Browsing Department of Horticulture by browse.metadata.advisor "Cronje, P. J. R."
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- ItemCreasing studies in citrus(Stellenbosch : University of Stellenbosch, 2010-03) Phiri, Zanele Penelope; Verreynne, J. S.; Cronje, P. J. R.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: Creasing, also known as albedo breakdown, is a preharvest disorder that affects the albedo of citrus fruit causing creases on the surface of the fruit. It is a recurrent problem in Navel and Valencia oranges and can cause individual orchard losses which often exceed 50%. Although the contributing factors are known, the physiological basis of creasing development is unresolved and the current control measures do not prevent creasing satisfactory. Hence, better control measures and further understanding of the physiology of creasing development is required. The objective of this two-year study was to determine if the position of fruit in a tree, light and carbohydrate manipulation techniques, and albedo mineral nutrients influence creasing development. Furthermore, the most effective application timing of gibberellic acid (GA3) with the least negative effect on fruit rind colour development and the effectiveness of cytokinins, other products and different root biostimulants to reduce creasing incidence were evaluated. The position of fruit in the tree and light influenced the development of creasing and the distribution of mineral nutrients in the albedo. Creasing incidence was higher on the south side than on the north side of the tree and fruit from the inside sub-sectors had a greater creasing incidence compared to fruit from the outside sub-sectors. The shady part of outside fruit was more creased compared to the sunny part of the fruit and covering fruit with brown paper bags increased creasing severity. The light manipulation techniques used on the leaves and fruit increased the nitrogen (N), phosphorus (P), potassium (K) and manganese (Mn) concentrations in the albedo and differences in the albedo mineral nutrients amongst the sub-sectors evaluated were observed, but creasing severity or creasing incidence was not significantly correlated with the albedo mineral concentrations at harvest. Albedo mineral concentrations earlier in the season may play a role in creasing development, as creasing severity was significantly correlated with copper (Cu), K, and Mn concentrations in the albedo during stage II of fruit development. Creasing incidence and albedo mineral concentrations were not affected by any of the carbohydrate manipulation techniques used in this study. The incidence and severity of creasing was significantly reduced, with a minor negative effect on fruit rind colour development, by the application of GA3, from mid November to mid January. Localised fruit application of CPPU [N-(2-chloro-4-pyridyl)-N-phenylurea], MaxCel (6- Benzyladenine) and CPPU in combination with calcium after physiological fruit drop reduced the incidence and severity of creasing, although creasing incidence was not significantly different from the control. The application of Messenger®, AVG (aminoethoxyvinylglycine) and different root biostimulants did not reduce creasing incidence. The results showed that cytokinins could reduce creasing incidence and justify further studies on application and uptake efficiency. The use of different root biostimulants are not recommended, but it is suggested that treatment effects may be more pronounced over a longer period.
- ItemCritical factors concomitant to the physiological development of alternate bearing in citrus (Citrus spp.)(Stellenbosch : Stellenbosch University, 2018-12) Stander, Ockert Petrus Jacobus; Cronje, P. J. R.; Barry, G. H.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: The significance of carbohydrates, mineral nutrients and phyto-hormones was investigated in relation to their possible roles in selected phenological events in alternate bearing ‘Nadorcott’ mandarin (C. reticultata Blanco) trees. Crop load in ‘Nadorcott’ mandarin trees was influenced by flowering intensity. The most important determinants of flowering intensity were the amount of new vegetative shoot growth and resulting number of new potential floral buds that developed during summer, and the influence of fruit on floral bud development during winter. The lack of development of summer vegetative shoots in “on” trees was not related to leaf carbohydrate concentration. In “off” trees, root sugar concentration peaked during full bloom and high root growth activity was observed prior to the vegetative shoot flush in summer. In “on” trees, fruit were the major carbohydrate sinks and probably disturbed the balance between vegetative shoot development and root growth. Sugar concentration in roots in “on” trees was 3-fold lower, root growth was absent, and shoot growth was halved. The concentration of mineral nutrients in leaves was a response to fruit load and not related to parameters of flowering or vegetative shoot growth. Measurements of phyto-hormones in leaves and roots confirmed that the inhibition of summer vegetative shoots was related to a high concentration of 1 H-indole-3-acetic acid (IAA) in leaves. High concentrations of dihydrophaseic acid and the abscisic acid (ABA) glucose ester suggested that IAA might have acted synergistically with ABA to create a growth inhibition in fruiting shoots. As a result, cytokinins did not contribute to the development of new summer vegetative shoots. High gibberellin concentration in leaves in May and June contributed to limited flowering in “on” trees. Consistent with this interpretation, treatment of “off” trees with 40 mg·L-1 gibberellic acid inhibited flowering, whereas soil and foliar treatments of “on” trees with 1000 mg·L-1 paclobutrazol or uniconazole, gibberellin biosynthesis inhibitors, increased flowering and resulted in fruit development from buds of “on” shoots.
