Department of Viticulture and Oenology

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Bunch quality and fertility of Vitis vinifera L. cv. Prime as affected by gibberellic acid (GA3) and s-abscisic acid (s-ABA).
(Stellenbosch : Stellenbosch University, 2022-12) Masikane, Nonkululeko Nosipho; Avenant, Eunice; Avenant, J. H. ; Stellenbosch University. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: Table grapes are one of the most cultivated, economically important fruit crops in the world and is one of the major fruit industries in South Africa. There is an increase competition in the market that leads to an increase production costs for South African table grape industry, to produce quality grapes that meet market requirements nationally and internationally. Market requirements for grapes are consumer driven, and are based on bunch structure, berry size, colour uniformity, flavour, texture, seedlessness and firmness. Plant growth regulators (PGRs) have become an important tool in producing grapes of high export quality, and to meet these market requirements. Gibberellic acid (GA3) is one of the registered plant growth regulators that is used as a chemical thinner in the grapevine, it is has been found to play a role in berry set, thinning and development. The effectiveness of PGRs dependent on cultivar, application time and concentration. This study aimed at establishing the effect of gibberellic acid (GA3) and abscisic acid (s-ABA) for thinning, combined with GA3 for berry sizing (applied at the recommended dosages, using different application volumes obtained with commercial spray equipment), on fertility of a 5-year-old Vitis vinifera cv. Prime block, grafted onto Ramsey rootstock on the farm Newgro at Kanoneiland in the Orange River Region. Four thinning treatments were combined with four berry sizing treatments, resulting in 16 treatment combinations. The thinning treatments comprised of a Control (No thinning); 1 ppm GA3 at 10% set and 3 days later; 1 ppm of GA3 x 5, with the first application at 10% set, followed by four more applications at 3-day intervals; and 1 ppm GA3 plus 400 ppm s-ABA at 10% set, followed by 1 ppm GA3 3 days later. All thinning treatments were applied at a volume of 1000 L.ha-1 with a Nobili 2000 T mist blower spray pump. The berry sizing treatments, applied at 7 to 8 mm and 8 to 10 mm berry diameter, respectively, comprised of a Control (Dipping) and three treatments applied by spraying. The same active ingredient dosage (2 x 20g GA3 ha⁻¹) was used for all treatments, however different application volumes were used for the spraying treatments (90 L ha-1 with an electrostatic spray pump (ESS); 250 L.ha-1 and 500 L.ha- 1 respectively with a Cima A T50S2.15.L11 mist blower spray pump. Bunches of all treatments obtained commercially acceptable bunch lengths (>20 cm) and Extra-Large berry size (> 18 mm in diameter). The control had the most compact bunches, but there was no significant difference between the three chemical thinning treatments, regarding bunch length, bunch compactness, berry size and berry size distribution. Therefore, producers could use any of these three chemical thinning treatments for Prime. There was a tendency that the largest berry size was obtained with the Dipping and ESS berry sizing treatments. Although there were significant differences, the percentage of extra small berries of all sizing treatments was very low (< 1) in both seasons. Potential fertility was assessed through forced budding in a glasshouse and bud dissections. Actual fertility was assessed in the vineyard. None of the thinning or sizing treatments negatively affected fertility. All treatments obtained commercially acceptable levels (> 80%) of bud break (ranging from 92 % to 97%) and fertility (more than one bunch per sprouted bud). Fertility was not affected by the GA3 treatments applied over three consecutive seasons, probably due to the dosage range used (1 to 5 g.ha-1 for thinning and 40 g.ha-1 for berry sizing). When making the final decision of which chemical thinning treatment and which berry sizing application technique to use, the effect on berry size and bunch quality should also be considered. None of the treatments evaluated negatively affected berry size and bunch quality. Any of the application techniques evaluated could be used, if GA3 within the recommended dosage range is used and applied at the recommended phenological stages for Prime. This study also provided data on the efficacy of GA3+ABA as thinning agent for table grapes, to be used to extend the registration of s-ABA on table grapes to include its use as a thinning agent.
