Alternative rest-breaking agents evaluated on ‘Golden Delicious’ apple trees

Abstract

To reduce the growth abnormalities associated with insufficient winter chilling, most of the apple (Malus domestica Borkh.) trees growing in the Western Cape of South Africa receive an annual application of a chemical restbreaking agent (RBA) at the end of winter. Studies were initiated to find safer alternatives to the commercially used dinitro-o-cresol/oil (DNOC/oil) and hydrogen cyanamide/oil (HC/oil). Mixtures of inorganic and organic nitrogen sources, together with an alkoxylated fatty amine (Acer®) adjuvant, providing various total N concentrations, without (2000/2001) or with (2001/2002) petroleum-oil or vegetable-oil adjuvants, were evaluated against the commercially used RBA's on mature and potted ‘Golden Delicious’ apple trees over two seasons. A total N concentration of 1.0% of the 75:25 inorganic: organic mixture with Acerreg but without petroleum-oil adjuvant gave the best results. The N mixtures gave as good a response for fruit set and fruit size, but a lower bud break relative to the DNOC and HC after the relatively warm winter of the first season (2000/2001) but there were no differences in any of the parameters monitored after the relatively cold second season (2001/2002). On the potted trees in the very marginal Stellenbosch region, the 1.5% N mixture alone or with 1 % oil gave as good a total bud break as the HC/oil. More studies are necessary to investigate the relationship between the efficacy of winter chilling and use of RBAs.