- ItemEffect of permanent shade netting on ‘Nadorcott’ mandarin tree phenology and productivity(Stellenbosch : Stellenbosch University, 2018-03) Brown, Robert; Cronje, P. J. R.; Stander, Ockert P. J.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: Permanent shade netting in citrus (Citrus spp.) is implemented to protect high-value fruit and trees from damaging natural elements. However, the use of the technology accompanies inevitable changes in orchard microclimate that impacts on the physiology and phenology of a citrus tree. In this study a 20% white permanent shade netting treatment was evaluated for its effects on citrus tree phenology, its impact on the efficacy of chemical fruit thinning agents, and the long-term profitability of the technology in a young ‘Nadorcott’ mandarin (C. reticulata Blanco) orchard. Shade netting did not enhance the growth of individual vegetative shoots but did increase tree volume over time. In general, flowering was not affected by the shade net treatment, but during the second season, flowering intensity on summer vegetative shoots was higher in the shade net treatment. Fruit set, fruit yield and fruit internal quality were not affected by the shade net treatment, but fruit diameter was increased in the second season. Shade netting did not influence the ability of uniconazole soil-drench treatment to reduce vegetative growth. The shade net treatment did not influence the efficacy of synthetic auxin fruit thinning agents to thin fruit. The synthetic auxin fruit thinning treatments increased the concentration of selected mineral elements in fruit, and treatments resulted in a shift in fruit size distribution, with higher number of large, premium-sized fruit per tree. The effect on fruit size distribution was more pronounced in the shade net treatment. Apart from fruit size, a combination of shade netting and chemical fruit thinning treatments had no effects on other important fruit quality attributes. From the budget model generated in this study, it can be concluded that 20% white permanent shade netting resulted in increased orchard profitability, despite a high establishment cost and increase in production costs. It can therefore be concluded that under typical Mediterranean-type production conditions, 20% white permanent shade netting increased the productivity and profitability of a ‘Nadorcott’ mandarin orchard. The use of the technology can be recommended in areas that experience extensive yield losses due to climatic conditions and possibly also permit citrus production in non-traditional areas.
- ItemFruit split and fruit size studies on Citrus(Stellenbosch : Stellenbosch University, 2013-03) Stander, Ockert Petrus Jacobus; Cronje, P. J. R.; Theron, K. I.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.ENGLISH ABSTRACT: Fruit size and the integrity of the rind are key components that determine the value of a citrus fruit. The application of 2,4-dichlorophenoxy acetic acid (2,4-D) to reduce splitting, a physiological disorder which entails cracking of the rind as well as to increase fruit size was conducted on three different split-susceptible mandarin and two split-susceptible orange cultivars. Treatments were applied directly after the physiological fruit drop period, as well as in January and February at 10 mg·L-1, alone or in combination with calcium (Ca), potassium (K) or gibberellic acid (GA3). Application of 2,4-D directly after physiological fruit drop, either alone or in a tank-mix with K, consistently reduced the number of split mandarin fruit, with later applications in January and February generally being ineffective. Post physiological fruit drop application of 10 mg·L-1 2,4-D significantly increased growth rate (mm.day-1) of all the mandarin cultivars, resulting in increased fruit size. Differences in sensitivity of cultivars to 2,4-D were evident, with the January application reducing the splitting in ‘Midknight’ Valencia. However, all the 2,4-D treatments reduced the fruit growth rate of the orange cultivars. The 2,4-D treatments, in terms of splitting, increased rind thickness, -strength and -coarseness of ‘Marisol’ Clementine, throughout fruit development. In addition fruit diameter and –length increased to such an extent that the fruit shape was altered (reduced d/l-ratio), reducing the potential of the rind to crack and the fruit to split, however rind coarseness of treated fruit was also increased. There were no major negative side effects on internal and external fruit quality, except for a possible reduction in juice content (%). Therefore, 10 mg·L-1 2,4-D can be applied directly after physiological fruit drop on ‘Marisol’ Clementine and ‘Mor’ mandarin to reduce fruit splitting.
