Predicting the Date of Bud Burst in Grapevines

Abstract

It was possible to forecast the date of bud burst under South African winter temperatures (Region III) using the Pouget-formulae and principles, and to establish a scale of bud burst for cultivars grown in South Africa. The sum of daily temperature effects was higher under the warmer South African conditions which resulted in changes in the formulae for determining the daily effect of temperature and the cultivar coefficient on the bud burst date. Highly significant linear relationships were, however, obtained and it was possible to predict the date of bud burst fairly accurately. Predicting the date of bud burst of grapevines has a number of practical advantages in that, inter alia, pruning can be planned more efficiently (e.g. the best time for pruning is 2-3 weeks before bud burst), while treatments against diseases (e.g. bud mite) and to homogenise and increase the percentage of bud burst (hydrogen cyanamide) can be timed more accurate! y. From bud burst dates obtained over 25 years for 22 cul-tivars in France (Bordeaux, Region II according to Winkler et al., 1974) as well as minimum and maximum temperatures during the period prior to bud burst, Pouget (1988) established a time scale for bud burst. Moreover, he indicated that it was possible to calculate the potential date of bud burst 2-3 weeks in advance. These calculations were based on "the law of the effect of temperature on rate of bud burst" (Pouget, 1967) and the sum of daily minimum and maximum temperature values from a fixed date during ecodormancy (post-dormancy). The aim of this investigation was to determine the possibility of using Pou get' s formulae and cul ti var coefficients to predict the date of bud burst of cultivars grown in South Africa under conditions of higher winter temperatures. This would then also make it possible to determine similar cultivar coefficients for cultivars commonly grown in South Africa. MATERIALS AND METHODS Bud burst dates were recorded over 8 years on 15 Vitis vinifera L. cultivars grown on the YORI experimental farm, Stellenbosch (Region III). Dates used in this study were recorded when 50% of the buds allocated during pruning (spur pruned and long bearers) showed green colouring. The cultivars were grafted onto 99 Richter (cl RY 30) with a planting width of 3,0 x l,5m, trellised on a l,5m slanting trellis and pruned during August. Cultivars of which coefficients were determined by Pouget (1988), viz. Gewtirztra-miner, Chasselas blanc (Chasselas dare), Shiraz, Sauvignon blanc and Ugni blanc were used as reference cultivars. Duration to bud burst: The duration to bud burst (D) was initially calculated as the number of days from 1 June, 15 June, 1 July and 15 July (during ecodormancy) up to and including the date of bud burst. These dates are sufficiently distant from bud burst for the buds of the various cultivars to be considered as being in a very similar physiological state regardless of date of bud burst. Sum of daily temperature effects: According to Pouget (1988) temperature has a specific action on a cultivar which is proportional to the rate of bud burst (number of days required to 50% bud burst): where: V 1 =rate of bud burst, t = temperature, c = cultivar bud burst coefficient, K = cultivar coefficient. (1) This temperature effect, which differs according to cul-tivar, is calculated from the effect of daily maximum and minimum temperatures and can be expressed as follows (Pouget, 1988): where: (2) aj = temperature effect for day j with j representing each day from 1 July up to and including date of bud burst. tM =maximum temperature on day j, 1m = minimum temperature on day j, Acknowledgement: The recording of bud burst dates by J.C.D. Theron is acknowledged.