A survey of eight chilling models for apple trees in Southern Brazil under mild winters

ABSTRACT The main models used for estimating chill accumulation throughout the dormancy period of temperate fruit trees in Brazil were developed in temperate regions in the northern hemisphere. Thus, they exhibit low precision when they are used in regions with mild winters. However, there are several little-known models that are more precise. This study aimed at evaluating the precision of eight different estimation models of chill accumulation applied to five apple cultivars which were grafted on two distinct rootstocks in a region with mild winters. Based on the coefficient of variation, the Microsoft Excel® program was used for evaluating the precision of the following models: Utah, modified Utah, CH < 7.2°C, CH < 10°C, CH < 15°C, Low chill, Taiwan low chill and Daily positive Utah chill unit (PCU). The lowest precision was attributed to both models Utah and CH < 7.2°C (the most common model in Brazil). Models CH < 15°C, Taiwan low chill, Low chill and modified Utah were considered precise (coefficient of variation  < 15% in annual chill accumulation and to evaluate requirement to overcome dormancy in cultivars). No important differences were observed in models precision between ‘M-26’ and ‘M-9’ rootstocks.