Bunch structure, rudimentary seed size and return fertility of Vitis vinifera L. ‘Sunred Seedless’ as affected by GA3 and GA4+7 thinning treatments
(Stellenbosch : Stellenbosch University, 2020-12) Claassen, Talana; Eunice, Avenant; Avenant, J. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: Market requirements for export grapes are consumer-driven and based on characteristics such as bunch size, bunch colour uniformity, berry size and distribution, seedlessness, flavour profile, texture and eating quality. In order to meet these requirements, the use of plant growth regulators (PGRs) has become an essential tool in producing grapes of high export quality, while contributing to reducing labour costs required for manual thinning or girdling to increase berry size. Increased costs associated with the production of table grapes, along with high expectations to meet increasing market demands, require attention to minimise input costs with the effective use of PGRs. The response of cultivars poses a challenge, as cultivars react differently towards a PGR application. Apart from cultivar response, the application timing and concentration used for the specific PGR also contribute towards the efficacy of the treatment applied. Limited research publications are available on the effect of GA4+7 used for thinning on table grapes, as well as the effect of GA3 and GA4+7 applications on rudimentary seed size and return fertility of specifically Sunred Seedless, but table grapes in general as well. The study aimed to determine whether an alternative gibberellic acid structure, GA4+7, could be used as a chemical thinning agent for cultivars that respond poorly to GA3 in order to improve bunch quality without negatively affecting the return fertility. The study was performed during the 2015/2016 and 2016/2017 growing seasons on 15-year-old Vitis vinifera L. cv. ‘Sunred Seedless’ vines, grafted onto Ramsey (Vitis champinii). The experimental site is situated in a commercial vineyard located on the premises of the ARC Infruitec-Nietvoorbij experimental farm at De Doorns, in the Hex River Valley, South Africa. A standard GA3 concentration of 5 parts per million (ppm) was evaluated against different concentrations of GA4+7, ranging from 7.5 ppm to 120 ppm, adjusted over the two seasons. These treatments were applied at different phenological stages in order to determine the most effective timing for a thinning application on Sunred Seedless. Eight treatments and an untreated control were evaluated during the 2015/2016 season. The treatments consisted of four early thinning applications applied 31 October 2015 and four late thinning applications applied 4 November 2015. Both the early and late treatments were applied at 5 ppm GA3, 7.5 ppm GA4+7,15 ppm GA4+7 and 30 ppm GA4+7. The two application dates refer to a difference in the predominant phenological stage of the vineyard, which a producer would have used to determine the timing of a thinning application. The early application timing represents a predominant phenological stage of 10% berry set (10%BS) and the late application represents berry set (BS). The treatment layout for the 2016/2017 season was adjusted to accommodate increased GA4+7 concentrations, as well as two sizing treatments. The nine treatments applied in this particular season consisted of an untreated control, six thinning (T) treatments (5 ppm GA3; 7.5 ppm GA4+7; 15 ppm GA4+7; 30 ppm GA4+7; 60 ppm GA4+7; 120 ppm GA4+7), a thinning and sizing (T+S) treatment (60 ppm GA4+7 + 60 ppm GA4+7) and a sizing (S) only treatment (60 ppm GA4+7). Each treatment had four replicates and each replicate consisted of four vines, referred to as an experimental unit. Within each experimental unit the two centre vines were used as the experimental data unit. Field sampling was performed in the experimental data unit. Additionally, within each experimental data unit, bunches were categorised and marked at four phenological stages to determine the optimal phenological stage for application. The stages for the 2015/2016 season included 80-100% flowering (80-100%F), 10% berry set (10%BS), berry set (BS) and berry set plus four days (BS+4D). The stages for the 2016/2017 season included 50% flowering (50%F), 80- 100%F, 10%BS and BS. Five bunches per experimental data unit were marked according to the phenological stages identified for each season. These marked bunches were used for bunch and berry evaluations at harvest and were therefore left in their natural state, with no bunch preparations applied or any berry sampling performed on them. Bunch structure assessments were performed in line with a protocol developed and applied by the Viticulture Division of ARC Infruitec-Nietvoorbij. Applications during