Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Thinning of stone fruit, just as in any other deciduous fruit crop, plays an important role in producing fruit of the right size and quality. Hand thinning is highly labor intensive and time consuming, thus an alternative method of thinning is important to the industry. Chemical and mechanical thinning either alone or in combination could be the alternative. Two chemicals, 1-aminocyclopropane-1-carboxylic acid (ACC) and 6-benzyladenine (6-BA) were evaluated on Japanese plums, cling peaches and nectarines. In addition, the Darwin 300™, a mechanical string thinner, was also included in trials on early maturing ‘Alpine’ nectarine and ‘African Rose™’ plum. In all trials the objective was to reduce the required hand thinning during commercial hand thinning without compromising on yield and fruit quality. In Japanese plums we were able to reduce the hand thinning requirement significantly with both the ACC thinning and mechanical thinning strategies. Regarding ACC, cultivars differed in their sensitivity to the chemical and the recommended rate will differ for cultivars. ACC consistently reduced the required hand thinning linearly with increasing rate. The recommended rate of ACC for ‘African Rose™’ is 600 μl.L-1 and for ‘Laetitia’ 400 μl.L-1. For ‘Fortune’ a recommended rate could not be determined at this stage, thus further trials should be conducted. The Darwin 300™ reduced hand thinning significantly without reducing the yield significantly. Combining the Darwin 300™ with ACC 600 μl.L-1 in ‘African Rose™’ gave promising results with regard to hand thinning requirement and fruit size, without reducing yield efficiency significantly. No leaf drop was observed on Japanese plums, except in the pilot trial when applications were made at high temperatures, which should therefore be avoided. ACC was effective as thinning agent in cling peaches. In ‘Keisie’, the results were positive during both seasons, and ACC reduced the hand thinning requirement without reducing yield efficiency. The recommended rate of ACC for ‘Keisie’ is 600 μl.L-1. Slight leaf drop was observed. In ‘Sandvliet’, there was a significant reduction in fruit set, without reducing the required hand thinning. The reduction in fruit set led to a significant reduction in yield. Severe leaf drop was observed, indicating that cultivars differ in sensitivity to ACC. ACC would not currently be recommended for ‘Sandvliet’. In nectarines, ACC only thinned ‘Turquoise’ but not ‘Alpine’ or ‘August Red’ at the rates and phenological stage used, again indicating cultivar differences in sensitivity. In ‘Turquoise’, the highest ACC rate (500 μl.L-1) reduced fruit set per tagged shoot, as well as the hand thinning requirement, but this rate also reduced the total yield. The Darwin 300™ evaluated on ‘Alpine’ reduced fruit set significantly and the hand thinning requirement without reducing yield efficiency, indicating that mechanical thinning is a viable option in nectarines. Slight leaf drop was observed in all nectarine trials and ACC would not currently be recommended for nectarines. 