Evaluation of alternative rest breaking agents for apples

Insufficient winter chilling (<1000 Utah chilling units) is experienced in apple (Malus × domestica Borkh.) growing regions with a Mediterranean-type climate, such as the Western Cape Province in South Africa. The use of chemical rest-breaking agents (RBAs) is currently the most effective method used in regions with insufficient chilling to increase budbreak, condense bloom period and advance vegetative bud development. The aims of this project included: (1) Evaluation of alternative rest-breaking agents on ‘Fuji; and ‘Rosy Glow’ apple trees; (2) Evaluation of 6-benzyl adenine + gibberellins (GA4+7), mineral oil (MO), and hydrogen cyanamide (HC) and timing of application to enhance budbreak of ‘Fuji’ apple trees.

Evaluation of alternative rest-breaking agents on ‘Fuji; and ‘Rosy Glow’ apple trees: The following RBAs were used: HC (Dormex®), MO (Budbreak®), 6-BA + GA4+7 (Promalin®), KNO3 and NaNO2. The 1.5 % HC served as the control treatment on ‘Fuji’ as the objective of the study was to find a safer but equally effective alternative RBA to HC. The 3.4% MO treatment was chosen as the control on ‘Rosy Glow’ as this treatment is the standard RBA used commercially on ‘Rosy Glow’. A 0.5% HC treatment was however included to compare the efficacy of 3.4% MO to 0.5% HC. Evaluation of 6-benzyl adenine + gibberellins (GA4+7), mineral oil, and hydrogen cyanamide and timing of application to enhance budbreak of ‘Fuji’ apple trees: The treatments, HC, MO, and 6-BA + GA4+7, were applied once on different trees at seven, six, and five weeks before full bloom (WBFB). The 1.5% HC application served as the control treatment.

Evaluation of alternative rest-breaking agents on ‘Fuji; and ‘Rosy Glow’ apple trees: The1.5% HC treatment was the most effective in advancing total budbreak on ‘Fuji’ in the second season. The treatments 0.25% HC plus 2.6% MO and 3.4% NaNO2 plus 2.6% MO did not differ significantly from 1.5% HC in stimulating reproductive and vegetative budbreak over time, and final total budbreak in ‘Fuji’ in the second season. No beneficial effects were found with the addition of 3.4% MO or 3.4% MO plus 3.8 mg·L-1 6-BA + GA4+7 to 0.5% HC in ‘Rosy Glow’ throughout two seasons. Applying 0.5% HC on its own was more effective in advancing budbreak than the treatments without HC, but not significantly different from 3.4% NaNO2 plus 3.4% MO in ‘Rosy Glow’ in the second season. Some differences were found in fruit yield and fruit quality traits, but these differences were not consistent with treatment effects earlier in the season. Evaluation of 6-benzyl adenine + gibberellins (GA4+7), mineral oil, and hydrogen cyanamide and timing of application to enhance budbreak of ‘Fuji’ apple trees: The recommended RBA for the South African apple industry of 0.25% HC plus 2.6% MO, was not as effective as 1.5% HC in advancing and increasing vegetative budbreak in ‘Fuji’ but increased reproductive budbreak percentage at full bloom compared to 1.5% HC in the second season. The 1.5% HC (control) treatment had a negative effect on fruit set per flower cluster in the first season, possibly due to increased competition between vegetative and reproductive development. Total fruit yield or total fruit yield efficiency was however not affected in the same season. In the second season, a significantly lower total fruit yield efficiency was recorded with 1.5% HC (control) relative to other treatments. Fruit set per flower cluster was however not affected by 1.5% HC (control) earlier in the season. It could have been that excessive vegetative growth competed with fruit development, causing fruit drop later in the season, after fruit set was evaluated.

Evaluation of alternative rest-breaking agents on ‘Fuji; and ‘Rosy Glow’ apple trees: Our results confirmed that a reduced rate of HC (0.25%) can be used in combination with 3.4% MO to induce budbreak in ‘Fuji’. The 3.4% NaNO2 and MO treatment combinations showed promising results in ‘Fuji’ and ‘Rosy Glow’, but 3.4% NaNO2 is expensive and not readily available in South Africa. From our results the recommended RBAs are 1.5% HC in ‘Fuji’, and 0.5% HC in ‘Rosy Glow’. Further research is needed to find a safe, cost-effective, alternative RBA to HC. The long-term effect of RBAs applied on the same trees should also be investigated.

Evaluation of 6-benzyl adenine + gibberellins (GA4+7), mineral oil, and hydrogen cyanamide and timing of application to enhance budbreak of ‘Fuji’ apple trees: Our results suggest that 2.6% MO plus 3.8 mg·L-1 6-BA + GA4+7 needs to be applied in combination with of HC (0.25%) to effectively induce budbreak. Inconsistent results were found between the rates of 6-BA + GA4+7 applied in combination with 0.25% HC and 2.6% MO. Further research is required to find a safe, reliable, and cost-effective RBA to replace HC. Future research should include the evaluation of long-term effects of RBAs applied to the same trees.

Hydrogen cyanamide (HC) at 0.25% can be used in combination with 3.4% mineral oil (MO) to induce budbreak in ‘Fuji’. The 3.4% NaNO2 and MO treatment combinations showed promising results in ‘Fuji’ and ‘Rosy Glow’. However, NaNO2 may be expensive and not readily available in South Africa. The recommended rest breaking agents are 1.5% HC in ‘Fuji’, and 0.5% HC in ‘Rosy Glow’. The 2.6% MO plus 3.8 mg·L-1 6-BA + GA₄₊₇ may be applied in combination with 0.25% HC to induce budbreak effectively.