Thesis (PhD(Agric))--Stellenbosch University, 2009. / ENGLISH ABSTRACT: The global movement in agriculture is towards more environmentally friendly, sustainable production practices, since the role of soil microbial functions in ensuring crop production and soil fertility has become more evident in agricultural systems. Furthermore, with the impeding phase-out of methyl bromide, apple replant disease (ARD) is becoming an increasingly important problem and biological management practises are needed. Since microbial activity is generally carbon-limited in agricultural soil, it is widely accepted that management practices providing a range of organic compounds on a regular basis will tend to maintain an active and diverse microbial population. It was hypothesised that the application of various biological amendments can affect soil microbial numbers and function, thereby having a positive effect on fruit tree growth and yield. The effect of continued applications of organic material, various microbial inoculants and biostimulants on tree performance were evaluated in conventional management systems. Field trials were established in a conventional pear orchard, potential apple replant disease sites, as well as an optimally managed, high density apple orchard under controlled fertigation. The use of compost, compost extracts, a Bacillus inoculant and humates were investigated intensively. Furthermore, to improve our understanding of soil biological systems a combination of simple, practical methods were used to evaluate the effect of biological amendments on soil microbial properties and effects were related to tree performance. Regular application of compost extract in combination with compost showed the most significant effect in improving tree performance in commercial pome fruit orchards under various conditions. In the pear orchard, cumulative yield over the first two seasons was improved by more than 50% compared to controls, while in the fertigated orchard yield was improved by 22%. Biological amendments also showed improved growth in orchards suffering from stunted growth symptoms typical of ARD. However, in severe ARD cases methyl bromide fumigation showed the most consistent effects. Other biological amendments which showed positive effects on yield were application of Bacillus inoculants (Biostart®) in combination with a labile C source and a low dosage humate product, as well as a combination of compost and humates. It was clear that a combination of labile organic matter and a diverse group of microorganisms showed most promise. Although for some specific treatments increased microbial numbers and activity may have resulted in improved tree performance, in general, changes in culture-based plate counts, soil enzyme activity and carbon utilisation profiles could not be used as an indicator of yield. It was suggested that improved synchronisation of nutrient release and plant uptake, as well as microbial phytohormone production, may play an important role in improving tree performance with application of biological amendments. More research is needed on the exact mechanisms through which compost extracts improve yield and studies on root growth proliferation, as well as effects in the rhizosphere are recommended. / AFRIKAANSE OPSOMMING: Binne lanbouverband is daar tans wêreldwyd die neiging om die uitwerking van produksie-praktykte op die omgewing in ag te neem en sodoende meer verantwoordelik op te tree. Omdat die belangrike rol wat grondmikro-organisme funksionering in volhoubare verbouingspraktyke speel nou deeglik besef word, word meer volhoubare bestuurspraktyke bepleit. Hiermee saam, noodsaak aspekte soos die uitfasering van metielbromied vir die beheer van appelhervestigingsiekte, dat biologiese bestuurspraktyke meer aandag geniet. Daar word geredelik aanvaar dat gereelde toediening en aanvulling van organiese materiaal ‘n aktiewe, diverse mikrobe populasie in die grond tot gevolg sal hê. Die hipotese is gestel dat die toediening van ‘n verskeidenheid biologiese produkte grondmikrobe getalle en werking gunstig kan beïnvloed. Dit kan moontlik weer aanleiding gee tot positiewe reaskies wat die groei en drag van vrugtebome betref. In hierdie studie is die uitwerking van voortgesette toedienings van organiese materiaal, mikrobiese inokulante, asook biostimulante, op die prestasievermoë van vrugtebome ondersoek. Veldproewe is uitgelê in ‘n konvensionele peerboord, verskeie boorde met moontlike appelhervestigingsiekte probleme, asook ‘n hoëdigtheidsaanplanting appelboord onder optimale bestuur. ‘n Deeglike ondersoek is gedoen met betrekking tot die gebruik van kompos, komposekstrak, Bacillus-inokulante en humate. Eenvoudige, praktiese metodes is aangewend om vas te stel hoe biologiese toevoegings grondmikrobe eienskappe beïnvloed en of dit verband hou met veranderinge in boomprestasie. Die studie het aangetoon dat die gereelde toediening van komposekstrak saammet kompos, betekenisvolle verbetering in boomprestasie van kernvrugboorde teweeg bring onder verskeie omstandighede. Die kumulatiewe opbrengs van ‘n