Chilling requirements of apple cultivars grown in mild Australian winter conditions

Future climate change is predicted to lead to reduced winter chill accumulation in many temperate fruit-tree growing regions of the world. Reliable measures of chilling requirement (CR) are necessary for growers in mild winter production locations to determine phenotypic suitability of cultivars for profitable production under current and future climates. This study has produced a collection of CRs for apple (Malus domestica) in chill portions (CP), chill units (CU) and chill hours (CH) for the mild climatic region of Applethorpe, Queensland, Australia. Total seasonal chill accumulation (1 March to 31 August) for 2014 and 2015 in Applethorpe was 75 and 78 CP; 1341 and 1242 CU and; 899 and 882 CH, respectively. All cultivars met their CRs by 10 September (2014) and 2 September (2015). Cultivar CR was estimated using forced bud methods over two seasons. The range of CRs measured for each of the three chill models was 57±2.9 to 77±1.5 CP, 976±40.3 to 1307±86.6 CU and 662±44.5 to 908±23.3 CH. Regardless of chill model choice, the nine cultivars were ranked the same from lowest (‘Cripps Red’) to the highest requirement (‘Fuji and Hi Early’). Analysis of historical climate data showed that the CRs of ‘Cripps Pink’ and ‘Granny Smith’ have been met in 56% and 58% of years respectively. By 2030, this was projected to fall to 2–25% of years for both varieties, and 0% of years by 2050 for ‘Cripps Pink’, and 0–2% by 2050 for ‘Granny Smith’. The results emphasise the need for better understanding of bud progression through the phases of dormancy. In particular, well-defined methods for identifying the transition from endo- to ecodormancy, and improved capacity of chill models to capture the chill accumulation process are necessary to provide valuable CR information to temperate fruit industries for adaptation to climate change