Thesis (MScAgric)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Fruit trees are exposed to various factors that can adversely affect the production of quality fruit. These factors can directly affect the fruit and the health of the tree and can be classified according to their biotic or abiotic nature, such as pathogenic attacks and drought stress respectively. One of the cultural practices used commercially to address these stresses is the application of fungicides and bactericides. The fruit production industry is under severe pressure from consumers, retailers and environmentalists, locally and internationally, to reduce chemical applications to fruit and fruit trees. The use of natural plant defence elicitor compounds and nutrients offer a potential alternative to fungicide and bactericide sprays and may also increase fruit quality and size as result of a reduction of plant stress. Trials were conducted to evaluate the efficiency of natural plant defence elicitors i.e. salicylic acid (SA) and flavonoids, in addition to mineral nutrients and bactericide/fungicides, on peach (incidence of Xanthomonas infection), plum (induced drought stress and Mg/Mn deficiencies) and apple (Mg/Mn deficiencies) fruit and trees against specified biotic or abiotic stress factors. Trial on Prunus persica cv. ‘Sandvliet’were conducted over two seasons (2008/2009 and 2011/2012) on a commercial site, Protea Farm, in the Worcester area in the Western Cape Province. During the 2008/2009 season the SA (AlexinTM, AlexiboostTM) containing treatments were applied first at 75% petal drop at concentrations of 125 and 250 ml. 100 L-1. The copper (StCu, Cu)-containing treatment was applied at 50% petal drop, while dichlorophen (XanbacTM) treatments were applied at fruit set at concentrations of 150, 300 and 200 ml. 100 L-1. The flavonoid (CroplifeTM) treatment was applied at the start of petal drop at a concentration of 150 ml.100 L-1. During the 2011/2012 season, a new flavonoid (CropbiolifeTM) treatment, as well as potassium (K-MaxTM) treatment, were incorporated into the trial and applied at concentrations of 150 and 500 ml. 100 L-1 respectively. Additionally a SA (AlexinTM) and dichlorophen (XanbacTM) treatments that performed well during the first season, were incorporated into the second season with application times and rates similar to the first season’s protocol. In addition to fruit size and quality measurements, the percentage Xanthomonas infection was determined on the leaves and fruit of the experimental trees. The SA (AlexinTM) containing treatment significantly reduced the incidence of Xanthomonas infection on leaves and fruit compared to the control in the first season. However, results varied between the two seasons, as no significant difference from the control could be obtained in the following season. The AlexinTM treatments also significantly increased the fruit size and quality. The flavonoid (CropbiolifeTM) and K (K-MaxTM) containing treatments similarly reduced the Xanthomonas infection on leaves and fruit, as well as increasing the fruit size and quality in the second season. The dichlorophen (XanbacTM) containing treatment recorded varying results as it significantly reduced the Xanthomonas infection on the fruit only in the second season. The plum trials were conducted over the 2011/2012 season on ‘Laetitia’ and ‘Songold’ plum trees, Welgevallen Experimental Farm, Stellenbosch University. Three SA (AlexinTM, AlexSal and RezistTM) containing foliar treatments were applied on the ‘Laetitia’ trees. Only two SA (AlexinTM, AlexSal) containing foliar treatments were applied on the ‘Songold’ trees. Additionally, a foliar treatment containing only K, Ca, Mg and B, was applied in both the ‘Laetitia’ and ‘Songold’ trials. All the treatments were first applied at 75% petal drop, at the same concentration of 250 ml. 100 