- ItemImpact of shade netting on internal and external quality of ‘Nadorcott’ mandarin fruit(Stellenbosch : Stellenbosch University, 2018-12) Botes, Johane; Cronje, P. J. R.; Hoffman, L. (Lynn) (Horticulturalist); Zacarias, Lorenzo; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: In the export-focused citrus industry of southern Africa, the production of high quality fruit i.e. good size, well-developed rind color, blemish-free fruit and good taste is of utmost importance to remain sustainable. Shade nets are a preharvest technique implemented to protect crops against excessive sunlight, wind and hail damage. Shade nets are effective in reducing sunburn, but inconsistent results from previous studies arise about fruit size, rind color and internal quality. Furthermore, the impact on postharvest fruit quality with regard to developing rind physiological disorders and to maintain the physical integrity of the rind as a protection layer against moisture loss and decay development, is not known. Three experiments were conducted to determine how 20% white permanent shade netting would influence, firstly the fruit quality development, secondly the postharvest behavior and lastly the rind physical properties of ‘Nadorcott’ mandarin fruit produced in Citrusdal, over two seasons (2016 and 2017). In the first experiment, results indicated that the fruit size, rind color and internal quality development patterns were similar for both shade-net exposed and control fruit. Shade netting, however, resulted in an increased fruit size over the 2017 development period, but with no influence on rind color or maturity. Sunburn incidence was effectively reduced by shade nets. During cold storage at 4 and -0.6 °C over a period of 34 days, storage duration did not influence the postharvest quality of shade net fruit differently compared to control fruit in terms of rind color, internal quality parameters and fruit weight loss. In addition, no negative effect of shade netting was evident for the above-mentioned parameters or the incidence of staining. In the third experiment inconsistency occurred with regards to the effect of shade netting on fruit rind strength at harvest. A higher firmness was recorded for shaded fruit in the first season. However, during cold storage, there was some indication that the shade net fruit was more susceptible to deformation and required a lower force over the whole storage duration to puncture the rind, compared to the control fruit. However, a lower force required may also be beneficial as it could be indicative that the fruit may be easier to peel. The firmness of shade net-produced fruit was differently influenced by cold storage in 2016, within the first 14 days of storage, compared to the control. In 2017, control fruit were recorded to have a higher firmness over the storage duration. Both results indicated that the control fruit possibly stored better than shade-net fruit. To conclude, shade net was effective in reducing sunburn without negatively affecting any external and internal quality parameters. The postharvest storage potential of fruit from shade netting did not differ from the control at both storage regimes. Results regarding the impact of shade netting on the physical properties of the rind provides some first guideline threshold force values required before damage is inflicted on the fruit. Knowledge of typical forces applied during the commercial harvest- and pack house processes is, however, required before these values can be compared to commercial practices to determine its importance. The use of shade-netting shows potential as a preventative technology ensuring high quality, unblemished fruit, but requires future studies taking into account the effect of various cultivars, tree age, bearing positions and the microclimatic effect on fruit production and postharvest storage behavior.
