S16 - Session P3 - Investigating the effect of different autumn, winter, and spring temperature scenarios on bud break in apple trees

Authors: Esme Louw *, Laura Allderman, Willem Steyn, Nigel Cook

Deciduous trees require sufficient winter chill to have a synchronised and condensed bloom period in spring. When winter chilling is incomplete spring bud break is delayed and reduced and rest-breaking agents are commonly used in warm winter production areas to mitigate this problem. However, historical commercial data indicates that bud break success can differ significantly across seasons despite comparable and sufficient chill accumulation. In the same way, we often see differences in the efficacy of rest-breaking applications (timing and dosage) even with similar deficiencies in winter chill. This indicates that the total amount of chill received during the dormant period is not the only determining factor for good bud break. The aim of this study was to investigate the effects of different autumn, winter and spring temperature scenarios on the onset, rate and total percentage bud break in apple trees. For two consecutive years, intact, whip nursery trees (M793 rootstock), grown in bags, were moved between two production sites with contrasting autumn and winter climates, warm and cold spring conditions were simulated in a glasshouse. This resulted in eight different temperature scenarios across the three seasons. Bud break was monitored for each separately. Furthermore, dormancy progression was quantified for each scenario by forcing trees at numerous occasions and expressing the time to bud break as a two linear joint line model. Cold winter temperatures contributed most to bud break success. Autumn chill mitigated some of the lack of winter chill but could not fully replace it. Warm spring temperature are less important after sufficient winter chill has accumulated. Therefore, successful bud break under mild winter conditions is not solely dependent on the amount of chill accumulated, but rather the distribution of the chill accumulation.