flowering resulted in a better thinning effect of Sunred Seedless, based on the bunch and berry mass measurements. Bunch and berry mass measurements at harvest didn’t result in a specific trend concerning a specific GA concentration and application timing combination that could be recommended for effective thinning of Sunred Seedless. Based on the subjective visual assessment of bunch compactness, applying a GA thinning treatment at 50% flowering is too early for Sunred Seedless, as it resulted in straggly bunches. However, the longer a GA thinning treatment was delayed from flowering to berry set, the less effective the thinning results were, resulting in more compact bunches if applied around berry set. These findings correspond with the results obtained for the quantitative bunch compactness measurements. The mean total and normal berries per cm of lateral length were reduced significantly by GA treatments applied during flowering. The 5 ppm GA3 treatment applied at 80-100%F resulted in the most effective thinning, with a significantly reduced number of total berries per cm of lateral compared to the untreated control. There was a significant increase in the mean percentage of shot berries at the 50%F and 80-100%F stages compared to the 10%BS and BS stages, for GA treatments applied during the 2016/2017 season. These results indicate that Sunred Seedless has a higher sensitivity for the formation of shot berries when GA is applied during flowering. An increase in shot berry occurrence was observed with the use of higher GA4+7 concentrations and double applications at the 50%F stage. The sensitivity of Sunred Seedless towards GA applications applied during early flowering, along with poor response for GA applications applied after flowering observed in this study, confirms why GA thinning treatments for this particular cultivar do not give economically acceptable results. Reoccurring trends regarding the bunch phenological stage at the time of application were observed in this study, rather than trends regarding a specific GA treatment and treatment rates. These results confirm that the timing of a GA applications play a fundamental role in the treatment outcome for a specific cultivar. A trend was observed that applying GA treatments during flowering resulted in decreased average rudimentary seed mass per berry as well as an improved rudimentary seed size distribution with an increased percentage of small rudimentary seeds compared to GA applied during the early stages of berry development. No consistent trend regarding the effect of different GA3 or GA4+7 application timing and rates on rudimentary seed size could be concluded over two seasons. Commercially acceptable bud break percentages of ≥ 80% were obtained for all treatments, determined through forced budding in June 2016 and 2017 as well as through actual fertility assessments in October 2016. A reduction in the mean number of bunches per sprouted bud was reported from June 2016 to June 2017 for the potential fertility assessed through forced budding. Potential fertility assessed through bud dissections did not follow the same trend from June 2016 to June 2017 as mentioned above for forced budding. The use of GA3 reduced the actual fertility of Sunred Seedless in this study, after one season of GA treatment application compared to the untreated control. Similar results were not observed for GA4+7 treatments. There was a poor correlation between the potential fertility determined through bud dissection and forced budding were reported, compared to the actual fertility determined in the vineyard. Potential fertility assessments are therefore not advised for crop estimations, but rather to be used for verifying the pruning system used for a specific cultivar.
Canopy manipulation practices for optimum colour of redglobe (V.Vinifera L.)
(Stellenbosch : University of Stellenbosch, 2006-03) Strydom, Janene; Raath, P. J.; Avenant, J. H.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
Under certain South African conditions, Redglobe develops a colour that is too dark and thus unacceptable for the Far Eastern markets. These markets require a pink colour instead of a dark red colour. The cultivation of grapes with an acceptable colour involves amongst other, canopy management practices. This generally includes the removal of leaves and/or lateral shoots. Hereby, the leaf area and the microclimatic conditions in the canopy are altered. The aim of this study was to test the usefulness of leaf and lateral shoot removal at different defoliation times after anthesis in order to obtain a pink coloured Redglobe crop. Other quality aspects, namely total soluble solids (TSS), total titratable acidity (TTA), berry mass and total yield, were