ix, 158 leaves : ill. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Pathology, 1972

Graduation date: 1990

Rhynchosporium commune is one of the most destructive pathogens of barley worldwide. It can cause crop yield losses of up to 40% in the UK and decrease in grain quality. Populations of R. commune can change rapidly, defeating new barley resistance (R) genes and fungicides after just a few seasons of their use. Fungicide use is one of the major modes of management of Rhynchosporium and is heavily relied on the agricultural industry. Fungicides that were effective in the past are no longer effective in controlling the disease and many are only effective when used in mixtures. Beyond the currently effective fungicides there is limited new chemistry available so there is a very real need for development in this area. In pathogenic fungi, the cell wall components play a key role in the establishment of pathogenesis. The cell wall forms the outer structure protecting the fungus from the host defence mechanisms. It is involved in initiating the direct contact with the host cells by adhering to their surface. The fungal cell wall also contains important antigens and other compounds modulating host immune responses. R. commune germinated conidia and interaction transcriptome sequencing generated a list of over 30 different cell wall proteins (CWPs) potentially involved in pathogenicity. R. commune genome and interaction transcriptome sequencing provided further information about the extent of CWP families as well as a subset of genes expressed during barley colonisation by R. commune. The use of bioinformatic techniques allowed for the analysis of gene sequences. Putative cell wall associated genes were compared to the sequences from the fungal database via sequence similarity, sequence alignments 15 and conserved domain searches to better understand their function. Phylogenetic analysis also allowed us to understand the evolutionary relationship between R. commune genes and related genes in other organisms. Transcription profiling of R. commune CWPs during the development of infection helped to prioritise them for functional characterisation. Targeted gene disruption unfortunately did not yield mutants but has furthered our understanding of this technique in R. commune for future attempts. Functional complementation was successful however and allowed the uncovering of the function of RSA9. The results show that R. commune RSA9 functions as an allantoicase, an enzyme which breaks down purines as a source of nitrogen when conditions are nitrogen limited. The use of chemical cell wall inhibitors allowed us to better understand the role of carbohydrate cell wall components in R. commune fitness and virulence. Inhibition of cellulose production by DCB showed reduced growth, germination and pathogenicity of R. commune. Similar results were observed when beta-glucan synthesis was impaired; as inhibitor concentration increased, growth and germination of the fungus decreased. The composition of R. commune cell wall was also uncovered during this research. Techniques such as HPLC and FTIR eluded the composition of monosaccharides and polysaccharides respectively. In addition the structure of R. commune cell wall was observed by microscopy, namely TEM. This project revealed some much needed information on the R. commune cell wall and the relation of its components to fitness and virulence during infection of barley.

<i>Rhynchosporium commune</i> is one of the most destructive fungal pathogens of barley worldwide. It causes scald, responsible for reduced grain quality and yield losses of up to 40%. This project aimed to identify genetic resistance in barley using two different approaches: an effector approach through the identification of important pathogen virulence factors and their barley targets, and a genomics association approach. Numerous secreted effectors have been identified in many phytopathogens including <i>R. commune</i>. <i>Rrs1</i> resistance, recognising the <i>R. commune </i>avirulence protein - AvrRrs1 (NIP1) has been deployed in the field to prevent infection but has soon proven ineffective. <i>R. commune </i>has managed to overcome this resistance by alteration or deletion of the <i>NIP1</i> gene as it is not essential for pathogenicity. However, our field trial data suggests that <i>Rrs1</i> remains an important component of resistance to <i>R. commune</i> in the field. Resistance genes recognising more essential <i>Avr</i> genes are likely to be more durable and as a consequence, the discovery of novel <i>R. commune Avr</i> genes is fundamental for the implementation of an integrated pest management approach to prevent this disease. Recent sequencing of the<i> R. commune</i> genome allowed identification of putative effectors. Expression of 26 potential effectors with low sequence variability in 9 sequenced <i>R. commune</i> strains have been analysed during barley infection. The best genes were selected for gene disruption and individual