Browsing by Author "Van der Vyver, L."

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    The influence of gibberellic acid (GA₃) for berry thinning and berry sizing on table grape production, quality and fertility of Prime
    (Stellenbosch : Stellenbosch University, 2016-12) Van der Vyver, L.; Avenant, Eunice; Avenant, J. H.; Strever, A. E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Table grapes are one of the major commercially grown non-climacteric fresh fruit crops worldwide. Over centuries the table grape industry became a niche market with increasing competition on the markets, putting pressure on table grape growers to produce quality grapes that meet market requirements nationally and internationally. To meet market requirements regarding bunch size and compactness, as well as berry size, colour, flavour, texture and firmness, viticultural practices for table grape production include the use of plant growth regulators (PGRs). Higher input costs are invested to meet these requirements. This lead to the critical focus on labour-intensive cultivation practices and whether alternative methods could be found to maintain high levels of fertility, production and quality. The the aim of this study was to identify GA₃ application methods and volumes for thinning and sizing treatments of table grapes without negatively affecting fertility. The study was done in a commercial Prime vineyard, grafted onto Ramsey, in Paarl, Berg River Valley, South Africa. There are limited scientific publications reporting research results on this cultivar, specifically regarding the effect of different GA3 application methods and volumes on production, quality and fertility. Thinning and berry sizing treatments were applied according to commercial concentrations recommended for Prime. In this trial, different GA3 application methods and volumes were evaluated. Two GA3 treatments were applied during two phenological stages. The first application was the thinning treatment which was applied at 80-100% full bloom. The second application was the berry sizing treatment which was applied when the berries where at 7-8 mm diameter. Six treatments were applied: Treatment 1 (NoThin + Dip (Control)), comprised of a no thinning application, followed by the berry sizing treatment applied by dipping. Treatment 2 (Thin + Dip) comprised a conventional thinning spray application, followed by a berry sizing treatment applied by dipping. Treatment 3 (Thin + 250 L/ha), Treatment 4 (Thin + 500 L/ha) and Treatment 5 (Thin + 1000 L/ha) comprised conventional thinning spray application, followed by berry sizing treatments applied by spraying with a mist blower with spray volumes of 250 L/ha, 500 L/ha and 1000 L/ha respectively. Treatment 6 (Thin + ESS) comprised a conventional thinning spray, followed by a berry sizing treatment, applied at 72 L/ha with an electrostatic spray pump (ESS). In both seasons, before the thinning application was applied, 15 inflorescences per data experimental unit were marked according to a phenological stage. In the first season, ten inflorescences per data experimental unit were marked at 80-100% full bloom (FB) and five inflorescences were marked at 10% set (referred to in the table grape indusy as 110% full bloom). In the second season five inflorescences per data experimental unit were marked at 80-100% FB, five inflorescences were marked at 10% set and five inflorescences were marked at 100% set. No manual bunch preparation actions were applied to these marked bunches and no berry sampling were done from them. These clusters were evaluated for bunch structure/compactness at harvest. This method was used to determine the optimum time for application of thinning treatments in terms of bunch structure at harvest. The bud break percentage determined through forced budding in June 2015 and June 2016, as well as through assessment in the vineyard (November 2015) did not differ significantly between treatments and was above 80% for all treatments. Commercially acceptable levels of bud break were obtained in both seasons. The potential and actual fertility decreased over the two seasons. In June 2015 Treatment 1 had a significantly higher potential fertility and Treatment 6 had a significantly lower potential fertility compared to the other treatments. In June 2016, no significant differences were found between treatments, although Treatment 6 again had the lowest potential fertility. It seems that Treatments 3 and 6 with lower application volumes and smaller droplet size are associated with lower fertility, possibly due to more effective coverage obtained on bunches (the target organs for berry sizing treatments), but also on the shoots and buds and that GA3 applied to the buds had a negative effect on potential fertility. This was also reflected by the actual fertility and yield obtained in the November 2015, where Treatment 2 had the lowest yield as compared to Treatments 3 and 6 (only significant for Treatment 3). Regarding manual thinning in both seasons, Treatment 1 required the longest time spent per ha and Treatments 2 and 5 required significantly less time, which can be ascribed to the larger berry size and % normal berries obtained with Treatment 2. No significant difference was found between the different spray applications (volumes). Therefore, the “best” method for application will depend on the effect on return fertility. In both seasons, Treatment 1 required the most hours for manual thinning and consequently had the highest cost, verifying the need for chemical thinning of Prime, to save labour cost. Time and cost of manual thinning of Prime using Treatment 2, can be up to 40% lower than with Treatment 1. Time required and cost for Treatments 2 and 5 ranged from 942 to 2578 hours and R12 595 and R31 992, which were in line with the time and cost required for commercial Prime blocks. Berry juice composition was not negatively affected by any of the treatments. The expected berry development and ripening patterns were found. Although a few significant differences were found regarding post-harvest quality, it did not practically impact the marketability of the grapes. Regarding the bunch structure in the 2014/2015 season, there were few significant differences between treatments. With the thinning application applied at 80-100% FB the number of berries per cm lateral length, as well as the number of normal berries per cm lateral length of Treatment 1 was significantly higher compared to Treatment 2, indicating that the bunches of Treatment 1 were more compact than the bunches of Treatment 2. The 80-100% FB Treatment 1 had a significantly higher number of small berries per cm lateral length compared to the other two treatments which can be linked to the longer time required for manual thinning of this treatment. In both seasons a trend was observed that a lower number of berries per cm lateral length (less compact bunches) was obtained with the thinning application applied at 80-100% FB compared to the later applications. The results of this study contribute to the available published scientific results regarding the effect of GA3 application methods (volumes) on fertility of table grapes. Based on the results after the first two seasons of the trial, the following are recommended regarding identifying GA₃ application methods and volumes for effective thinning and sizing treatments of table grapes without negatively affecting fertility: . Treatment 2 (Chemical thinning with a standard GA3 spray application, followed by a GA3 dipping treatment for berry sizing) had the largest berries, less compact bunches and the highest percentage normal berries. This treatment also required the least time for manual thinning. . Practical implementation of Treatment 2 (Chemical thinning with a standard GA3 spray application, followed by a GA3 dipping treatment for berry sizing) in commercial table grape production requires availability of sufficient labour. This is already practically applied by several producers in the industry in situations where they have practical experience of a decline in fertility after GA3 applications. . Current available results indicate that the lower spray application volumes Treatment 3 (250 L/ha) and Treatment 6 (ESS 72 L/ha) were associated with a decrease in fertility, while with Treatment 4 (500 L/ha) no indication of a negative effect on fertility was found. Therefore, repetition of the trial is needed to verify these results and to investigate whether the 500 L/ha spray application volume could be used instead of the current standard industry practice of using 1000 L/ha for the majority PGRs and other spray applications. Using an application volume of 500 L/ha instead of 1000 L/ha will have several practical and economic benefits, in terms of more hectares being sprayed with a one tank mix, decreasing the water foot print as well as the carbon foot print. . It is recommended to repeat the trial for at least one more season to verify results obtained and test repeatability.