Evaluation of dormancy-breaking agents on apple trees grown in warm-winter regions of South Africa

South Africa has a Mediterranean-type climate associated with insufficient winter chilling for commercial apple (Malus x domestica Borkh.) production. Insufficient winter chilling (<1000 chilling units per year, Utah Model) is usually associated with an abnormal bud break pattern, delayed foliation, and protracted flowering. Commercial apple growers depend on dormancy-breaking agents, including hydrogen cyanamide (HC) and winter oil, to compensate for insufficient winter chilling. HC is not environmentally friendly and has been banned in many apple producing countries. There is an urgent need for the South African apple industry to reduce the concentration of HC or find alternative dormancy-breaking agents that are more environmentally friendly. The aim of this study was to evaluate alternative or supplemental dormancy-breaking agents, including 6-benzyl adenine + gibberellins (6-BA + GA(4+7)), potassium nitrate (KNO3), sodium nitrite (NaNO2), urea and winter oil on apple trees to improve bud break. The field study was conducted in commercial 'Rosy Glow' and 'Fuji' apple orchards from 2016 to 2019. Uniform individual trees were randomly selected and allocated to dormancy-breaking agents. The 'Fuji' and 'Rosy Glow' trials were laid out as a randomized complete block design with 10 single tree replicate trees per six treatments and one guard tree and guard rows on either side of treatment rows. Two branches per tree representing one-year- and two-year-old shoots were randomly selected and tagged for counting the total number of dormant buds before the application of dormancy-breaking agents. 3% HC promoted more vegetative buds than reproductive buds because of a phytotoxic effect on floral buds. All the alternative treatments were as good as the industry standard 3% HC on 'Fuji' or 1% HC + 4% Oil on 'Rosy Glow'.