expression in barley cultivars and landraces using the Barley Stripe Mosaic Virus (BSMV) – based expression system to see if they are recognised by the plant. The work also focused on candidate effectors with putative functions. A putative protease inhibitor was chosen for functional characterisation but its function and importance for pathogenicity could not be confirmed. In addition, high amount of the candidate protein appeared to be toxic for barley and <i>Nicotinana benthamiana</i>. Two SA (salicylic acid)-related putative effectors were also chosen for further characterisation and revealed a direct link between the SA pathway of barley and <i>R. commune</i>. The results of this project suggest that <i>R. commune</i> might be able to manipulate the SA pathway of the host confirming the existence of a biotrophic phase of the fungus. The genomics association approach to identify resistance genes against <i>R. commune</i> in barley used a Genome Wide Association Scan (GWAS) using a combination of three years of disease nursery field trial data for a collection of over 500 elite spring barley cultivars. This analysis identified a number of quantitative trait loci (QTL) in barley genome regions previously shown to contain major resistance genes such as <i>Rrs1</i> on chromosome 3H, <i>Rrs2</i> on chromosome 7H, <i>Rrs3</i> on chromosome 4H, <i>Rrs4</i> on chromosome 3H, <i>Rrs13</i> on chromosome 6H, <i>Rrs14</i> on chromosome 1H and<i> Rrs16</i> on chromosome 4H, as well as novel QTL. The work was focused on <i>Rrs1</i> resistance.<i> R. commune</i> strains producing a type of NIP1 effector, recognised by barley lines containing <i>Rrs1</i>, were used to confirm the resistance in predicted <i>Rrs1</i> barley cultivars. The <i>Rrs1</i> interval has been narrowed down to 3 Mbp, and high resolution mapping led to the identification of 3 SNP markers which perfectly discriminated <i>Rrs1Rh4</i> lines from susceptible lines. These diagnostic markers will provide a useful breeding tool for improving the design of new varieties allowing the incorporation of the<i> Rrs1</i> resistance. This research takes us a step closer towards cloning the first barley major resistance (R) gene against <i>R. commune</i>, which is likely to be present only in<i> Rrs1</i> lines and have a kinase domain very similar to the one in a putative wall associated kinase found within the <i>Rrs1 </i>interval in the genome assembly of susceptible cultivar Morex. It will also help us to better understand <i>R. commune</i>-barley pathosystem and to identify further R genes.

Thesis (M. Ag. Sci)--University of Adelaide, Dept. of Plant Science, 1992. / Includes bibliographical references.

The overall objective of the work described in this thesis was to characterise the three genes Hv14.3.3c, HvMAPKK1 and HvFKBP41, in terms of a role in defence and stress response signalling. These genes had previously been found to be differentially expressed in compatible versus incompatible interactions of barley with the fungus Rhynchosporium secalis, suggesting a possible role in the plant defence response, while current literature suggests these genes may also play a role in signal transduction, possibly under a broad range of stresses, including abiotic as well as biotic. Two main approaches were undertaken to characterise gene function: expression analysis and the identification of protein-protein interactions. To facilitate expression analysis, full length cDNA fragments of each gene were first obtained using bioinformatics, RACE and genomic walking techniques. Expression was then investigated using quantitative real-time RT-PCR. The results of the expression analysis confirmed that the candidate genes were in fact differentially expressed during infection, suggesting a role in the defence response of barley against R. secalis. Analysing their expression in the context of other stresses and treatments, namely frost, drought and ABA, indicated their role may not be limited only to biotic stress, but include abiotic stress as well. To investigate the possibility that these genes are involved in signalling during the defence response, protein-protein interaction techniques such as yeast two-hybrid and affinity pulldowns were used to identify interacting proteins in an attempt to place the genes within a known signalling network and build and extend on these networks. Y2H screening was used successfully to identify two putative interactors of Hv14.3.3c; an EPSP (5-enolpyruvylshikimate-3-phosphate) synthase and a putative wound-induced protein, and two interactors of HvFKBP41; a Rab-type GTPase and the same wound-induced protein. From what is known about the function of these genes in the literature, they fit well with a role in stress response signalling and the potential to be involved in signalling networks with the candidate gene products and also with each other. Through the trial of many different affinity pulldown techniques, a method for identifying interacting proteins from plant extracts