- ItemThe impact of shade netting on the microclimate of a citrus orchard and the tree's physiology(Stellenbosch : Stellenbosch University, 2018-12) Prins, Michael du Toit; Cronje, P. J. R.; Graham, G. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.ENGLISH ABSTRACT: This study aimed to quantify the change in microclimate under shade netting and its effect on the leaf physiology of ‘Nadorcott’ mandarin (Citrus reticulata Blanco) in a winter rainfall production area. The first experiment quantified the change in orchard microclimate. The reduction in solar radiation affected the ambient, soil temperature and the number of hours for specific physiological and phenological temperature ranges under the shade netting. A reduced wind speed under the netting potentially led to less removal of moisture from the air and slightly increased relative humidity leading to a decrease in vapour pressure deficit (VPD) of the air. Therefore, the atmospheric evaporative demand was reduced and increased the volumetric soil water content. The second experiment focused on how the changes in microclimate affected a citrus leaf’s physiology. A reduction in VPD, especially in summer, led to increased stomatal conductance and resulted in increased CO2 assimilation rates of leaves under the shade netting. Therefore, the shade netting did not influence the leaf physiology negatively. Thirdly the total carbohydrate assimilation and distribution of different carbohydrate components, i.e., reducing sugars, polysaccharides, and starch, in leaves and roots were investigated as glucose equivalents. The most notable change under the shade netting in carbohydrate levels was in leaves, with an increased starch content especially noted after harvest. Root carbohydrates showed some differences between treatments, however, these could not only be attributed to the shade net treatment. The fourth experiment investigated how the change in microclimate under shade netting influenced the tree water status or tree water potential as well as fruit and canopy growth. This was accomplished by conducting pre-dawn pressure chamber readings of the main treatments as well as additional irrigation treatments. An increased tree water potential for trees under the shade netting was recorded as well as increased tree canopy volume and final fruit size. It can, therefore, be concluded that 20% white shade netting altered the orchard microclimate without negatively affecting the leaf physiology associated with photosynthesis in ‘Nadorcott’ mandarin trees. Leaves under the shade netting favoured the production of storage carbohydrates and the reduction in solar radiation and increased soil water content that led to a less water stressed environment, increasing the tree water potential. Further research should include how the microclimate can be altered in citrus production areas with different climatic conditions in South Africa.
- ItemInfluence of rind water content on mandarin citrus fruit quality(Stellenbosch : Stellenbosch University, 2016-03) Joubert, Jeanine; Cronje, P. J. R.; Hoffman, L. (Lynn) (Horticulturalist); Zacarias, L.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: ‘Nules Clementine’ and ‘Nadorcott’ mandarin are commercially important Citrus cultivars in South Africa. Both cultivars are prone to develop rind breakdown and pitting which are considered non-chilling related postharvest physiological rind disorders. The progressive and erratic nature of these rind disorders result in high financial losses. The incidence of a rind disorder is thought to be associated firstly with an increased susceptibility, as influenced by pre-harvest aspects; and secondly with a trigger in the postharvest environment. A study was conducted over two seasons a study to determine the effect of late nitrogen (stage II and after summer flush) application, pre-harvest water stress and postharvest handling were conducted. The rind quality of ‘Nules Clementine’ and ‘Nadorcott’ mandarin fruit harvested from Citrusdal and Riebeeck Kasteel was evaluated. Soil applications of nitrogen at 20 kg·ha-1 and 40 kg·ha-1 were done on 21 January and 26 March 2014/2015, respectively. This was in addition to the standard 300 kg·ha-1 nitrogen provided by the producer. During 2015 a 1% urea foliar application was sprayed on 26 March. During the post-harvest period all fruit were dehydrated at 25 °C and 60 to 80% RH (0.7 to 1.1 kPa vapour pressure deficit) for two days, followed by rehydration at 100% RH for one day. Subsequently fruit were stored at either -0.6 °C or 4 °C for a 30-day period. There were no significant differences in fruit colour or size between the different nitrogen treatments. No increase in rind disorders and no negative impacts on internal fruit quality were noted. To determine the impact of pre-harvest water stress, the soil below the tree was covered with plastic sheets three weeks prior to harvest to exclude rainfall or irrigation. The effect of postharvest stress was established by dehydrating and rehydrating fruit at 0.7-1.1 kPa vapour pressure deficit for different periods after harvest. Wax was applied on day 5 and thereafter fruit was stored at 4 °C for 30 days. The results indicated that pre-harvest water stress did not have a detrimental effect on fruit susceptibility to disorders. By early wax application, however, a decrease in moisture loss was recorded, coinciding with lower incidences of rind disorders. The final part of the study was aimed at determining if postharvest handling lending to water loss, as induced by high vapour pressure, could increase disorders. From the results of the trials it can be concluded that exposing fruit to dehydration increased rind disorder susceptibility. Dehydration prior to wax application on day 5 also increased pitting and rind breakdown, whereas an early wax application, 2 to 3 days after harvest, reduced incidence. This study serves as a step to resolve the impact of factors predisposing the citrus fruit rind to progressive postharvest disorders.
