Assessing phenological data to classify newly bred peach and nectarine selections according to chilling requirement
Most of the peach growing regions in South Africa have inadequate winter chilling to break dormancy of high chilling requirement (CR) cultivars and only medium (400-800 Utah chilling units [CU]) to low (< 400 CU) CR cultivars can be produced successfully in these regions. To breed new peach and nectarine cultivars adapted to local growing conditions, an accurate classification system is needed to identify low and medium chill requiring seedlings. This study was conducted on 31 peach and nectarine selections during an advanced stage of evaluation to test the accuracy of the phonological data based on the date of 10% and 80% bud break, flowering period, occurrence and severity of delayed foliation and CU accumulation. Two replicate bundles of shoots of each selection were prepared (in two consecutive seasons) and forced at 25°C with continuous illumination until no further changes in bud burst occurred. CU in the orchard was calculated according to the Utah and Infruitec models. The hours below 12°C and 7°C were calculated. For each selection the number of days until 20% vegetative and reproductive bud break was plotted over day of year, Utah CU, Infruitec CU, hours below 12°C, and hours below 7°C. The inverse of the number of days until 20% bud break was plotted against all the above variables. The data obtained was expressed as parabolas. The areas under these parabolas was statistically analysed using the CANDISC procedure of SAS Release 8.1. The groupings of the CANDISC procedure were more or less consistent with the preliminary groupings obtained with the phenological classification method. It can be recommended that the phenological classification method can be used to identify selections according to their CR.