was successfully established, however, issues with protein identification meant that interacting proteins were not identified using this technique. Steps were then made towards confirming the interactions identified using the Y2H system. Full length cDNA sequences of the identified interactors were obtained and expression analysis performed, in the aim of investigating co-expression patterns between the genes encoding the interacting proteins and the three candidate genes, to support a potential interaction. To confirm the Hv14.3.3c-HvEPSP interaction, co-immunoprecipitation and BRET were then used, however confirmation was unsuccessful due to issues with non-specific binding in co-immunoprecipitation and technical issues trying to establish the BRET analysis system in barley. In summary, the results of this study place the candidate genes Hv14.3.3c, HvMAPKK1 and HvFKBP41 as players in signal transduction during the plant defence/stress response. With the identification of previously uncharacterised protein interactions, some progress has also been made towards placing these genes within known signalling networks and identifying potential downstream genes that could possibly play a more specific role in defence response signalling and therefore be potential targets for the generation of resistant or stress tolerant plants. / Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2009

Biochar is the solid, carbon-rich by-product obtained from pyrolysis. It offers the prospect of long-term carbon sequestration and soil conditioning with agronomic benefits, often referred to as the ‘biochar effect‘. These multiple direct or indirect changes in the soil plant interface have also been associated with the control of plant diseases by influencing the host’s systemic induced resistance. The biological impact of biochar on the phytopathology of a major cereal grain has not yet been investigated. The most damaging foliar disease of barley in the UK is Rhynchosporium leaf blotch caused by the hemibiotrophic fungal pathogen Rhynchosporium commune. The aim of this project was to evaluate biochar performance and effectiveness as a disease control agent in the barley – R. commune – pathosystem, and attempted to identify underlying mechanisms by which biochar may function in the interaction between barley and the causal pathogen. Therefore, a unique set of nine slow pyrolysis biochars were established along a 350 to 800°C pyrolysis temperature gradient, with eight of these made from pelleted softwoods and a single one made from Miscanthus straw. A comprehensive biochar quality assessment was undertaken and established that the biochars differed in their chemical composition, which largely depended on production parameters, predominantly temperature (P < 0.05). The analysis proposed that biochar 9, made from Miscanthus at 800°C, showed added value as a soil conditioner over softwood biochars, due to higher pH, mineral ash and macronutrient recoveries, which pointed towards a possible liming potential. Regardless of the feedstock, biochars pyrolysed above 600°C indicated potential use for carbon sequestration purposes, due to higher carbon stability. Short-term controlled bioassays showed significant restricted growth of R. commune mycelium on defined medium to direct (1.0% w/w) and indirect volatile exposure from certain biochars (P < 0.001). The findings suggested a synergistic effect of the softwood biochars acidic nature and presence of fungicidal compounds, with observed inhibition of 100% attributed to re-condensation of tarry vapours onto biochar surfaces during pyrolysis. Qualitative biochar volatile organic compound analysis was conducted and identified biocide active phenolic and organic acid compounds, similar to those commonly found in smoke, bio-oils or wood vinegars. These findings proposed possible application for mitigation of inoculum pressure in field-grown barley, but the toxic nature of volatiles raised concerns over risks to human and environmental health, as also evidenced by detrimental barley growth effects. Subsequent controlled in vivo and in planta experiments revealed significant (P < 0.05) symptomatic barley leaf blotch reduction effects of up to 100%, following 5% (w/w) application of biochars 4, 5, 8 and 9. Barley plants transcriptional changes in ISR-dependent LOX2 and SAR-dependent PR1-b expression in planta verified systemic induced resistance as mechanisms behind the significant disease suppression of barley plants grown in soil amended with biochar 5 and 8. Disease reduction and biochar mediated induced resistance was attributed to either low concentrations of phytotoxic compounds, a direct toxicity effect from fungicidal compounds or indirect promotion of beneficial microbes. The results provided evidence, that in the case of the studied pathosystem, there is potential for biochar with specific characteristics to be considered as a soil amendment, offering not only carbon sequestration, but also possible improved disease resistance.

