COMPARISON OF 2 MODELS FOR THE PREDICTION OF REST COMPLETION IN PEACHES

Phenological and climatic data were collected from nectarine orchards containing the cultivars 'Sunlite', 'Flavortop' and 'Fantasia', growin g in six climatically divergent areas in the Western Cape, South Afric a. Shoots were forced at 25-degrees-C and bud volumes measured in orde r to develop an index of when the winter chilling requirement for each cultivar in each area had been satisfied. Flower bud break gave the o nly consistent indication of when the chilling requirement had been sa tisfied and 10% flower bud break after 14 days at 25-degrees-C was ado pted as the end of rest index. The 'dynamic' rest completion predictio n model was considerably more accurate than the currently used 'Utah' model, for estimating the rest requirement of the nectarine cultivars 'Sunlite', 'Flavortop' and 'Fantasia'. The 'dynamic' model not only ta kes into account the positive effect of cool temperatures and negative effect of high temperatures on rest completion that the 'Utah' model does, but also the positive effect of moderate temperatures and the fa ct that chilling negation is subject to time constraints. The cultivar s tested required about 33, 41 and 41 'dynamic' portions of chilling r espectively, followed by an average of 2000+/-1067 degree growing hour s above a 10-degrees-C base temperature in order to reach full bloom i n spring. Vegetative bud break was enhanced in the colder locations, w hile reproductive development was better in the warmer locations.