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ISHS Acta Horticulturae 1221: EUFRIN Thinning Working Group Symposia

Comparison between 6-benzyladenine and metamitron as thinning agents in 'Royal Gala', 'Cripps Pink' and'Red Delicious' apple cultivars

Authors:   N. Rosa, W. Verjans, C. Oliveira, D. Bylemans, S. Remy
Keywords:   abscission, chemical thinning, fruit growth rate, Greene model, Malus × domestica
DOI:   10.17660/ActaHortic.2018.1221.8
The research described here was conducted in 2015 on the cultivars ‘Royal Gala’, ‘Cripps Pink’, and ‘Red Delicious’ at Pcfruit Research Centre in Sint-Truiden, Belgium. Four treatments were compared: untreated, manual thinning, two applications of 165 ppm of metamitron at 8 and 12 mm fruit diameter and one application of 150 ppm of 6-benzyladenine (6-BA) between 8 and 12 mm fruit diameter. At each application, all cultivars were sprayed on the same day. Fruit drop, phytotoxicity, fruit growth rate and photosynthetic efficiency were measured. Fruit growth was monitored in all the treatments using a simplified version of the Greene model to also predict the persisting and the abscising fruit percentage. Fruit drop was higher in all cultivars treated with metamitron than with 6-BA. Fruits of trees treated with metamitron showed the highest growth rate. A relation between the duration of the blocking effect of photosynthesis by metamitron and the phytotoxicity level in the trees was observed. At harvest, metamitron and manually thinned fruits reached a higher average weight compared to the other treatments. The results obtained with 6-BA at 150 ppm were not as good as expected in terms of crop load and optimal fruit size in all the three cultivars tested, probably because of the non-optimal weather conditions after application. On the other hand, the results obtained with a double application of 165 ppm of metamitron were similar to hand-thinned trees. ‘Cripps Pink’ was the easiest-to-thin cultivar, followed by ‘Royal Gala’, achieving the ideal crop load, and ‘Red Delicious’, which was the most difficult to thin

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