Includes bibliographical references

The barley scald pathogen (Rhynchosporium secalis) causes extensive economic losses, not only through lost product and quality, but also due to costs associated with chemical control. Economic and environmental impacts and the emerging resistance to fungicides and dominant resistance genes are reasons to understand molecular defence responses in order to develop new strategies to increase resistance of barley to this pathogen. In most pathosystems, defence gene expression in susceptible or resistant genotypes commonly differs quantitatively. Thus, differentially expressed genes between genotypes contrasting for response to infection by pathogens are considered candidate genes that have a role in resistance. This thesis presents functional analysis of a subset of genes isolated from a Suppression Subtractive Hybridisation library. The library was previously established and enriched for differentially expressed genes in epidermis of resistant and susceptible near-isogenic barley cultivars inoculated with R. secalis. Functional characterisation involved both investigating their putitative biochemical function as well as the genes‟ role(s) in biotic and abiotic stress responses. Three cDNA clones from the library were selected based on the putative function of the encoded proteins and the full length of the clones and their homologues were isolated from cDNA and genomic DNA. One of the clones represented a member of the pathogenesis-related protein family 17 (PR-17). Southern hybridisation showed that a small multigene family encodes the barley PR-17 proteins. Three members were cloned with two of them being novel. The second clone was homologous to galactinol synthases (GolS) and Southern blot analysis indicated existence of two GolS genes in the barley genome and subsequently two HvGolS members were isolated. The last clone (a single gene) showed similarity to very long chain fatty acid elongases, which indicates its involvement in synthesis of cuticular waxes. A characterised Arabidopsis mutant named fiddlehead (Atfdh) was highly similar to this gene and it was named HvFdh. Detailed expression analysis using Q-PCR, Northern blot analysis and publically available microarray data revealed that the isolated genes are regulated in response to a variety of abiotic and biotic stresses as well as different tissues during barley development. Under some treatments expression patterns were consistent with their putative roles and in agreement with results of other studies. Nevertheless, in other treatments expression profiles were not in agreement with previous findings in other plants indicating potentially different stress adaptation mechanisms between species. Further insight into the function of the encoded proteins was gained by their subcellular localisation using transient expression as GFP fusion proteins followed by confocal laser scanning microscopy. The results were in agreement with in silico predictions and their putative cellular function. In addition, a comprehensive list of homologous genes from other species was compiled for each gene by using public EST databases. Analyses of phylogenetic relationship and multiple sequence alignment of the homologues provided further clues to their function and conserved regions of the proteins. HvPR-17 anti-fungal properties were investigated by heterologous protein expression in E. coli and subsequent in vitro bioassays using purified protein under different conditions against a number of phytopathogenic fungi. However, no anti-fungal activity was observed. A construct with the AtFdh promoter driving the coding region of barley Fiddlehead was used for complementation of the Arabidopsis fiddlehead mutant to investigate functional orthology between these genes from dicots and monocots. The Arabidopsis fiddlehead mutant phenotype that shows contact-mediated organ fusion, germination of spore on epidermis and reduced number of trichomes was completely reverted by HvFdh. Finally, more than fifty transgenic barley lines were regenerated over-expressing or suppressing one of the three genes. The analyses of the transgenic progeny exhibited some interesting developmental phenotypes and resistance to scald and drought tolerance. These lines are awaiting further experiments to investigate the effect of altered expression in conferring resistance to other pathogens and abiotic stress tolerance as well as biochemical analysis. Collectively, in this work six barley genes were cloned and characterised by a variety of in silico techniques, temporal and transient expression analyses, subcellular localisation, in vitro bioassays and mutant complementation in Arabidopsis and loss- and gain-of-function transgenic barley plants. This work has provided insight into the function of these gene families in barley. Furthermore, the data suggest that they are regulated by the defence response to pathogenic fungi as well as drought, salinity and frost in barley. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1375755 / Thesis (Ph.D.) - University of Adelaide, School of Agriculture, Food and Wine, 2009