also evaluated. A canopy management trial was conducted on six year old Redglobe vines with moderate vigour. The treatment design was a 2 x 3 x 4 factorial and involved two leaf removal (L) levels (L0 = 0% leaf removal; L33 = 33% leaf removal) in combination with three lateral shoot removal (LS) levels (LS0 = 0 % lateral shoot removal; LS50 = 50% lateral shoot removal; LS100 = 100% lateral shoot removal). Four defoliation times (DT) were selected: 36 (pea berry size), 69 (véraison), 76 (one week after véraison) and 83 (two weeks after véraison) days after anthesis (DAA). A total of 24 treatment combinations, replicated in four blocks, were applied. Generally, treatment combinations involving 33% leaf removal lowered the main shoot leaf area. Likewise, the lateral shoot leaf area was decreased by increasing levels of lateral shoot removal at any defoliation time. As expected, 33% leaf removal applied in combination with any level of lateral shoot removal, always resulted in a lower total vine leaf area compared to where 0% leaf removal was part of the treatment combination. Compensation reactions occurred and in this regard the main shoot leaf size increased due to 33% leaf removal applied at 1 week after véraison and 2 weeks after véraison. Treatment combinations involving lateral shoot removal increased the ratio of main shoot leaf area to the total leaf area. On the other hand, the main shoot leaf area percentage was lowered by the application of 33% leaf removal at 2 weeks after véraison compared to no leaf removal at the same defoliation time. It can therefore be assumed that the contribution of lateral shoot leaves to grape composition might have increased in cases where the main shoot leaf area was lowered at a later stage (e.g. 2 weeks after véraison). The bunches were visually evaluated and divided into classes from dark (class one) to light (class nine). This visual bunch evaluation showed that the mean bunch colour was in class three (lighter than class two) due to the defoliation time. The lateral shoot removal x leaf removal interaction resulted in a mean bunch colour that was in classes 2 and 3. However, within these classes, there was a tendency that bunch colour decreased for defoliation times later than pea berry size. The lateral shoot removal x leaf removal interactions showed that bunch colour was darker when the treatment combinations involved 0% leaf removal. The percentage of bunches with the desired colour was increased by application of the treatments at véraison, compared to the other defoliation times, and also with 50% lateral shoot removal and 100% lateral shoot removal compared to 0% lateral shoot removal. Biochemical analyses confirmed that increased levels of lateral shoot removal generally lowered the anthocyanin concentration regardless of defoliation time. A similar effect on TSS was observed, i.e. from véraison onwards, the application of 50% lateral shoot removal and 100% lateral shoot removal tended to lower TSS. The effect of these levels of lateral shoot removal at véraison was significant. The role of the lateral shoots in colour development and sugar accumulation is therefore emphasized. Furthermore, the special role that lateral shoots also play in berry development is illustrated in that berry mass tended to decrease when 100% lateral shoot removal in combination with 33% leaf removal and 100% lateral shoot removal in combination with 0% leaf removal were applied at véraison. This, together with the positive relationship obtained between grape colour and the lateral shoot leaf area:fruit mass ratio, accentuates the role of active leaf area during the ripening period. The possible effect of the microclimatic light environment on colour must also be considered. However, although the light intensity increased with increased levels of LS, the colour that was obtained was probably not associated with the differences in light intensity. It was found that it is possible to manipulate the colour of Redglobe grapes with defoliation treatments. However, the treatments that have a decreasing effect on grape colour also affected other quality parameters like TSS and berry size negatively. Although, it is possible to reduce the colour of Redglobe through the application of leaf and lateral shoot removal at different defoliation times, the question arises whether the treatment combinations used in this study are worthwhile to pursue because the mean bunch colour that was obtained was still too dark. However, it was possible to increase the percentage of bunches with the desired colour. Therefore, if such treatments are applied, it must be approached cautiously, keeping in mind that assimilate supply has to be sustained throughout the ripening period.
