Fine genetic characterization of the Co locus responsible for the columnar habit in apple (Malus x domestica Borkh.)

Apple (Malus x domestica Borkh.) is one of the more widely cultivated fruit tree. It is therefore not surprising that a lot of efforts are being made worldwide to obtain new varieties with improved characteristics especially regarding fruit quality, pathogen resistance and tree architecture. In case of fruit trees the plant architecture is considered a very important trait, because it can influence fruit quality, plant density, production and labour requirements. The columnar growth habit in apple was discovered in 1970 as a dominant mutation of the cultivar McIntosh (Fisher 1970). It is characterized by short internodes, a thick stem with little difference in diameter between the base and the top and a reduced plant height and branching (Hemmat et al. 1997). The axillary buds grow mostly into spurs, resulting in a relatively sturdy tree with few and mainly short side shoots. These characteristics are considered useful for the minimal pruning interventions required and for the possibility of high-density cultivation. At present different apple cultivars carrying the mutated allele (Co) responsible of the columnar phenotype have been released such as Wijcick, Telamon, Tuscan, Trajan, Maple and Charlotte