Thesis (PhDAgric)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: Barley leaf scald, caused by Rhynchosporium secalis, is the most important disease of barley (Hordeum vulgare) in the Western Cape province of South Africa. The disease was first reported from South Africa in 1937. The present study is the first attempt to characterise the South African R. secalis population. Topics such as pathogenesisrelated proteins, virulence spectra, variability of pathotypes, sources of variation, host resistance, breeding strategies, molecular characterisation and fungicide sensitivity are summarised in Part 1 of this dissertation. In succeeding Parts the focus is on the characteristics of the local R. secalis population regarding virulence spectrum, DNA polymorphisms, in vitro as well as in vivo fungicide sensitivity. These aspects are treated as separate entities, leading to some duplication which is unavoidable. In Part 2 the virulence spectra of 50 R. secalis isolates from a population in the. Western Cape province were determined. Twenty-one races were detected using 17 differential barley cultivars. The two most prevalent races, namely races 4 and 7 had three and four virulence genes respectively. Both race 4 and 7 were virulent on the most susceptible cultivars, namely West China, Steudelli, C.I.8618 and C.I.2226. Considering the resistance genes reported for cultivars Atlas 46, Turk, and C.I.3515 which showed no susceptible cultivar-pathogen interaction, it would appear that the Rh- Rh3-Rh4 complex is primarily involved in conferring resistance to the local R. secalis isolates. A total of 20 races (47 isolates) characterised in Part 2 were selected for further characterisation by means of DNA fingerprinting. In Part 3 an anonymous multilocus DNA probe was used to characterise the genotypic structure of these isolates by means of RFLP analysis. No correlation between any particular fingerprint pattern, race, district, field or lesion was found. The two most prevalent races, 4 and 7, did not share the same genotypes, even when isolated from the same field or lesion. The genotypic diversity of the isolates studied was 46.5% of the theoretical maximum diversity. The high level of genotypic variation observed in the South African R. secalis population resembled the genotypic diversity observed in other cereal pathogens with known sexual structures. Although no teleomorph has yet been observed, these data suggest that sexual recombination may operate within the local population of R. secalis. In South Africa barley scald is primarily controlled by means of fungicides. The continued use of fungicides on cereal crops results in the build-up of fungicide resistance in the population, which could lower the efficacy of these compounds. These aspects were investigated in Part 4, where isolates (collected during 1993 to 1995) were evaluated in vitro for sensitivity to triadimenol, tebuconazole, flusilazole and propiconazole. The sensitivity fluctuated but in 1995 isolates were significantly less sensitive towards triadimenol than in the previous two years. In a second experiment, isolates collected from two fields with a 5-6 year-history of triadimenol seed treatments and tebuconazole applications, were evaluated for their fungicide sensitivity. A significant positive correlation was observed between tebuconazole and triadimenol sensitivity among,R. secalis populations from these fields. However, such a correlation was not found within the R. secalis population collected during 1993-1995 where shorter crop rotation patterns and a range of fungicides was applied. In a third experiment, the fungicide sensitivity of local R. secalis isolates was evaluated towards two new triazole fungicides, namely bromuconazole and triticonazole. Correlation coefficients observed between these new triazoles and those previously applied in South Africa were not significantly positive. The lack of significant cross-resistance has important practical implications regarding the management of fungicide