Tank-mixing boron with 6-benzyladenine to enhance ‘Gala’ apple fruit size may promote fruit splitting.

Peryea, F.J. and Neilsen, G.H. (2013). "Tank-mixing boron with 6-benzyladenine to enhance ‘Gala’ apple fruit size may promote fruit splitting.", Acta Horticulturae (ISHS), 984, pp. 401-408.

Abstract

Because ‘Gala’ apple fruit (Malus ×domestica Borkh.) are naturally small, considerable commercial effort is made to increase their size. The plant bioregulator 6-benzyladenine (6-BA) often is applied for the express purpose of enhancing ‘Gala’ apple fruit size. ‘Gala’ apples also are prone to a physiological disorder known as splitting, manifest as longitudinal and radical cracking of the fruit originating at the stem end. Although the 6-BA label recommends against tank-mixing other substances with the product, many growers believe that adding boron (B) enhances the activity of 6-BA. This practice simultaneously provides B for nutritional purposes, which is recommended to be applied annually under Washington State (USA) conditions. In 2005, we conducted a field study examining the effect of adding various formulations of B to 6-BA sprays made to increase fruit size. The 6-BA sprays had little effect on average fruit mass but changed the distribution of fruit size within the total fruit population. Rather than shifting the fruit size population curve to larger fruit, it reduced kurtosis, with the increase in the proportion of large fruits being offset by a complementary increase in the proportion of small fruits. Adding B did not appre¬ciably alter fruit size grouping frequencies compared to 6-BA alone. The B-containing sprays had no effect on mid-summer leaf B but slightly increased at-harvest fruit B. There was a tendency for the B-containing spray treatments to increase the incidence of splitting. There was a significant linear relationship between splitting and fruit B, suggesting that splitting may be exacerbated by small increases in fruit B within the concentration range normally considered optimal for preventing B deficiency symptoms and avoiding B toxicity problems. It is possible that splitting in ‘Gala’ apple is particularly sensitive to B excess, suggesting that B fertilization recommendations should be cultivar-specific.

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