Virulence spectrum, molecular characterisation and fungicide sensitivity of the South African Rhynchosporium secalis population

[摘要] ENGLISH ABSTRACT:Barley leaf scald, caused by Rhynchosporium secalis, is the most important disease ofbarley (Hordeum vulgare) in the Western Cape province of South Africa. The diseasewas first reported from South Africa in 1937. The present study is the first attempt tocharacterise the South African R. secalis population. Topics such as pathogenesisrelatedproteins, virulence spectra, variability of pathotypes, sources of variation, hostresistance, breeding strategies, molecular characterisation and fungicide sensitivity aresummarised in Part 1 of this dissertation. In succeeding Parts the focus is on thecharacteristics of the local R. secalis population regarding virulence spectrum, DNApolymorphisms, in vitro as well as in vivo fungicide sensitivity. These aspects aretreated 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 17differential barley cultivars. The two most prevalent races, namely races 4 and 7 hadthree and four virulence genes respectively. Both race 4 and 7 were virulent on themost 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.3515which 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. secalisisolates.A total of 20 races (47 isolates) characterised in Part 2 were selected for furthercharacterisation by means of DNA fingerprinting. In Part 3 an anonymous multilocusDNA probe was used to characterise the genotypic structure of these isolates by meansof 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 sharethe same genotypes, even when isolated from the same field or lesion. The genotypicdiversity of the isolates studied was 46.5% of the theoretical maximum diversity. Thehigh level of genotypic variation observed in the South African R. secalis populationresembled the genotypic diversity observed in other cereal pathogens with knownsexual structures. Although no teleomorph has yet been observed, these data suggestthat sexual recombination may operate within the local population of R. secalis.In South Africa barley scald is primarily controlled by means of fungicides. Thecontinued use of fungicides on cereal crops results in the build-up of fungicideresistance in the population, which could lower the efficacy of these compounds. Theseaspects were investigated in Part 4, where isolates (collected during 1993 to 1995) wereevaluated in vitro for sensitivity to triadimenol, tebuconazole, flusilazole andpropiconazole. The sensitivity fluctuated but in 1995 isolates were significantly lesssensitive 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 treatmentsand tebuconazole applications, were evaluated for their fungicide sensitivity. Asignificant positive correlation was observed between tebuconazole and triadimenolsensitivity among,R. secalis populations from these fields. However, such a correlationwas not found within the R. secalis population collected during 1993-1995 whereshorter crop rotation patterns and a range of fungicides was applied. In a thirdexperiment, the fungicide sensitivity of local R. secalis isolates was evaluated towardstwo new triazole fungicides, namely bromuconazole and triticonazole. Correlationcoefficients observed between these new triazoles and those previously applied in SouthAfrica were not significantly positive. The lack of significant cross-resistance hasimportant 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 ofthis study was to determine how MIC values would influence virulence (leaf areaaffected) and sporulation. Results indicated that all isolates were equally fit to inducelesions and sporulate in the absence of tebuconazole. Thus no fitness cost wasassociated with the degree of tebuconazole sensitivity in the present study. All R.secalis isolates were able to induce lesions on tebuconazole treated leaves, but differedsignificantly with respect to the percentage leaf area affected. Isolates, least sensitive(MIC = 10 ug/rnl) towards tebuconazole were more adapted on tebuconazole treatedleaves, being able to repeatedly cause larger lesions than sensitive R. secalis isolates(MIC = 1 ug/rnl), