Determining the dormancy progression of commercial apple cultivars and clones available to the South African Fruit Industry

Healthy dormancy progression synchronises budbreak in the next season and increasing the fruiting potential. Dormancy progression is influenced by, and dependant on, environmental factors, especially low temperatures and chill accumulation, both factors that vary greatly between production areas. This makes the selection of well suited cultivars and rootstock/scion combinations important, but also challenging in South African’s diverse apple producing conditions. Understanding the progression of dormancy of a specific cultivar (and its clones) and its robustness to annual changes in autumn/winter temperatures is not just beneficial when establishing a new orchard, but also in managing existing orchards. This project aims at determining the dormancy progression patterns of commercially important apple scions and their respective clones, in two climatically contrasting production regions over two winter seasons.

Shoots from 22 different apple clones were collected every 14 days from two apple producing areas (Elgin and Koue Bokkeveld) in 2016 and 2017. Shoot collection started at budset until budbreak or application of rest breaking agent. The shoots were exposed to standard forcing conditions (25˚C, continues light) and monitored until 50% budbreak occurred. From this dormancy progression curves were constructed and a two linear joint line model was fitted to present 11 variables that describe dormancy progression. The means of the variables were compared and subjected to a principle component analysis followed by a cluster analysis to rank the cultivars according to their dormancy behaviour.

The dormancy progression of the most important commercial apple cultivars (and their respective clones) are presented for two of the biggest production areas in South Africa. The cultivars were ranked according to their dormancy start date, maximum dormancy level and dormancy end date. Results show that clones have different dormancy progression patterns that change when subjected to different climatic conditions (season and region). Some cultivar specific behaviour is discussed and the results of a case study are presented to illustrate different modes of action in chill/cold perception cultivars.

In the Koue Bokkeveld the dormancy patterns show annual differences that relate to Utah chill unit accumulation. In warmer years their dormancy patterns start later and exiting later compared to cooler years. For similar cultivars grown in warmer climates such as Elgin, the cultivars behave more erratic during the entrance into dormancy possibly due to different autumn conditions. These trees also have extended dormancy release periods that end late in the year. In Elgin, dormancy progression differences are seen between years despite similar Utah chill accumulation, expressing again the unsuitability and risk of using the Utah model in warm winter climates. The results can be used by growers to match cultivars with climatic areas when planning new orchards and in managing expectations of budbreak in existing orchards based on the chill conditions of the preceding autumn and winter. The data also cautions against expecting similar dormancy behaviour in clones selected for non-dormancy related traits such as colour.