Persimmon cultivation in South Africa is expanding rapidly as a result of lucrative marketing opportunities that exist in the northern hemisphere for out-of-season southern hemisphere fruit (Rabe, 2003). Since 1998, ≈750 ha of the astringent Triumph cultivar has been planted. ‘Triumph’, like many important persimmon cultivars, is dioecious and orchards consist entirely of female trees (Kitagawa and Glucina, 1984). Parthenocarpic fruit set in persimmons may vary considerably from year to year (Yamada et al., 1987) and poor fruit set is a major limitation to the early onset of regular, high production (George et al., 1997). Gibberellins (GAs) stimulate fruit set in various fruit species, i.e., peach (Stutte and Gage, 1990), ‘Clementine’ mandarin (Talon et al., 1992), apple (Bangerth and Schröder, 1994), and pear (Deckers and Schoofs, 2002). Application of GA3 during or shortly after anthesis increases parthenocarpic fruit set in various persimmon cultivars (George et al., 1997; Yamamura et al., 1989). In South Africa, application of 20 mg·L−1 GA3 at 30% and 70% full bloom (FB) is the recommended industry standard treatment to increase parthenocarpic fruit set in ‘Triumph’.
Despite rigorous application of GA3, parthenocarpic fruit set in ‘Triumph’ is erratic under South African conditions and producers have struggled to obtain regular high yields. Young ‘Triumph’ orchards are also late to come into production despite prolific flowering (Hodgson, 1938) and the use of GA3 in Israel (Blumenfeld, 1981). In addition, there is concern that GA3 application may reduce return bloom. A decrease in flower initiation in response to GAs has been reported for various fruit species (Bangerth, 2006; Goldschmidt et al., 1997), including persimmon (Yamamura et al., 1989). This decrease in return bloom may decrease cumulative yields and induce the onset of alternate bearing.
As a result of the apparent ineffectiveness and perceived shortcomings of GA3, producers are considering scoring or girdling (a knife-cut through the phloem and cambium or the removal of a 2-mm strip of phloem and cambium to the depth of the secondary xylem in a complete circle around the trunk, respectively) as a possible substitute to increase fruit set and yield in ‘Triumph’. Girdling during flowering has been shown to improve fruit set in various fruit species (Goren et al., 2004), including persimmons (Hasegawa et al., 2003; Hodgson, 1938). In addition, girdling and scoring may have a lesser effect on return bloom compared with GA3 through its stimulating effect on flower initiation (Goren et al., 2004; Hodgson, 1938). Blumenfeld (1981) reports that a combination of girdling and GA3 application was the most effective treatment to increase fruit set in young ‘Triumph’ orchards in Israel.
Scoring and girdling were equally effective in decreasing vegetative growth in apple (Autio and Greene, 1994) and increasing fruit size in peach and nectarine (Agustí et al., 1998) and loquat (Agustí et al., 2005). However, the potential for adverse effects on tree health is much less with scoring. Girdling may impair tree health if callusing is slow or inadequate (Fernandez-Escobar et al., 1987). Because the rate of wound repair is related to girdling width (Fernandez-Escobar et al., 1987), vascular connectivity is re-established much faster after scoring than after girdling (Furr et al., 1945). Furthermore, girdling with a hand saw carries the risk of cutting into the secondary xylem, resulting in severe water stress, abscission of all the fruit on affected trees, and even tree death (personal observation).
The research reported here evaluated the effect of GA3 application during FB, scoring or girdling during FB, and interactions between GA3 and girdling or scoring on fruit set, yield, return bloom, and fruit mass in ‘Triumph’ persimmon. The ultimate aims of this work are to develop guidelines for South African producers to improve fruit set of ‘Triumph’ persimmon and to obtain early and regular high yields of good-quality fruit.
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