- ItemNon-destructive prediction and monitoring of postharvest quality of citrus fruit(Stellenbosch : Stellenbosch University, 2013-12) Magwaza, Lembe Samukelo; Opara, U. L.; Terry, L. A.; Cronje, P. J. R.; Nieuwoudt, Helene; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticultural Science.ENGLISH ABSTRACT: The aim of this study was to develop non-destructive methods to predict external and internal quality of citrus fruit. A critical review of the literature identified presymptomatic biochemical markers associated with non-chilling rind physiological disorders. The prospects for the use of visible to near infrared spectroscopy (Vis/NIRS) as non-destructive technology to sort affected fruit were also reviewed. Initial studies were conducted to determine the optimum condition for NIRS measurements and to evaluate the accuracy of this technique and associated chemometric analysis. It was found that the emission head spectroscopy in diffuse reflectance mode could predict fruit mass, colour index, total soluble solids, and vitamin C with high accuracy. Vis/NIRS was used to predict postharvest rind physico-chemical properties related to rind quality and susceptibility of ‘Nules Clementine’ to RBD. Partial least squares (PLS) statistics demonstrated that rind colour index, dry matter (DM) content, total carbohydrates, and water loss were predicted accurately. Chemometric analysis showed that optimal PLS model performances for DM, sucrose, glucose, and fructose were obtained using models based on multiple scatter correction (MSC) spectral pre-processing. The critical step in evaluating the feasibility of Vis/NIRS was to test the robustness of the calibration models across orchards from four growing regions in South Africa over two seasons. Studies on the effects of microclimatic conditions predisposing fruit to RBD showed that fruit inside the canopy, especially artificially bagged fruit, had lower DM, higher mass loss, and were more susceptible to RBD. The study suggested that variations in microclimatic conditions between seasons, as well as within the tree canopy, affect the biochemical profile of the rind, which in turn influences fruit response to postharvest stresses associated with senescence and susceptibility to RBD. Principal component analysis (PCA) and PLS discriminant analysis (PLS-DA) models were applied to distinguish between fruit from respectively, inside and outside tree canopy, using Vis/NIRS signal, suggesting the possibility of using this technology to discriminate between fruit based on their susceptibility to RBD. Results from the application of optical coherence tomography (OCT), a novel non-destructive technology for imaging histological changes in biological tissues, showed promise as a potential technique for immediate, real-time acquisition of images of rind anatomical features of citrus fruit. The study also demonstrated the potential of Vis/NIRS as a non-destructive tool for sorting citrus fruit based on external and internal quality.
- ItemPost-harvest rind pitting studies on ‘Valencia’ orange(Stellenbosch : Stellenbosch University, 2016-03) Ehlers, Jacques Louis; Cronje, P. J. R.; Theron, K. I.; Alferez, F.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: Post-harvest rind pitting is a non-chilling related physiological rind disorder that affects various citrus cultivars and reduces fruit value. This disorder is characterised by the collapse of the flavedo sub-epidermal cells, however the main cause of this disorder is unknown, but it is aggravated by changes in relative humidity (RH) and rind water status. Studies were conducted on ‘Turkey’ and more susceptible ‘Benny’ valencia oranges in Limpopo and Mpumalanga South Africa. The effect of fruit position, maturity and size on fruit susceptibility to this disorder was investigated and it was found that fruit from the outside of the canopy are more susceptible to this disorder probably due to greater exposure to variation in environmental conditions than fruit from the inside of the canopy. More mature fruit were also found to be slightly more susceptible, however size did not influence incidence of this disorder. Various plant growth regulators were also evaluated to prevent pitting. The application of the synthetic auxins 2,4-dichlorophenoxy acetic acid (2,4-D) and 3,5,6 trichloro-2-pyridiloxyacetic acid (3,5,6 TPA) at 50 % petal drop (2,4-D) or after physiological fruit drop (2,4-D or 3,5,6-TPA) reduced the incidence of post-harvest pitting. Application of s-abscisic acid 1 week before harvest was also found to reduce incidence of this disorder, however gibberellic acid applied in January did not reduce the incidence of post-harvest rind pitting. A systemic fungicide thiabendazole (TBZ) which reduces the incidence of chilling injury also reduced post-harvest pitting incidence if applied before fruit were subjected to stress inducing environmental conditions. Pre-harvest foliar application of TBZ 1 week before harvest and post-harvest dip treatments directly after harvest reduced post-harvest weight loss and incidence of this disorder. A citrus industry survey was conducted to estimate the financial impact of this disorder at foreign and local markets on producers. Markets generating higher prices had a lower tolerance for incidence of post-harvest rind pitting than lower priced markets and are therefore seen as high-risk. Due to this large reduction in market price for fruit with the disorder, treatments found during this study might be cost effective.