L-1. Additionally to fruit size and quality, the mineral nutrient content of the leaves and fruit was determined. The ascorbic acid and glutathione content was determined in fruit at harvest and again after storage. None of the treatments had a positive effect on the parameters measured, except the SA (AlexinTM) containing treatments which increased the titratible acidity (TA) in both at harvest and after storage. The treatments also did not alleviate the induced stress compared to the control. The apple and plum tree trials were conducted over the 2011/2012 season in a semi-closed greenhouse, at the Welgevallen Experimental Farm, Stellenbosch. Magnesium (Mg) and Manganese (Mn) deficiencies were induced in one-year-old ‘Royal Beaut’ apple and ‘Laetitia’ plum trees planted in 10 L nursery bags, by omitting these nutrients from a standard Long Ashton soil application. Foliar treatments of Mg (MagMaxTM) and Mn (ManMaxTM) containing sprays were subsequently applied at concentrations of 250 and 75 ml. 100 L-1 respectively, after deficiency symptoms for these nutrients were visually observed. Mineral nutrient analysis of the leaves were analysed on the 13th of February for the plums and 30th of March 2012, for the apples. The Mn (ManMaxTM) containing treatment successfully overcame the Mn induced deficiency. The Mg (MagMaxTM) containing treatment did not overcome the induced Mg deficiency and was probably due to the deficient nitrogen levels in the plants, caused by an error in the initial Long Ashton nutrient solution formulation. In conclusion AlexinTM, K-MaxTM and CropbiolifeTM have shown their ability to decrease Xanthomonas infection in peaches. Additionally to their positive effect on fruit size and quality on the peaches. SA was not able to overcome the induced stress on plums, but had a positive effect on the fruit quality and size. The ManMaxTM been proven to overcome the induced Mn deficiency, while MagMaxTM was unsuccessful to overcome the Mg deficiency. / AFRIKAANSE OPSOMMING: Vrugtebome word blootgestel aan verskeie faktore wat die produksie van kwaliteit vrugte nadelig kan beïnvloed. Hierdie faktore kan ‘n direkte invloed hê op die vrugte en op die gesondheid van die boom en kan geklassifiseer word op grond van hulle biotiese of abiotiese natuur, soos patogeen infeksie en droogte stres onderskeidelik. Van die produksie praktyke wat gebruik word sluit in die toepassing van verskillende swamdoders en bakterisiede. Die vrugtebedryf is onder geweldige druk van verbruikers, die kleinhandel en omgewingsbewustes om die toediening van chemikalieë aan vrugte en vrugtebome te verminder. Die gebruik van natuurlike plant verdediging stimulerende verbindings en nutriënte, bied 'n moontlike alternatief tot die spuit van swamdoders en bakterisiede, en kan ook moontlik ʼn bydrae maak tot verbeterde vrugkwaliteit en -grootte. Proewe is uitgevoer om die effektiwiteit van die natuurlike plant verdediging stimulante, salisielsuur (SA) en flavonoïede, addisioneel tot verskillende voedingstowwe en bakterieële / swamdoders op perske, pruim en appels teen Xanthomonas infeksie, droogte stres en Mg / Mn tekorte as biotiese en abiotiese stres faktore onderskeidelik te evalueer. Die Prunus persica ‘Sandvliet’ proewe is oor twee seisoene (2008/2009 en 2011/2012) op 'n kommersiële perseel, Protea Farm, in die Worcester-area in die Wes-Kaap Provinsie, uitgevoer. Gedurende die 2008/2009 seisoen is die SA (AlexinTM, AlexiboostTM) bevattende behandelings eers toegedien by 75% blomblaarval teen konsentrasies 125 en 250 ml. 100 L-1. Die koper (StCu, Cu) bevattende behandeling is toegedien by 50% blomblaarval, terwyl die dichlorofen (XanbacTM) bevattende behandelings toegedien is by vrugset, teen konsentrasies van 150, 300 en 200 ml. 100 L-1. Die flavonoïde (CroplifeTM) behandeling is toegedien by die begin van blomblaarval teen 'n konsentrasie van 150 