Research Matters - Fruit thinning in apple trees

Apple growers sometimes have to use fruit-thinning agents to stop their trees producing too many small fruit.

However, some of the established agents are now being phased out because they are thought to adversely affect human health.

In the experiments described here, 12-year-old 'Gold Rush' apple trees growing in North Carolina, USA, were sprayed with 1-naphthaleneacetic acid (NAA - 5mg per litre) when the fruit were 10mm in diameter. Once the spray had dried, uniform spurs from treated and untreated trees were removed and sprayed with solutions containing 0, 50, 100 or 200mg per litre of 1-aminocyclopropane carboxylic acid (ACC). These same treatments were applied in the following year.

The ACC is converted into ethylene by plant tissues and ethylene production was highest the day after treatment. The highest concentration of ACC produced the most ethylene and also considerably reduced the number of fruit set in both years. One disadvantage of using such a high concentration was that it caused some yellowing of leaves.

When NAA was applied alone, it too reduced fruit set but not by as much as ACC. When both sprays were applied to the same spur, their effects were usually additive. As NAA had little impact on leaf yellowing, one future approach might be to spray ACC in combination with NAA.

Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick

Effects of 1-Aminocyclopropane Carboxylic Acid on the Rate of Ethylene Release from Detached Fruiting Spurs & on Fruit Abscission in Apple by McArtney (2011). Journal of Horticultural Science & Biotechnology 86 (6): 640-644. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.


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