6-BA was included to combat ACC-induced leaf drop and was partially successful. The reason for the differences observed in response to ACC between cling peaches and plums on the one hand, and nectarines on the other, cannot currently be explained. / AFRIKAANSE OPSOMMING: Uitdun van steenvrugte, net soos vir enige ander sagtevrugte soort, speel 'n belangrike rol in die produksie van vrugte met die regte grootte en gehalte. Uitdun van steenvrugte is hoogs arbeidsintensief en tydrowend, dus is dit belangrik om ʼn alternatief te vind vir die bedryf. Chemiese of meganiese uitdunning alleen of in kombinasie kan die alternatiewe wees. Twee middels, 1-aminosiklopropaan-1-karboksielsuur (ACC) en 6-bensieladenien (6-BA) is geëvalueer op Japanese pruime, taaipitperskes en nektariens. Daarby is die Darwin 300™, ʼn meganiese uitdunmasjien, ingesluit vir twee vroeë kultivars, nl. Alpine nektarien en African Rose™ pruim. Die doel van die proewe was om handuitdunning tydens kommersiële handuitdun te verminder, sonder om die opbrengs en vrugkwaliteit negatief te beïnvloed. Vir Japanese pruime kon ons die nodige handuitdunning beduidend verminder met beide die ACC en meganiese uitdun strategieë. Daar was wel ʼn verskil tussen die kultivars se sensitiwiteit teenoor ACC en die aanbevole konsentrasie sal verskil tussen kultivars. ACC het die benodigde handuitdunning vir al drie kultivars lineêr verminder met ʼn toename in konsentrasie. Die aanbevole konsentrasie van ACC vir ‘African Rose ™’ is 600 μl.L-1 en vir ‘Laetitia’ 400 μl.L-1. Vir ‘Fortune’ kan daar nog nie op hierdie stadium 'n konsentrasie aanbeveling gemaak word nie. Die Darwin 300™ behandeling het die benodigde handuitdunning beduidend verminder sonder om die opbrengs te beïnvloed. Die kombinasie van die Darwin 300 ™ met ACC 600 μl.L-1 het ook goeie resultate opgelewer wat handuitdunning en vruggrootte aanbetref sonder om die opbrengsdoeltreffendheid te verlaag. Geen blaarval was opgemerk by die pruime nie, behalwe in ʼn voorlopige proef toe die ACC toegedien is by hoë temperature, wat dus vermy moet word. Die effektiwiteit van ACC as uitdunmiddel van taaipitperskes was belowend. Vir ‘Keisie’ was die resultate positief vir beide seisoene, en ACC het handuitdunning verminder sonder om die opbrengs te beïnvloed. Die aanbevole ACC konsentrasie vir ‘Keisie’ is 600 μl.L-1. Effense blaarval is wel waargeneem. Vir ‘Sandvliet’ was daar 'n beduidende vermindering in vrugset, sonder dat handuitdunning verminder is. Daar was ook 'n beduidende afname in opbrengs en erge blaarval in die proef waargeneem. ACC sal tans nie aanbeveel word vir 'Sandvliet’ nie. Met nektariens het ACC net ‘n uitduneffek op ‘Turquoise’ getoon, maar nie teen die aangewende dosisse en ontwikkelingstadium op ‘Alpine’ of ‘Augustus Red’ nie. Dit dui daarop dat ACC kultivarspesifiek mag wees. In ‘Turquoise’ het die hoogste konsentrasie (500 μl.L-1) vrugset van gemerkte lote en die handuitdunning verminder, maar ook die totale opbrengs. Die Darwin 300 ™ het die vrugset van ‘Alpine’ asook die benodigde handuitdunning aansienlik verminder sonder om die opbrengs te verlaag. Effense blaarval was opgemerk in alle nektarien proewe. ACC sal nie aanbeveel word as uitdunmiddel vir nektariens nie. 6-BA was in die studie ingesluit om ACC-geïnduseerde blaarval teen te werk en was slegs gedeeltelik suksesvol. Die rede vir die verskille in respons tot ACC tussen pruime, perskes en nektariens kan nie tans verklaar word nie.

