Research Matters - Fruit thinning in apple trees: update

Fruit thinning is often required to produce a commercially profitable apple crop, yet fewer chemicals are now approved for this purpose.

(See Research Matters from 13 January).

In the experiments discussed here, a mechanical thinning device with three adjustable horizontal rotors, each with vertically-rotating string brushes, was used to remove flowers. A photograph of the tractor-driven device can be seen on the website referred to below.

The device was used to remove a proportion of flowers from seven-year-old slender spindles of 'Buckeye Gala' grown on a NIC29 rootstock in Washington State, USA. The thinning was done when the trees were in full bloom.

Two rotor speeds were used (260rpm and 360rpm) alone and in combination with either a post-bloom-thinning chemical spray or additional fruit thinning by hand. A control treatment was hand thinned to one fruit per 10cm.

Mechanical flower thinning alone at the higher speed produced large fruit but the total yield was lower than with hand thinning. Mechanical flower thinning with subsequent manual fruit thinning gave high-quality fruit of good size, with an acceptable overall yield and return performance.

The best treatment employed a combination of four chemicals, some of which are not approved for use in the UK and soon will not be approved in the USA either.

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

Mechanical Flower Thinning Improves Fruit Quality of Apples and Promotes Consistent Bearing by Hehnen, Hanrahan, Lewis, McFerson and Blanke (2012). Scientia Horticulturae 134: 241-244. The contents of issues of Scientia Horticulturae and abstracts of papers are provided at

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