The effect of different irrigation frequencies in combination with boron and calcium bunch applications on berry split of SoutherngrapeOne
(Stellenbosch : University of Stellenbosch, 2010-03) Koekemoer, Abraham Leander; Raath, P. J.; Avenant, J. H.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: The table grape industry employ a wide range of viticultural management practices in order to produce the high quality grapes demanded by the export market. A common contributor to degrading the quality of table grapes is the occurrence of berry split, which not only has an unattractive visual effect, but also increases the berries’ susceptibility to infection by spoilage organisms. A number of environmental conditions such as rainfall and humidity, and/or agricultural practices, such as irrigation, and high density canopies, can lead to higher plant cell water content. This in turn, can increase the potential of berry split to occur. To date, the main method of berry split prevention has been the management of plant water status by; (i) regulating irrigation withdrawal times, and (ii) covering of canopies if rainfall is predicted prior to harvest. The aim of this study was to determine the effect that irrigation frequency, as induced by irrigation withdrawals; as well as boron (B) and calcium (Ca) treatments, applied as bunch directed sprays, have on pre- and post-harvest berry split. To this end, a newly released late ripening, white seedless cultivar, SoutherngrapeOne was chosen as a model cultivar as it has a high susceptibility to berry split. SoutherngrapeOne vines were subsequently subjected to a range of irrigation frequencies based on typical irrigation scheduling used in the table grape industry, which comprised of a low, medium and high frequency. The low frequency was duplicated in order to demonstrate the effect that a heavy irrigation, just before harvest may have on berry split. These treatments were further subdivided to investigate the effect that B and Ca may have on berry split. For the B treatment, four Solubor1 bunch directed sprays were applied from 8mm berry size to véraison. The Ca treatment consisted of Stopit R 2 and Caltrac R 3 bunch directed sprays applied over the same period. In addition, a combination of the B and Ca treatment were applied to investigate any possible interaction. To account for the effect of water as solvent in the B and Ca treatments, and the spraying effect, pure water as treatment was also evaluated. Control vines received no sprays. The applied irrigation treatments resulted in different plant water status conditions. Separate applications of B and Ca treatments resulted in a decrease of B and Ca content in the flesh respectively. The control and combination treatment, of B and Ca resulted in the same of B and Ca content in the flesh. Furthermore, none of the applied treatments resulted in an increase of either B or Ca content in the berry skin. It was found that the medium frequency irrigation resulted in the best irrigation strategy to prevent pre-harvest berry split. Surprisingly, all the subtreatments: B, Ca, and combination of B and Ca, resulted in an increased incidence of pre-harvest berry split when compared to the control group for the 2006/07 season. However, in the 2007/08 season only the B treatment resulted in an increase of pre-harvest berry split. Concerning post-cold-storage physiological disorders, Ca treatments appear to have reduced berry drop, but increased decay. In the 2006/07 season, the B treatment resulted in reduced post-cold-storage berry split, whereas Btreatment in the 2007/08 season had no effect. Both B and Ca should be considered as having the potential to increase the appearance of hairline cracking. Calcium treatment also led to an increase in decay which may have been as a result of the splitting it contributed to. Low frequency irrigation recieving irrigation before harvest was found to result in browner stems. Low irrigation frequencies decreased the cell size of the berry skin. The Ca treatment gave rise to thicker (weaker) cell walls, this morphological change may be responsible for the physiological disorders it caused. From these findings, it can be deduced that poorly managed irrigation, together with unnecessary application of B and/or Ca may result in an increase of berry split and other physiological disorders, with subsequent financial losses for the producer.
Effect of post-harvest summer pruning on carbohydrate reserve status, bud break and fertility of Sultanina H5 in the Lower Orange River region.
(Stellenbosch : Stellenbosch University, 2022-12) Toolo, Keboneilwe Boitumelo; Avenant, Eunice; Avenant, J. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
ENGLISH ABSTRACT: Carbohydrate reserves produced from photosynthesis are stored in perennial tissues of the grapevine in the form of starch and free-sugar fractions or soluble sugars, mostly sucrose, glucose, and fructose. These reserves are highly affected by viticultural practices altering the source-sink relationship in the grapevine. Post-harvest pruning is a practice widely applied by several table grape producers in South Africa aiming to channel carbohydrate reserve accumulation to the remaining shoots. Due to the high input costs of table grape production, any manipulation, including post-harvest summer pruning, should be applied only if it is scientifically proven to have practical and economic benefits. This study, comprising of two trials, aimed to determine whether post-harvest pruning results in increased carbohydrate reserve status, improved bud break and fertility, as well as to establish a base for quantifying and practically assessing the carbohydrate reserve status of grapevines. The first trial focused on establishing the seasonal dynamics of non-structural carbohydrate (NSC) reserves of Vitis vinifera L. cv. Sultanina H5 in the semi-arid Lower Orange River (LOR) and the Mediterranean Hex River Valley (HRV). Root, trunk, cane and/or shoot tissues