Shoot growth control of pear trees (Pyrus communis L.) with prohexadione-calcium

Prohexadione-calcium (ProCa) is a gibberellin biosynthesis inhibitor with limited persistence and low toxicity that has recently been registered as a shoot growth retardant for apples in North America (Apogee®) and Europe (Regalis®). We investigated the response of six different pear cultivars ('Rosemarie', 'Forelle', 'Packham's Triumph', 'Flamingo', 'Early Bon Chretien' and 'Golden Russet Bosc') to ProCa in the Western Cape, South Africa, over the period 1999-2003. ProCa treatments ranging from 50 to 250 mg l−1 were applied between one and three times per season and were compared with girdling and control treatments. ProCa application reduced shoot growth in all of the cultivars. 'Forelle', was least responsive and showed a reduction in 2002/2003 but not 2001/2002. 'Rosemarie', 'Flamingo', 'Packham's Triumph' and 'Golden Russet Bosc' were most responsive, with shoot growth being reduced by up to 50% of the control for the latter two cultivars. The response to number of applications and ProCa concentration was not consistent across cultivars. 'Early Bon Chretien', for example, only responded to high ProCa concentration, whereas 'Rosemarie' was very responsive at the lowest ProCa concentration. ProCa application improved fruit set but decreased final fruit size in some of the cultivars, particularly in 'Rosemarie'. It also caused a decrease in return bloom in 'Forelle' and 'Packham's Triumph'. The response of different cultivars to girdling, in terms of shoot growth and fruit set was highly variable. Girdling tended to increase final fruit size and return bloom. ProCa shows potential as a shoot growth retardant for the pear cultivars investigated, but the response of reduced fruit size in some cultivars requires mitigation. Future research on combined girdling and ProCa treatments may be instructive.