ml. 100 L-1. Gedurende die 2011/2012 seisoen was 'n nuwe flavonoïd (CropbiolifeTM) en ‘n kalium (K-MaxTM) behandeling toegevoeg tot die eksperiment, met ʼn toediening teen konsentrasies van onderskeidelik 150 en 500 ml. 100 L-1. Daarbenewens is die SA (AlexinTM) en dichlorofen (XanbacTM) behandeling van die 2008/2009 seisoen herhaal teen dieselfde konsentrasies en toedieningstye soos in die protokol van die eerste seisoen. Behalwe vir die bepaling van vruggrootte en –kwaliteit, is die persentasie Xanthomonas infeksie op blare en vrugte ook bepaal. Die SA (AlexinTM) bevattende behandeling het die voorkoms van Xanthomonas infeksie op die blare en vrugte betekenisvol verminder in vergelyking met die kontrole. Resultate het egter gewissel in die daaropvolgende seisoen en geen beduidende verskille tussen die behandelings is waargeneem nie. Hierdie SA-bevattende behandelings het ook tot ‘n toename in vruggrootte en -kwaliteit gelei. Die flavonoïde bevattende behandelings, (CropbiolifeTM) en K (K-MaxTM), het soortgelyke afnames in Xanthomonas infeksie op die blare en vrugte in die tweede seisoen getoon, sowel as ‘n toename in vruggrootte en -kwaliteit. Die dichlorofen (XanbacTM) bevattende behandeling het variërende resultate getoon aangesien dit slegs tot ‘n beduidende afname in Xanthomonas infeksie op die blare en vrugte in die tweede seisoen kon lei. Pruim proewe is uitgevoer in die 2011/2012 seisoen op ‘Laetitia’ en ‘Songold’ pruimbome te Welgevallen Proefplaas, Universiteit van Stellenbosch. Drie SA (AlexinTM, AlexSal en RezistTM) bevattende blaar behandelings is toegedien op die ‘Laetitia’ bome. Slegs twee SA (AlexinTM, AlexSal) blaar behandelinge is toegedien op die ‘Songold’ bome. ʼn Verdere K, Ca, Mg en B blaar behandeling is ook toegedien in beide die ‘Laetitia’ en ‘Songold’ proewe. Al die behandelings se eerste toediening het saamgeval met 75% blomblaarval, teen dieselfde konsentrasie van 250 ml. 100 L-1. Addisioneel tot vruggrootte en –kwaliteit, is die mineraal element inhoud van die blare en vrugte bepaal. Die askorbiensuur en glutatioon inhoud is bepaal in die vrugte met oes asook na opberging. Geen behandeling het 'n positiewe uitwerking op die parameters wat gemeet is getoon nie, behalwe een van die SA (AlexinTM) bevattende behandelings wat die titreerbare sure (TS) verhoog het in beide kultivars. Die behandelings kon ook nie die geïnduseerde stres verlig in vergelyking met die kontrole nie. Die appel- en pruim proewe is uitgevoer gedurende die 2011/2012 seisoen in 'n semi-geslote glashuis te Welgevallen Proefplaas, Universiteit van Stellenbosch. Magnesium (Mg) en Mangaan (Mn) tekorte is geïnduseer in een-jaar-oue ‘Royal Beaut’ appel en ‘Laetitia’ pruim bome, aangeplant in 10L kwekerysakke, deur dié elemente uit ʼn toediening van standaard Long-Ashton voedingsoplossing aan die grond weg te laat. Mg (MagMaxTM) en Mn (ManMaxTM) bevattende blaarspuite is daarna toegepas teen onderskeidelik konsentrasies van 250 en 75 ml. 100 L-1. ʼn Minerale analise van die blare is uitgevoer op 13 Februarie, op die pruime en 30 Maart 2012, op die appels. Die Mn (ManMaxTM) bevattend behandeling het die Mn-geïnduseerde tekort verlig. Die Mg (MagMaxTM) bevattende behandeling het nie die geïnduseerde Mg-tekort verlig nie. Dit is moontlik toe te skryf aan die stikstof tekort in die plante wat te wyte was aan ʼn foutiewe Long Ashton voedingsoplossing formulasie wat aanvanklik toegedien is. Ten slotte het AlexinTM, K-MaxTM en CropbiolifeTM getoon dat hul die vermoë het om Xanthomonas infeksie te verminder, asook om vruggrootte en kwaliteit in perskes te verbeter. SA was nie in staat om die geïnduseerde stres op pruime te oorkom nie, maar het 'n positiewe uitwerking op die vruggroote en kwaliteit gehad. ManMaxTM het getoon dat dit ʼn geïnduseerde, visuele Mn tekort kan oorkom, terwyl MagMaxTM onsuksesvol was om die Mg-tekort te oorkom