were sampled monthly and analysed for NSC. The Anthrone method was used to analyse soluble sugars and starch, while enzymatic analysis was used to quantify specific sugars (sucrose, d-fructose and d- glucose). Starch and sucrose were the most abundant forms of NSC in all tissues in both regions. In both regions, soluble sugars in permanent tissues (roots, trunks, canes) reached their highest concentration during dormancy (June-July). The starch concentration was low in all tissues in winter (July), during grapevine dormancy, whereafter it increased to a peak occurring in August (before bud break). A steep decrease in starch concentration was recorded from dormancy to flowering in both regions, indicating a dependency of the vine on carbohydrate reserves during that period. Accumulation of NSC reserves began after flowering to the post-harvest period, reaching their second peaks in autumn. The overall higher soluble sugars and starch (roots and canes) concentrations in the tissues of the Mediterranean region is ascribed to the earlier accumulation of reserves, lower crop load and a shorter post-harvest period characteristic of this region. A basis was established for sampling grapevine tissues for qualitative assessment of grapevine NSC reserve status, linking sampling time to occurrence of peaks in soluble sugars and starch concentrations. It is recommended that sampling for qualitative assessment of soluble sugars should be done after leaf fall, during dormancy (June-July under the conditions of this study). Starch concentrations should be assessed before bud break (August under the conditions of this study). Based on significant positive correlations between NSC concentrations of different tissue types, tissue types that could be sampled for indication of the overall NSC status of the grapevine were identified. Canes and shoots could be sampled and analysed to indicate the overall NSC reserve status of the vine. These tissues are considered the most practical tissue types to sample for determination of grapevine NSC reserve status. The second trial investigated the effect of post-harvest summer pruning applied in the semi-arid Lower Orange River region on NSC reserve status, bud break and fertility of Sultanina H5 grapevines. Five post-harvest pruning treatments were applied, namely an early 33% and a 66% shoot removal pruning treatment one day after harvest (33_1dAH and 66_1dAH respectively), a late 33% and a 66% shoot removal pruning treatment 45 days after harvest (33_45dAH and 66_45dAH respectively) and a control (Ctr), in which no post-harvest summer pruning was applied. To quantify pruning severity, the number and length of removed shoots, as well as the number of leaves and leaf area removed were determined at the time of the post-harvest summer pruning treatment application. After winter pruning was applied, the removed canes and shoots were measured to calculate the overall shoot length and leaf area removed per vine. The day after pruning treatments were applied, photosynthetically active radiation (PAR), as well as photosynthetic activity and related physiological parameters were measured. Cane and/or shoot, stem and root tissue, were sampled on 4 dates for assessing the effect of pruning treatments on NSC reserve status. Bud break and fertility were assessed through forced bud break and bud dissection for potential bud break and fertility, while actual bud break and fertility were assessed in the vineyard. Post-harvest pruning proved to be beneficial for light penetration, but it did not improve the photosynthetic rate of the leaves. A few significant differences were recorded on the impact of the treatments on TNC. These however, do not show a clear trend. Post-harvest summer pruning did not have a significant effect on final bud break and potential fertility of grapevines in the season following the treatment. Based on this one season’s results, post-harvest pruning did not have overall practical benefits. Repeating the treatments for two more seasons on the same data vines, would indicate whether there is a carry-over effect of the practice on NSC, bud break and fertility. It is recommended that in a further phase of this project, available rapid and accurate methods to quantify carbohydrate reserves should be used and/or evaluated for use in grapevine studies, including Near-Infrared spectroscopy, as well as the starch iodine test (already commercially used in the apple and forestry industries).
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.
Manipulation of the taste of Regal Seedless (Vitis vinifera L.) table grapes
(Stellenbosch : University of Stellenbosch, 2007-03) Fraser, W. J.; Huysamer, M.; Oberholster, A.; Avenant, J. H.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology.
Regal Seedless is a white, seedless grape which has the potential to become a profitable cultivar for the table grape producer since it has the advantages of early season harvesting and inherently large berries. There is, however, a downside to this cultivar, namely the seasonal occurrence of an unacceptable, astringent taste. This negative taste affects the demand by local and international markets. The astringency perception is due to the presence of phenolic compounds. It is well known that the phenolic composition and concentration change during the ripening of the grape. Different postharvest treatments are applied to fresh fruit like persimmons to remove astringency. These treatments include the use of carbon dioxide, nitrogen and ethanol. The aim of this study was to determine the optimum maturity level for Regal Seedless where the phenolic concentration is the lowest and the astringent taste acceptable. The use of postharvest treatments to manipulate the taste and the phenolic content, were also investigated. The effect on other quality parameters like total soluble solids (TSS), pH and total titratable acidity (TTA) were also evaluated.

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