| Summary: | Thesis (PhD(Agric) (Horticulture))--Stellenbosch University, 2008. === Lack of winter chilling is a major problem in producing temperate-zone fruit in warm climates.
Delayed foliation and protracted bud burst and flowering are the main problems necessitating
artificial means to break dormancy. In South Africa (SA), most apple production areas receive
insufficient winter chilling, and an annual application of rest breaking (RB) agents is included as
standard practice. The most used RB agent in SA was dinitro-o-cresol (DNOC) but its use was
discontinued. Hydrogen cyanamide (HC) became the replacement. It has been effective in apple,
but variable effects on fruit set, blossom, yield and fruit quality have been reported. Thidiazuron
(TDZ) has also shown the ability to break dormancy in apples. Another important practice in apple
production is chemical thinning (CT). However, results are highly influenced by the type of
chemical, weather conditions, cultivar and blossom pattern.
With the increasing efficacy of RB and by identifying its effects on vegetative and reproductive
development, it will be possible to determine more effective chemical thinning treatments. The
objective of this study was to determine appropriate RB treatments for apple trees in a warm winter
climate, identifying their effect on vegetative and reproductive development and the influence on
CT efficacy. The research was performed in the Elgin area (34°S, 300 m) SA, over a period of
three years, on ‘Golden Delicious’ and ‘Royal Gala’.
In evaluating the effect of different HC concentrations and oil, no synergistic or antagonistic effects
were observed on budburst and yield. Mineral oil at 4% plus 1 to 2% Dormex® combined were
sufficient to break dormancy. Dormex® at 4% (2.08% HC) reduced fruit set and yield. In general,
the rest breaking treatments (DNOC, HC and TDZ) enhanced the final vegetative bud burst
compared to the control, while reproductive bud burst in 2002 and 2003 was not significantly
influenced. The treatments compressed and advanced flowering periods, but this effect was not
always evident when the spring was warm. The treatments synchronised flowering on the tree and
between the two cultivars. The mixture of 0.245% HC and 4% oil was less effective in terms of
increasing bud burst in ‘Royal Gala’ compared to other rest-breaking treatments. The mixture of
0.49% HC and 4% oil effectively compressed and synchronised flowering in ‘Golden Delicious’.
TDZ-oil used at the lower rates also increased bud burst and concentrated flowering. However, it appears that after a cooler winter, higher rates could result in an exacerbated bud burst effect with
excessive vegetative growth.
The rate and timing of TDZ-oil application influenced the reproductive development of apples and
therefore fruit quality. In ‘Golden Delicious’ increased fruit set, number of seeds, and reduced fruit
russeting appear as beneficial results of TDZ-oil, whereas fruit set and russeting was not affected in
‘Granny Smith’. TDZ-oil, when applied late and at increasing rates, led to an increase in the
malformation of calyx cavities, especially when chemical thinning was performed using the
cytokinin-like compound benzyladenine. The effect seemed to be cultivar specific, with ‘Golden
Delicious’ being the most severely affected. Increased return bloom in response to late TDZ
application in ‘Golden Delicious’ and ‘Royal Gala’ appeared to be beneficial.
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