- ItemStudies on the phenology and carbohydrate status of alternate bearing ‘Nadorcott’ mandarin trees(Stellenbosch : Stellenbosch University, 2012-12) Van der Merwe, Izak Schalk; Theron, K. I.; Cronje, P. J. R.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.ENGLISH ABSTRACT: Alternate bearing is a common phenomenon in most commercial perennial fruit trees. In citrus, the “on” year consists of a heavy crop load with mostly small fruit, often followed by an “off” year with few, large and coarse fruit. Carbohydrates play an important role in affecting alternate bearing, especially during fruit set, but also flowering and fruit maturation, and are essential in maintaining a regular bearing habit. Changes in starch and total sugar accumulation in the leaves of the alternate bearing prone „Nadorcott‟ mandarin were followed over an entire season for both “on” and “off” trees to evaluate the possibility of using carbohydrate levels to predict bearing potential. Starch accumulation followed a distinct pattern with differences between “on” and “off” trees visible in April and May. Starch concentrations in April showed a moderate negative correlation with yield and a moderate positive correlation with return bloom. Rapid starch accumulation started prior to harvest with a peak at the beginning of flowering. Thereafter a sharp decrease in starch levels occurred until after full bloom followed by a steady decrease from physiological fruit drop towards fruit maturity. “On” trees bore 53% more fruit than “off” trees, but the return bloom of “off” trees was 140% more than “on” trees, thus illustrating the negative effect that a large crop has on the next season‟s bloom. It was concluded that for „Nadorcott‟ mandarin, leaf starch concentration in April can be used as an indication of bearing potential the following season. Pruning is a well-established management tool to control alternate bearing. Summer pruned trees had more spring flush vegetative shoots, more nodes per shoot and also more growth per parent shoot overall, compared to unpruned, control trees. Control trees had higher light levels inside the tree compared to summer pruned trees. However, no differences in leaf starch or total sugar levels during April were measured between treatments. Production of new bearing sites should therefore be considered in this experiment. It was concluded that pruning during November followed by early regrowth management gave the best balance between light penetration and production of new bearing units. Pruning in November, rather than during winter, also allowed selective pruning of shoots with or without flowers, depending on whether it was an “on” or an “off” year. When fruit thinning chemicals are applied at the optimum time and concentration, it is an effective way of moderating an alternate bearing cycle. Unfortunately no significant differences were obtained in this experiment even though the thinning treatments did show slightly higher starch levels in April 2012, indicating that the demand for energy was lower in these trees. This response was most likely due to the slightly lower yield and fruit number of the thinning treatments compared to the control. The dichlorprop treatment also showed a higher fruit growth rate, and future research should focus on timing of chemical thinning sprays in late mandarin cultivars
- ItemStudies to reduce the incidence of chilling injury in navel orange fruit(Stellenbosch : Stellenbosch University, 2013-03) Hordijk, Jeanine; Cronje, P. J. R.; Opara, U. L.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.ENGLISH ABSTRACT: Citrus fruit exported from South Africa to markets such as the USA and China undergo a mandatory 24 day exposure of -0.6°C during shipment to kill any insect larvae in the fruit, however, this protocol causes chilling injury (CI). The aim of this study was firstly to determine the influence of various preharvest factors on chilling sensitivity. In addition, Near-Infrared (NIR) spectroscopy was tested as a potential management tool to identify variation in CI susceptibility of fruit and lastly the efficacy of thiabendazole (TBZ) applied in the packline to reduce CI was determined. Various factors influence the susceptibility of a navel orange fruit to CI including cultivar, micro-climate, harvest date, fruit size and rind colour. In this study it was found that ‘Washington’ was more susceptible to CI compared to ‘Navelina’ navel orange. Fruit from the coldest part of Citrusdal (Tharakama) had the highest incidence of CI, which concurred with literature. The incidence of CI was overall less when fruit were harvested in the middle of the commercial harvest window; however, the internal maturity at harvest does not appear to be related to the sensitivity of orange fruit to CI. Near infrared (NIR) spectroscopy was tested as a potential tool to predict fruit quality parameters in relation to CI. Analysing the NIR data with principal components analysis (PCA), score plots were obtained that separate fruit in clusters from the inside and outside of the canopy positions as well as different sizes and rind colours (green vs. orange). However, analysing data with partial least square regression (PLS) using fruit quality parameters (firmness, rind colour and mass), the NIR spectra obtained with the integrated sphere did not provide a good prediction model for CI index. Thiabendazole (TBZ) is reported to reduce the incidence of CI of citrus fruit and this fungicide was applied in a semi-commercial packline in the wax as well as the drench. The results of the application of different fungicides from the TBZ chemical group indicated that the TBZ dip treatments had the highest efficacy in reducing both the incidence and severity of CI and in addition were more effective when applied in warm (40°C) than cold water (10°C). Applications at the commercial recommended rate (20mL.L-1 and half of the commercial recommended rate were both effective in reducing the incidence of CI. Wax application was effective in reducing the incidence of CI however, the application of TBZ in the wax reduced the incidence of CI even more. For the successful reduction of CI incidence in commercial shipments of citrus fruit the focus should not be on a single factor but rather a strategy that encompasses pre-harvest factors that would influence rind quality as well as specific postharvest technologies know to decrease the impact of CI.