resistance. In Part 5, isolates with different minimum inhibitory concentration (MIC) towards tebuconazole in vitro (1, 3 and 10 ug/ml) were compared in vivo. The aim of this study was to determine how MIC values would influence virulence (leaf area affected) and sporulation. Results indicated that all isolates were equally fit to induce lesions and sporulate in the absence of tebuconazole. Thus no fitness cost was associated with the degree of tebuconazole sensitivity in the present study. All R. secalis isolates were able to induce lesions on tebuconazole treated leaves, but differed significantly with respect to the percentage leaf area affected. Isolates, least sensitive (MIC = 10 ug/rnl) towards tebuconazole were more adapted on tebuconazole treated leaves, being able to repeatedly cause larger lesions than sensitive R. secalis isolates (MIC = 1 ug/rnl), Sporulation was not significantly different between isolates on lesions of untreated or tebuconazole treated leaves. Larger leaf areas affected and adequate sporulation suggest that a less sensitive population would result in more disease in tebuconazole treated fields. In conclusion, this study revealed the variability associated with the South African R. secalis population regarding virulence spectrum and genotypic structure. The data in this study suggest that it is likely that the local population will easily adapt to newly introduced, single gene resistance. For more durable resistance, higher levels of quantitative resistance should be introduced. This type of resistance is, however, more difficult to identify and incorporate than single gene resistance. Consequently, barley scald control will remain dependent on the efficacy of fungicide applications. Furthermore, the lack of cross-resistance and low frequency of resistant isolates indicates a low risk for the development of fungicide resistance in the local R. secalis population. Other factors such as current crop rotation practices and the range of fungicides being ~pplied also contribute to this low risk level. However, the status of these factors can change over time. The in vivo tebuconazole sensitivity study has indicated that a resistant field population of R. secalis may be able to build-up. It is, therefore, necessary to monitor the fungicide sensitivity of R. secalis isolates at timely intervals with view to successful barley cultivation in the future. / AFRIKAANSE OPSOMMING: Blaarvlek op gars (Hordeum vulgare), veroorsaak deur Rhynchosporium secalis, is die belangrikste siekte van gars in die Wes-Kaap provinsie van Suid-Afrika. Die voorkoms van R. secalis op gars is in Suid-Afrika vir die eertse keer in 1937 gerapporteer. Hierdie studie is die eerste poging tot karakterisering van die plaaslike R. secalis-populasie. Aspekte soos proteïene betrokke by patogenese, virulensiespektra, variabiliteit van patotipes, bronne van variasie, gasheerweerstand, teeltprogramme, molekulêre karakterisering en swamdodersensitiwiteit word in Deel I van die tesis opgesom. In die daaropvolgende gedeelte is die fokus op die karakterisering van die R. secalis-populasie en behels DNA karakterisering, virulensiespektrum, en swamdodersensitiwiteit in vitro asook in vivo. .. In Deel 2 is die virulensiespektra van 50 R. secalis isolate van 'n populasie in die. Wes-Kaap geëvalueer teenoor 17 differensiëel weerstandbiedende gars kultivars en hieruit is 21 rasse geïdentifiseer. Die twee mees algemene rasse (rasse 4 en 7), met onderskeidelik drie en vier virulensie gene, het virulent vertoon teenoor die mees vatbare kultivars soos West China, Steudelli, C.I.8618 en C.I.2226. Geen vatbare kultivar-patogeen interaksies is met kultivars Atlas 46, Turk en C.I.3515, wat al drie die Rh-Rh3-Rh4 kompleks dra, gevind nie. Dit wil dus voorkom asof hierdie genekompleks effektiewe gasheerweerstand teen die plaaslike R. secalis isolate kan bied. 'n Totaal van 20 rasse (47 isolate), gekarakteriseer in Deel 2, is geselekteer vir verdere karakterisering met behulp van DNA bandpatrone. In Deel 3 is 'n anonieme multilokus DNA peiler gebruik om deur middel van RFLP analise die genotipiese struktuur van hierdie R. secalis-isolate te bepaal. Geen assosiasie is gevind tussen DNA bandpatroon en ras, distrik, garsland of letsel nie. Die twee rasse (4 en 7) wat mees algemeen voorkom, het nie dieselfde bandpatroon vertoon nie, ook nie dié afkomstig vanuit dieselfde garsland of letsel nie. Die genotipiese diversiteit van isolate was 46.5% van die teoretiese maksimum diversiteit. Die hoë vlak van variasie waargeneem in die R. secalis populasie