approximately 70% of full bloom on 11 March, at which time all on treated shoots were counted and phytotoxic symptoms evaluated. Fruit were counted on treated shoots after small fruit drop, on 23 April. Standard procedures were followed for statistical analyses.

Results and Discussion:

Fruit set was significantly reduced by ATS and the PacificHort Penetrant (Table 1). Numerically, fruit set was reduced by ATS and Monterey Ag Super 7 and the horticultural vinegar and yucca solution. No phytotoxicity was observed of any kind, and I would suggest a larger scale trial next year, in which whole trees are treated, to get a large representation of treatment results. Concentrations of chosen treatments might depend on environmental conditions that can affect strength of bloom, such as amount of chilling received during the dormant season and weather conditions during bloom. In 2007 we experienced ideal conditions for a strong fruit set—good chill and good weather at bloom, which creates a ‘harder-to-thin’ crop. I would recommend retesting ATS + Pacific HortPenetrant, the horticultural vinegar (also useful for organic growers) and Monterey Ag Super 7 + ATS as the top candidates for thinning.

References:

Hasey, J., R. Duncan, M. Norton, K.M. Klonsky, and P. Livingston. 2004. Sample costs to produce cling peaches in the Sacramento and San Joaquin Valleys, Extra-Early Harvested Varieties. UCCE, Dept. Agric. Reso. Econ., UCDavis.

Table 1. Effects of bloom thinners on fruit set in ‘O’Henry’ peach, 2007. Treatment %Fruit set Untreated control 51.7 abX 0.1% ATS control 55.9 ab 0.1% ATS + 5% Activator 90 62.6 ab 2% Akzo-Nobel AB600 control 53.4 ab 2% Akzo-Nobel AB650 control 86.9 a 0.1% ATS +2% AB600 70.3 ab 0.1% ATS +2% AB650 63.8 ab 0.1% ATS + 2% CanHance 70.9 ab 0.1% ATS + 2% Super 7 39.1 bc 0.1% ATS + 2% Magnify 58.0 ab 0.1% ATS + 2% Rocket DL 53.1 ab 0.1% ATS + 2% Yucca AgAide 65.5 ab 0.1% ATS + 2% PacificHort Penetrant 16.2 c 10% Hort vinegar + yucca surfactant 47.2 bc XMean separation by Duncan’s Multiple Range Test, P = 0.05.