Thesis (MScAgric)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Producing fruit of the appropriate size and high quality is of the upmost importance to realize a profit in the fruit industry. This can be achieved through bloom or fruitlet thinning to reduce the number of fruit left on the tree. The cost of production is rising and labour cost forms a large part of the total production cost. Thinning of stone fruit is labour intensive and expensive, so an alternative to hand thinning needs to be found. Two alternatives are chemical and mechanical thinning. Chemical thinners are not routinely used in stone fruit as it is in pome fruit production and gibberellins were evaluated in this study. The Darwin 300TM was evaluated as a mechanical alternative to hand thinning. It thins flowers during bloom, before fruitlet thinning by hand is performed. In our trials on nectarines and Japanese plums the objective of reducing the time required for hand thinning was achieved, with the Darwin 300TM reducing the time required by up to 50%. When the time required to thin was reduced too much it also reduced the yield, but this could be overcome by lowering the rotor speed or using different strategies during supplementary hand thinning at the fruitlet stage. The bloom thinning and reduction in yield led to an increase in the fruit size. Care should be taken when using the Darwin 300TM as the earlier thinning could increase pit splitting and/or fruit cracking, especially in cultivars that are sensitive to these defects. The optimal rate of thinning needs to be determined for each cultivar individually. The application of gibberellic acid (GA3) and gibberellin A4+7 (GA4+7) at the pit hardening stage in the previous season could decrease the number of flowers for the following growing season. There was no effect on the yield at harvest or fruit size in the season of GA3 and GA4+7 applications, but the fruit firmness was increased. This effect was more pronounced for the GA4+7 applications. Our objective of reducing the time required for thinning was achieved in some but not all cultivars. The yield was not significantly reduced, with the fruit maturity only delayed in ‘African Rose’ plum. Again no increase in fruit size was found, but the fruit firmness was again increased. The GA-applications therefore were not satisfactory in their reduction of the time required for hand thinning. A positive effect is the increase in fruit firmness, which could possibly increase the storage potential of the fruit without having negative effects on the other aspects of fruit quality but this needs further evaluation. / AFRIKAANSE OPSOMMING: Produksie van vrugte met die verlangde vruggrootte en hoë vrug kwaliteit is baie belangrik vir die realisering van ‘n wins in die vrugte-industrie. Met hierdie mikpunt in gedagte, is blom- en vruguitdunning baie belangrik om die aantal vrugte per boom te verminder. Die kostes geassosieer met vrugte produksie is besig om te styg en arbeidskoste vorm ‘n groot deel van die totale produksiekostes. Uitdunning van steenvrugte is arbeidsintensief en baie duur, dus moet ‘n alternatief vir handuitdunning gevind word. Daar is twee alternatiewe naamlik chemiese en meganiese uitdunning. Chemiese uitdunmiddels word algemeen in kernvrugproduksie gebruik, maar daar is tans geen chemiese middels vir steenvrugte nie. In hierdie studie was gibberelliene ge-evalueer as potensiële uitdunmiddel. Die Darwin 300TM is ge-evalueer gedurende blomtyd as ‘n meganiese alternatief vir handuitdunning. Die masjien verwyder blomme en verminder so die vruguitdunning benodig. In ons eksperimente op nektarien- en Japanese pruimkultivars het ons gevind dat die tyd benodig vir handuitdunning met tot 50% verminder is deur die Darwin 300TM. Dit het ook daartoe gelei dat die totale oes per boom verlaag is. Hierdie effek kan vermy word deur die rotor spoed te verminder of die strategie vir aanvullende handuitdunning aan te pas. Die feit dat die grootste deel van die uitdunproses in blomtyd uitgevoer is en ook die feit dat die totale oes per boom verlaag is, het daartoe gelei dat die vrugte groter was. Die vroeër uitdunning met die Darwin 300TM kan egter lei tot ‘n verhoging in vrugkrake en gesplete pitte. Dit moet veral in gedagte gehou word by kultivars wat geneig is tot hierdie afwykings/defekte. Die optimum tempo van uitdunning moet vir elke kultivar individueel bepaal word. Wanneer gibberelliensuur (GA3) of gibberelien A4+7 (GA4+7) by pitverharding toegedien word in die vorige groeiseisoen, kan dit lei tot die vermindering van die hoeveelheid vrugte in die volgende seisoen. Daar was geen effek op die totale oes per boom en die vruggrootte tydens oes in die seisoen van aanwending nie, maar die vrugfermheid is verhoog. Die effek was hoër na die GA4+7 as na die GA3 aanwending. Die mikpunt om die tyd benodig vir handuitdunning te verminder, is in sommige kultivars bereik. Die oes per boom in die opvolgseisoen is weer eens nie verlaag nie, maar die vrug rypheid van ‘African Rose’ pruime is vertraag. Geen effek is op die vruggrootte opgemerk nie, maar die vrugfermheid was weer eens verhoog. Die GA-toedienings het dus nie bevredigend die tyd benodig vir handuitdunning verminder nie. ‘n Positiewe effek is die verhoging van die vrugfermheid, wat moontlik kan lei tot die verhoging van die opbergingspotensiaal van die vrugte sonder enige ander negatiewe effekte, maar hierdie aspek benodig verdere navorsing.