is soortgelyk aan variasie waargeneem in ander graanpatogene wat oor 'n geslagtelike stadium in die lewenssiklus beskik. Alhoewel geen geslagtelike stadium tot dusver geidentifiseer is nie, dui die vlak van variasie daarop dat geslagtelike rekombinasie moontlik wel plaasvind binne die plaaslike R. secalis populasie. In Suid-Afrika word blaarvlek op gars primêr deur swamdoders beheer. Die toenemende gebruik van swamdoders op graangewasse veroorsaak moontlik 'n opbou van swamdoderweerstand in die populasie. Dit kan die effektiwiteit van swamdoders verlaag. Hierdie veronderstelling is in Deel 4 ondersoek, waar die sensitiwiteit van isolate in vitro teenoor triadimenol, tebukonasool, flusilasool en propikonasool geëvalueer is. Die triasooi sensitiwiteit van R. secalis isolate wat gedurende die 1993- 1995 seisoen versamel is het gewissel en slegs vir triadimenol was daar 'n tendens na meer weerstandbiedenheid. 'n Swamdoder-evaluasie is in 'n aparte eksperiment op isolate gedoen wat versamel is vanaf twee garslande met 'n 5-6 jaar geskiedenis van triadimenol saadbehandelings en tebukonasool bespuitings. 'n Betekenisvolle positiewe korrelasie is waaJ~geneem tussen tebukonasool en triadimenol sensitiwiteit in R. secalis isolate afkomstig vanaf hierdie twee garslande. 'n Soortgelyke korrelasie is egter nie gevind in die populasie wat gedurende die 1993-1995 seisoene versamel IS me. Laasgenoemde kan moontlik toegeskryf word aan korter wisselboupatrone en die toediening van 'n verskeidenheid van swamdoders. In 'n derde eksperiment is die sensitiwiteit van plaaslike R. secalis isolate teenoor twee nuwe triasole, naamlik bromukonasool en tritikonasool getoets. Die korrelasie waargeneem tussen die twee nuwe triasole en triasooi swamdoders reeds voorheen in gebruik in die Wes-Kaap was me betekenisvol positief me. Die gebrek aan betekenisvolle kruisweerstandbiedendheid het belangrike praktiese implikasies vir die bestuur van swamdoder -weerstandbiedendheid. In Deel 5 is isolate met wisselende minimum inhiberende konsentrasies (MIKs) teenoor tebukonasool in vitro (1, 3 en 10 ug/ml) en in vivo vergelyk. Die doel van hierdie studie was om te bepaal hoe wisselende MIK-waardes virulensie (blaaroppervlakte geïnfekteer) en sporulasie sal beïnvloed. Resultate dui daarop dat alle R. secalis isolate in hierdie studie ewe fiks was om, in die afwesigheid van tebukonasool, letsels te induseer en te sporuleer. Die bevinding is dat die verlies in fiksheid nie geassosieer is met die mate van tebukonasool weerstand nie. Alle R. secalis isolate het die vermoë gehad om letsels op tebukonasool-behandelde blare te veroorsaak maar het betekenisvol verskil ten opsigte van die blaaroppervlakte geaffekteer. Isolate wat minder sensitief (MIK = 10 ug/rnl) teenoor tebukonasool in vitro is, het meer aangepastheid op tebukonasool-behandelde blare getoon. Gevolglik het hierdie isolate herhaaldelik meer letsels veroorsaak as sensitiewe isolate (MIK = 1 ug/ml), Sporulasie het nie betekenisvol verskil tussen isolate vanaf letsels op ondehandelde of tebukonsoolbehandelde blare nie. Hierdie resultate dui egter daarop dat 'n minder sensitiewe populasie tot meer siektevoorkoms in tebukonasool-bespuite lande kan lei. Die studie het die veranderlike karakter van die Suid-Afrikaanse R. secalispopulasie aangaande virulensiespektrum en genotipiese struktuur blootgelê. Dit is dus baie moontlik dat die R. secalis-populasie maklik sal aanpas by teelmateriaal met nuwe enkelgeen-weerstand. Vir volgehoue gasheerweerstand is dit egter nodig dat hoër vlakke van kwantitatiewe weerstand ingeteel moet word. In die praktyk is hierdie tipe weerstand egter baie moeiliker om te identifiseer en by nuwe teelmateriaal in te sluit as in die geval van enkelgeen-weerstand, Dit bring mee dat blaarvlekbeheer afhanklik bly van swamdodertoedienings as beheermaatreël. Die resultate van hierdie studie dui daarop dat daar tans 'n lae risiko vir die ontwikkeling van swamdoderweerstand in die plaaslike populasie is, as gevolg van die afwesigheid van kruisweerstandbiedendheid en die lae voorkoms van weerstandbiediende isolate. Ander faktore soos die wisselboustelsels wat toegepas word en die verskeidenheid van swamdoders toegedien dra ook daartoe by. Ten spyte hiervan kan die status van hierdie faktore egter oor tyd verander. Die in vivo tebukonasool studie het daarop gedui dat 'n weerstandbiedende veldpopulasie van R. secalis die potensiaal het om te vermeerder. Gevolglik is die tydige monitering van swamdodersenisitiwiteit van R. secalis isolate noodsaaklik om 'n volhoubare garsproduksie te verseker.