The determination of fruit growth curves for new apple cultivars Pink Lady and Fuji

There is a worldwide increasing demand for fruit quality with fruit size being one of the quality parameters. For a good fruit size, fruit thinning of apples and pears is a common practice throughout the fruit industry. It can improve quality, control crop load, increase final fruit size and reduce alternate bearing (Bergh, O.,1984; Bramardi, Castro & Zanelli (1998), Zhang, (1997); Greybe, E., Bergh, O. & Ferreira, D.I., 1997).

In order to assist in fruit thinning, it is necessary to know the seasonal growth pattern and growth curve of the fruit.  Williams et al. (1969) has shown that there is a relationship between fruit size during the growing season and the size reached at harvest. This enables one to relate the size of the fruit at harvest back to the size, for example at thinning time. It is thus possible to identify the small fruit that should be removed at hand thinning.  According to literature, several factors influence fruit size including climate and orchard practices. This indicates that specific fruit growth curves are required according to specific climate, cultivar and orchard practices. Therefor, although several models have been developed for pome fruit growth around the world (Batjer, et al., 1975;Bramardi,et al., 1998; Ortega, et al., 1996; Ortega, et al., 1998; Williams, et al.,1969),no models for the new apple and pear varieties existed under South African conditions. Two varieties,‘Pink Lady’ and ‘Fuji’, were identified to be studied under field conditions to determine their growth curves. Results were to be used during hand thinning under local conditions.

During the 1999/2000 and 2000/01 seasons the diameters of different apple varieties were recorded on a weekly basis, from approximately 40 days from full bloom (dffb) until harvest. The objectives were i) to determine the growth curve for each variety under these conditions and thereafter ii) to develop an accurate tool (growth table) for determining which fruitlets to hand thin at 40 dffb.

Differences in fruit diameter were observed between the varieties and areas as early as 40 dffb. These differences were related to variations in seasonal temperature, hours of sunlight and crop load in the present as well as in previous seasons (Bergh, 1984).  The differences, observed between the varieties, for fruitlets which had similar diameters at 40 dffb, showed that the varieties had inherently different growth rates, which have to be considered when determining the optimum thinning procedure. In all cultivars, mall fruit at 40 dffb remained small at harvest and could be accurately identified early in the season and eliminated.