peerboord is oor twee seisoene met meer as 50% verhoog teenoor die kontrole. In ‘n optimaal bestuurde appelboord onder sproeibemesting, is opbrengs met 22% verhoog in vergelyking met die kontrole. Biologiese toevoegings het ook groei verbeter in boorde waar appelhervestigingsiekte bome se groei vertraag het. In die geval van ernstige appelhervestigingsimptome het metielbromied egter steeds die mees konstante positiewe uitwerking gehad. Ander biologiese toevoegings wat ‘n gunstige uitwerking op opbrengs getoon het, was ‘n kombinasie van Bacillus inokulante, ‘n lae dosis humaat en ‘n aktiewe koolstofbron, asook kompos in kombinasie met humate. Dit is duidelik dat ‘n kombinasie van ‘n maklik afbreekbare koolstofbron (soos kompos) tesame met ‘n diverse groep mikroorganismes mees belowend is vir gebruik in biologiese verbouingssisteme. Resultate toon dat veranderings in aantal organismes gemeet deur plaattellings, die aktiwiteit van grondensieme, en verbruikspotensiaal van verskillende koolstofbronne, nie as ‘n aanduiding van boomprestasie gebruik kan word nie. Daar is voorgestel dat verbeterde sinkronisasie van voedingselementvrystelling en plantopname, sowel as produksie van plantgroeihormone deur mikrobe, moontlik ‘n rol speel by boomreaksies op biologiese toevoegings. Meer navorsing wat verband hou met die meganisme waardeur komposekstrak opbrengs verbeter, is nodig. Verder word studies op fynwortelontwikkeling sowel as aspekte van die wortelrisosfeer aanbeveel.

Thesis (MScAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Apple replant disease (ARD) is one of the major impediments to the establishment of an economically viable apple orchard on sites previously planted to apple. In spite of extensive research on ARD, the etiology remains to be fully elucidated. A possible biological origin of ARD etiology in South Africa was investigated by the dilution of replant field soil with sterilised soil. Commercial orchards with ARD were selected for use in pot trials and disease severity evaluated after three months, by measuring shoot length, dry mass of plants as well as root discolouration. Although diluting replant soil to 25 and 50% (v/v) significantly reduced the effects of ARD, symptoms were only absent in 0% replant soil. It was clear that seedlings planted in any mixture containing replant soil, even only 25% replant soil, consistently exhibited symptoms of stunted growth and root discolouration similar to those seedlings grown in 100% replant soil. This indicates that ARD in South Africa is primarily of a biological nature. As an initial step in formulating sustainable disease control alternatives to replace methyl bromide, pot trials were conducted to assess the impact of compost treatments as well as biological control products on ARD. Compost as well as sterilised and unsterilised compost teas (compost extract) significantly increased seedling growth even under optimum nutrient conditions when compared to the control, suggesting that they negate the effects of ARD. Results also indicated that applying high concentrations of compost does not necessarily provide additional growth benefits compared to lower concentrations. Results with biocontrol formulations were less favourable. Only one of the biocontrol formulations, a combination of Bacillus spp. (Biostart®) improved growth significantly compared to the control. There was, however, some inconsistency with results for the different trials conducted using this product. Fungal as well as nematode populations associated with ARD soils were characterised to the generic level to get a clearer understanding of the etiology of ARD in South Africa. Pythium and Cylindrocarpon spp. were consistently isolated from all six replant soils in all trials that formed part of this study, indicating that these fungi may have a role in ARD etiology in South Africa. Nematodes implicated in ARD development were inconsistently associated with ARD soils used in these studies. This suggests that nematodes do not have a primary causal role in ARD etiology in South Africa. Field trials were conducted in commercial orchards to assess the impact of organic amendments and promising biological control products, as indicated by the pot trials, on ARO severity under field conditions. These biological soil amendments were also compared with the standard chemical control methods for ARO, methyl bromide and chloropicrin. In all three trials established, compost and mulch as well as manure and mulch, consistently increased growth to the same extent as the standard chemical treatments and by combining these chemical treatments with organic amendments a significant, additional growth increase could be attained. Biocontrol formulations evaluated in field studies gave variable results. Biostart® improved growth when applied on its own, but not in combination with the chemical Herbifume (metham-sodium). Inoculating soil with effective microorganisms (EM), consisting primarily of photosynthetic bacteria, had no significant effect on growth. Results from this study indicate that application of