ISHS Acta Horticulturae 662: VII International Symposium on Temperate Zone Fruits in the Tropics and Subtropics
WINTER CHILLING IN AREAS WITH MILD WINTERS: ITS MEASUREMENT AND SUPPLEMENTATION |
| Author: | P. Allan |
| Keywords: | Actinidia deliciosa var. deliciosa, Prunus persica, Vitis vinifera |
| Abstract:
Early attempts to measure winter chilling, using the Utah Chill Unit model, were unsuccessful as negative totals were obtained at Pietermaritzburg, despite the fact that low chill peach trees produce well. This was due to high daytime temperatures (25 to 30°C) negating the beneficial effects of low night temperatures. Positive Utah Chill Units (PCU) can be accumulated, by ignoring negative daily totals. These PCU have been used to produce maps of South Africa showing how much winter chilling accumulates each month in different areas. Both the Israeli dynamic model and positive chill units have been shown to be equally accurate in estimating the amount of winter chilling in both cold and mild areas of South Africa. Production of low-chill ‘Allison’ kiwifruit at Pietermaritzburg has been improved (trebled) by evaporative cooling in winter, especially under 30 to 40% artificial shade. Dormex® (hydrogen cyanamide) sprays in late winter have had a synergistic effect with evaporative cooling in giving the greatest production. By intermittent overhead sprinkling, when daytime temperatures exceed 16°C in winter, wet bud temperatures have been kept below 19°C, with a beneficial effect on winter chilling. Evaporative cooling should start when daytime temperature exceeds 16 to 18°C provided the night temperature had been below 12°C. Ideally, sprinkling cycles should be long enough to wet all the buds effectively, and the OFF time should be short enough to prevent bud temperatures rising too high. Production of early table grapes has been improved (improved bud break and increased number of bunches produced) in the lower Orange river area of South Africa, in some years. Intermittent sprinkling is used on a large scale as insurance in case of inadequate winter chilling in mild winters. | |
Download Adobe Acrobat Reader (free software to read PDF files) | |
