Chill unit accumulation and necessity of rest breaking agents in South African table grape production regions

Abstract

Cultivation of table grapes in warm regions of South Africa is complicated by problems associated with delayed and uneven bud break. Rest breaking agents, mainly hydrogen cyanamide, are applied to overcome these problems. The chill unit accumulation of the five major table grape production regions in South Africa was compared with the minimum chilling required to obtain a high bud break percentage (>80%) and even bud break, defined as 200 hours between 0 and 10 ∘ C, or 400 hours at 3 ∘ C. The effect of chilling accumulation (0, 50, 100, 200, 400 and 800 hours at 3 ∘ C) on bud break of dormant cuttings of three table grape cultivars was investigated in controlled studies in a glass house over a three year period. Cuttings used in the study were collected from commercial vineyards in Saron in the Berg River Valley (representing a warm region) and the Hex River Valley (representing a cool region). Sultanina and Sugraone cuttings from Saron required at least 200 hours at 3 ∘ C to obtain a final bud break % of 80%+. A final bud break % of 80%+ was obtained with all Sultanina cuttings from Hex River Valley. Treatments receiving 400 and 800 hours at 3 ∘ C, showed the most rapid rate of bud break. Prime cuttings collected from Saron, receiving 0, 50, 100 and 400 hours at 3 ∘ C, showed a more rapid bud break process, as well as a higher final bud break % compared to Sugraone and Sultanina. It seems as if Prime has a lower chilling requirement than the other two cultivars. The model of Dokoozlian for quantifying chilling status is applicable to Saron and the Hex River Valley. The necessity of using rest breaking chemicals in the Saron area, for obtaining a high bud break % and even bud break, was confirmed. The Hex River Valley receives sufficient chilling temperatures to obtain a high bud break %, but rest breaking chemicals can be used to obtain more even bud break. The use of hydrogen cyanamide for improved and even bud break is a general practice in regions that receive less than 400 chill units. In regions receiving more than 400 chill units, several producers still use hydrogen cyanamide for even bud break. It is recommended that the chilling requirement of economic important cultivars should be determined simultaneously in field trials and glass house trials for the major table grape production regions of South Africa, to verify the necessity of using rest breaking chemicals for specific cultivars in specific regions.