Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Profitability in the export driven South African blueberry industry is dependent on early spring harvests. The George region in the Western Cape accumulates too few chill-units to release buds of some southern highbush (SHB) blueberry cultivars from endodormancy. This causes problems like delayed budbreak and extended harvests. Growers of other temperate fruit crops are also affected by these problems and chemical rest breaking agents (RBAs) are applied in orchards in an attempt to overcome these. Application of the RBA hydrogen cyanamide (HC) occurs commonly in commercial pome and stone fruit orchards while thidiazuron (TDZ), another RBA is applied on a limited scale in apple orchards. The effect of RBA application on berry ripening, berry size and yield in SHB cultivars Bluecrisp, Emerald and Star was investigated for two seasons in an orchard near George, in order to determine to what extent harvest scheduling with RBAs is possible. Following Dormex® (HC, 520 g L-1) application during 2010, when a warm winter was experienced, the berry ripening of ‘Bluecrisp’ was accelerated. Dormex® application before reproductive bud scales opened, but after some chilling, resulted in acceptable yield and berry size without damage to reproductive buds. A 1% rate gave similar results as a 2% rate, but at a lower risk of reproductive bud damage. Lift® (TDZ, 3 g L-1) application reduced the number of days to 75% harvest in ‘Star’ during 2010. Lower yielding plants produced larger berries than those from higher yielding plants. Lift® application after reproductive buds scales have opened caused malformed and damaged flowers. Delaying the initiation of reproductive buds could delay spring reproductive budbreak until after new leaves had formed. In turn, this should induce a faster berry ripening rate in some SHB cultivars than would otherwise be the case following unseasonably warm winters. Reproductive bud initiation in SHB blueberries occurs under long (16 hours) nights with the mediation of phytochrome. It is possible in a controlled environment to suppress SHB blueberry reproductive bud initiation by night interruption (NI). The effect of NI on berry ripening, berry size and yield in ‘Emerald’ and ‘Snowchaser’ was investigated for two seasons, to determine what degree of harvest scheduling is possible with this technique. NI did not suppress reproductive bud development under these trial conditions, since both cultivars flowered and produced fruit. The effect on berry size and yield was cultivar specific. During 2011 NI decreased the number of berries harvested and total yield per plant in ‘Emerald’, and this decrease was linear the longer the NI lasted. / AFRIKAANSE OPSOMMING: Winsgewendheid in die uitvoer-gedrewe Suid-Afrikaanse bloubessie-bedryf is van vroeë lente oeste afhanklik. In die George-omgewing in die Wes-Kaap bou te min winterkoue op om die endodormansie van sommige ‘southern highbush’ (SHB) bloubessie kultivars natuurlik op te hef, wat probleme soos vertraagde bot en uitgerekte oestye veroorsaak. Produsente van ander gematigde vrugtesoorte, word ook deur hierdie probleme geraak en chemiese rusbreekmiddels (RBs) word in boorde aangewend in ’n poging om dit te oorkom. In kern- en steenvrugboorde vind aanwending van die RB waterstofsianamied (WS) algemeen plaas. Thidiazuron (TDZ), ’n ander RB word op beperkte skaal in appelboorde aangewend. Die uitwerking van RBs op bessierypwording, -grootte en opbrengs van SHB kultivars Bluecrisp, Emerald en Star is oor twee seisoene in ’n boord naby George ondersoek, om vas te stel tot watter mate bloubessie-oesskedulering met behulp van RB aanwending moontlik is. Na Dormex® (WS, 520 g L-1) aanwending in 2010, waarin ’n warm winter ondervind is, is die bessierypwording van ‘Bluecrisp’ versnel. Dormex® aanwendingstye voordat blomknopskubblare oopmaak, maar nadat winterkoue opgebou het, het ’n aanvaarbare opbrengs en bessiegrootte met geen blomknopskade tot gevolg gehad nie. ’n 1% Konsentrasie gee soortgelyke reaksies as ’n 2% aanwending maar teen ’n laer risiko vir blomknopskade. Lift® (TDZ, 3 g L-1) aanwending het die aantal dae tot 75% oesinsameling van ‘Star’ in 2010 verminder. Plante wat ’n laer opbrengs lewer produseer groter bessies as die wat ’n hoër opbrengs lewer. Lift® aanwending nadat blomknopskubblare oopgemaak het, het misvormde en beskadigde blomme tot gevolg gehad. Vertraging van blomknopinisiasie kan die oopmaak van blomknoppe uitstel tot na nuwe blare in die lente gevorm het. Dit kan vinniger bessie rypwording meebring as wat die geval vir sommige SHB kultivars na warm winters is. Die aanvang van blomknopontwikkeling in SHB bloubessies vind tydens lang nagte (16 ure) plaas en staan onder beheer van fitochroom. Onder beheerde toestande kan bloubessie blomknopinisiasie deur onderbreking van die lang donker (nag) tydperk (ON) in ’n lig-donker siklus onderdruk word. Die uitwerking van ON op bessierypwording, -grootte en opbrengs van ‘Emerald’ en ‘Snowchaser’ is oor twee seisoene ondersoek, om die mate waartoe oesskedulering met hierdie tegniek in ’n boord moontlik is aan te spreek. ON het nie die blomknopinisiasie onder hierdie eksperimentele toestande onderdruk nie, aangesien beide kultivars in albei seisoene kon blom en opbrengste lewer. Die effek op bessiegrootte en opbrengs was kultivar spesifiek. In 2011 is die totale opbrengs en hoeveelheid bessies per plant geoes van ‘Emerald’, deur ON verminder en dié vermindering was liniêr met toename in aantal ON siklusse.