- ItemStudies to reduce the size of the navel-end opening of navel oranges(Stellenbosch : University of Stellenbosch, 2010-03) Mupambi, Giverson; Verreynne, J. S.; Cronje, P. J. R.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Horticulture.ENGLISH ABSTRACT: The size of the navel-end opening is an important parameter for external fruit quality in navel oranges [Citrus sinensis (L.) Osbeck]. The application of 2,4-dichlorophenoxyacetic acid (2,4-D) to increase the percentage of closed navel-ends and reduce the size of the navel-end opening was conducted on six different navel orange cultivars. Treatments were applied at full bloom (FB), 100% petal drop (PD), as well as 2 weeks (2 WAPD) and 4 weeks after 100% petal drop (4 WAPD), at 15 mg·L-1 to 45 mg·L-1, to determine the most effective timing and concentration. The application of 2,4-D at FB consistently decreased the average navel-end size (all fruit) and increased the percentage of closed navel-ends in all the cultivars, with later applications at PD, 2 WAPD and 4 WAPD being generally ineffective, regardless of the concentration applied. There were no major negative side effects on internal and external fruit quality, except for the reduction in juice content (%), especially with the later treatments. Therefore, 15 mg·L-1 2,4-D at FB can be applied to increase the percentage of closed navel-ends and possibly increase export packouts. Navel oranges have a small secondary fruit located inside the primary fruit at the stylar-end and an opening at the stylar-end called the navel-end opening or the stylar-end aperture. Fruit growth and development was studied in three navel orange cultivars by measuring the primary fruit diameter, the secondary fruit diameter and the navel-end opening fortnightly, using both destructive and non-destructive sampling methods. The relationships between the primary fruit size, the secondary fruit size and the navel-end opening size were studied using correlation analysis. In addition, the effect of 2,4-D on fruit morphology, when applied as a treatment to reduce the size of the navel-end opening, was also evaluated on the same cultivars. The primary fruit, the secondary fruit and the navel-end opening followed a similar developmental pattern, although the navel-end opening developed later, about six weeks after FB. The primary fruit size was not related to the size of the secondary fruit or the navel-end opening. Similarly, the size of the navel-end opening was not related to the size of the secondary fruit. No negative effects were noted on the primary fruit morphology when 2,4-D was applied. Fruit splitting is a major physiological disorder of ‘Marisol’ Clementine mandarin (Citrus reticulata) fruit. The effect of application of 2,4-D on fruit splitting and fruit quality was evaluated on ‘Marisol’ Clementine mandarin trees grafted on Troyer citrange rootstock. Treatments included an untreated control, 2,4-D applied at 15 mg·L-1 or 25 mg·L-1 at FB and 15 mg·L-1 or 25 mg·L-1 at PD. The application of 2,4-D reduced fruit splitting in ‘Marisol’ Clementine fruit. Internal fruit quality was not affected by the treatments, however, the fruit developed a coarse rind due to enlarged oil glands and the styles stayed attached on the fruit until harvest. Therefore, although 2,4-D reduced fruit splitting, it cannot be recommended at the timings and concentrations evaluated.