Girdling treatment to reduce vigor and increase production of high-quality Yellow-skinned ‘Koukou’ apples

Abstract

The production of yellow-skinned apples has increased in recent years; however, they are less acceptable to consumers than red-skinned apples. Therefore, the production of high-quality fruit is essential. In the present study, we applied a girdling treatment at 1) different times (June or August), and 2) different widths (2cm, 5cm, and 8cm). We compared the effect on tree growth, flower bud formation and fruit quality over a 4-year field trial using vigorous yellow-skinned ‘Koukou’ apple trees. The August girdling reduced the shoot growth by about 6 to 10cm per shoot and this reduction also restricted secondary extension of the shoots. The August treatment improved the fruit quality parameters, including the soluble solids content and skin color index, more effectively than the June treatment. Compared with the girdling timing, the effect of girdling widths in August was less obvious, but even the 2cm treatment in August was enough to reduce tree vigor and improve fruit quality. Apical flower bud formation was accelerated by the August girdling in both the treatment year and the following year. Regardless of the girdling width, the August girdling improved the fruit quality parameters: E.g. flesh firmness, soluble solids content, and skin color index. The August girding fruit also had higher sorbitol and sucrose concentrations than the control. Moreover, both the girdling treatments accelerated the incidence of watercore, which is a preferred condition for the Asian market. In conclusion, we found that all widths of girdling in August improved the harvested fruit qualities, including the sugar (sucrose) content, as well as watercore development in the treatment year and effectively controlled tree vigor, increased flower bud formation and increased yields.