organic amendments could possibly substitute for soil fumigation in replanted apple orchards. However, compost quality standards need to be implemented and because few types of compost are universally effective, different types of composts should be compared in specific soil environments before recommendations can be made. Oue to variable results with biocontrol products, ARO management with these biological soil amendments cannot be guaranteed at this stage and further studies are recommended. / AFRIKAANSE OPSOMMING: EVALUERING VAN DIE SPESIFIEKE APPELHERVESTIGING-PROBLEEM IN BOORDE IN DIE WES-KAAP Appelhervestiging-siekte (AHS) skep 'n groot probleem in die vestiging van jong appelbome op grond waar daar reeds voorheen appels verbou is. Ten spyte van omvangryke navorsing is die oorsaak van die probleem nog hoofsaaklik onbekend. 'n Moontlike biologiese oorsaakleer is in Suid-Afrika ondersoek deur die hervestigings-effek te probeer verminder deur die vermenging van hervestigingsgrond met gesteriliseerde grond. Kommersiële boorde met 'n appelhervestigingsprobleem is geselekteer en gebruik in potproewe. Die ernstigheidsgraad van die siekte is na drie maande se groei geevalueer deur lootlengte, droë massa en wortelverkleuring te meet. Alhoewel verdunning van die hervestigingsgrond tot 50 en 25% (vlv) die effek van AHS op groei betekenisvol verminder het, kon die skadelike effek van die veroorsakende faktor slegs uitgeskakel word deur saailinge in 100% gesteriliseerde grond te plant. Dit was duidelik dat saailinge wat in enige grondmengsel geplant is waarin hervestigingsgrond voorgekom het, selfs al was dit net 25%, konsekwent simptome van vertraagde groei en wortelverkleuring getoon het. Dit is 'n aanduiding dat AHS in Suid-Afrika hoofsaaklik biologies van aard is. Potproewe is uitgevoer as 'n eerste stap in die formulering van volhoubare siektebeheer-strategieë, om die impak van kompos-behandelings en biologiese beheer produkte op AHS te ondersoek. Kompos sowel as gesteriliseerde en ongesteriliseerde kompos-tee (kompos-water) het, selfs onder optimale voedingsomstandighede, die groei van saailinge betekenisvol verbeter. Dit dui aan dat hierdie behandelings die effek van AHS kan teenwerk. Resultate het ook daarop gedui dat hoër kompos konsentrasies nie noodwendig enige addisionele voordele vir groei inhou in vergelyking met laer konsentrasies nie. Resultate met biologiese beheer produkte was minder gunstig. Slegs een van die produkte wat geëvalueer is, 'n kombinasie van Bacillus spp. (Biostart®), het groei betekenisvol verbeter in vergelyking met die kontrole. Resultate was egter inkonsekwent vir die verskillende proewe waarin hierdie produk gebruik is. Swampopulasies sowel as aalwurmpopulasies wat met hervestigingsgrond geassosieer word, is geïdentifiseer tot op generiese vlak om vas te stel waardeur AHS in Suid-Afrika veroorsaak word. Pythium en Cylindrocarpon spp. is konsekwent van al ses hervestigingsgronde geïsoleer wat daarop dui dat hierdie twee swamgenera 'n beduidende rol in AHS ontwikkeling in Suid-Afrika mag hê. Aalwurms wat aangedui is in die literatuur om 'n moontlike rol in AHS te hê, was slegs in enkele gevalle geassosieer met hervestigingsgronde waarvan in hierdie studie gebruik gemaak is. Die gevolg-trekking is dus gemaak dat aalwurms nie 'n betekenisvolle rol speel as hoof-veroorsakende organisme onder Suid-Afrikaanse toestande nie. Veldproewe is uitgevoer in kommersiële appelboorde om vas te stel wat die effek van organiese materiaal, asook belowende biologiese beheermiddels, soos aangedui deur potproewe, op AHS onder veldtoestande is. Die biologiese grondtoedienings is ook vergelyk met die standaard chemiese beheermiddels (metielbromied en chloorpikrien). In al drie proewe wat gevestig is, het kompos met 'n deklaag, sowel as kraalmis met 'n deklaag, groei betekenisvol verbeter tot dieselfde mate as chemiese middels. Daar kon ook 'n beduidende, addisionele groeitoename gemeet word in gevalle waar chemiese middels met organiese materiaal gekombineer is. Resultate met biologiese beheer formulasies wat onder veldtoestande geëvalueer is, het gevarieer. Biostart® het groei verbeter wanneer dit alleen toegedien is, maar in kombinasie met die chemiese middel Herbifume (metham-sodium) het dit geen effek gehad nie. Die inokulering van grond met 'n oplossing van effektiewe mikroorganismes (EM) wat hoofsaaklik uit fotosinterende baterieë bestaan, het ook geen betekenisvolle effek op groei gehad nie. Die gevolgtrekking is gemaak dat toediening van organiese materiaal moonlik as plaasvervanger vir metielbromied-beroking kan dien in die beheer van AHS. Die nodige komposkwaliteit-standaarde moet egter eers geïmplimenteer word. Omdat feitlik geen kompos universeel effektief kan wees nie, is dit ook nodig dat verskillende tipes kompos met mekaar vergelyk moet word in spesifieke grondtoestande voordat verdere aanbevelings gemaak kan word. As gevolg van variërende resultate met biologiese beheer produkte kan AHS beheer met hierdie middels nie gewaarborg word op hierdie stadium nie en verdere studies word aanbeveel.

