Investigating the effect of different autumn/winter scenarios on the spring bud break in apple trees

Deciduous trees require sufficient winter chill to have a synchronised and condensed bloom period in spring. When winter chilling is incomplete spring bud burst 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 burst 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 burst. The aim of this study was to investigate the effects of different autumn, winter and spring temperature scenarios on the bud burst pattern of apple trees.

For two consecutive years, 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. Dormancy progression was quantified for each scenario by forcing trees at numerous occasions and expressing the time to bud burst as a two linear joint line model. Spring bud burst was also monitored.

While leaf drop may be an indication of maximum dormancy in colder regions, in warmer regions, leaf drop occurs long after maximum dormancy has been reached. Cold winter temperatures contributed most to bud burst success but autumn chill mitigated some of the lack of winter chill in years when winter chill is low. Warm spring temperatures are less important after sufficient winter chill has accumulated.

Leaf drop is not an indication of maximum dormancy in warm winter areas. Successful bud burst under mild winter conditions is not solely dependent on the amount of chill accumulated, but rather the distribution of the chill throughout the dormant period, the winter period being the most important.

Do not use leaf drop as an estimate for dormancy under conditions of inadequate chill. During dormancy progression, the chill accumulated during the winter is the overriding factor driving adequate bud burst. In the absence of sufficient winter chill, cold autumn and warm spring conditions can mitigate the lack of winter chill but cannot fully replace it.