Aufgrund des gesellschaftlichen Wandels, globaler klimatischer und marktlicher Veränderungen sowie zunehmender Ressourcenknappheit sind landwirtschaftliche Betriebe immer mehr gefordert, ihre Produktionsverfahren anzupassen. Doch sind solche Innovationen häufig komplex und weisen einen systemischen Charakter auf. KMU sind häufig nicht in der Lage, diese oft mit höherem Risiko behafteten Innovationen voranzutreiben oder zu übernehmen. Zur Förderung ihrer Diffusion ist es notwendig, die äußeren und inneren Rahmenbedingungen der Innovationsumgebung zu kennen. Am Fallbeispiel der Innovationsprozesse zur Überwindung der Nachbauproblematik im Apfelanbau werden potenzielle Einflussfaktoren auf die Adoption neuer Maßnahmen mithilfe des sektoralen Innovationssystemansatzes nach Malerba und einem erweiterten Technology Acceptance Model erfasst. Hierbei wurde deutlich, dass trotz der günstigen natürlichen Rahmenbedingungen in der Untersuchungsregion, Faktoren wie sinkende Erzeugerpreise, Lohnsteigerungen, Wettbewerbsdruck, Klimawandel, die abnehmende Verfügbarkeit jungfräulicher Flächen sowie steigende Anforderungen des LEH und der Gesellschaft einen wirtschaftlichen Anbau zunehmend erschweren. Vor allem aber für Baumschulen sind die Folgen der Bodenmüdigkeit aufgrund des hohen Produktionswertes pro Hektar Kulturfläche besonders schwerwiegend. Maßnahmen, die bisher gegen Bodenermüdung ergriffen wurden, sind wenig effektiv, oder unwirtschaftlich. Die Analyse der Akzeptanzbefragung potenzieller Adopter zum Einsatz von Mikroorganismen gegen Bodenmüdigkeit ergab, dass die Nützlichkeit des Verfahrens positiv wahrgenommen wird und sich dies in einer hohen Übernahmeabsicht widerspiegelt. Eine Strukturgleichungsanalyse des in dieser Studie entwickelten Akzeptanzmodells mittels SmartPLS mit einem Bestimmtheitsmaß von 68% verdeutlichen, dass nicht vorrangig ökonomische Faktoren, sondern in erster Linie die Kompatibilität der Maßnahme, Einfluss auf eine potenzielle Übernahme ausüben würde. / Due to social change, global climatic and market changes as well as increasing scarcity of resources, agricultural enterprises are increasingly required to adapt their production processes. However, innovations often have a systemic character. SMEs are often not in a position to promote or adopt these innovations, which are often associated with higher risks. In order to promote their diffusion, it is, therefore, advantageous to know the external and internal framework conditions of the environment for innovation. Using the case study of innovation processes to overcome the problem of apple replant disease, potential influencing factors on the adoption of new measures are identified on the basis of the Sectoral Innovation System Approach according to Malerba and an extended Technology Acceptance Model. It became clear that, despite the favourable natural conditions in the study region, factors such as falling producer prices, wage increases, competitive pressure, climate change, the decreasing availability of virgin land and the increasing demands of food retailers and society are making commercial cultivation increasingly challenging. However, the consequences of soil fatigue are particularly severe for tree nurseries due to the high production value per hectare of cultivated land. Measures that have so far been taken to combat soil fatigue are ineffective or uneconomical. The analysis of the acceptance survey of potential adopters of microorganisms against soil fatigue showed that the usefulness of the method is perceived positively and that this is reflected in a high intention to adopt it. A structural equation analysis of the acceptance model developed in this study using SmartPLS and a coefficient of determination of 68% make it clear that not primarily economic factors, but primarily the compatibility of the measure, would influence a potential adoption.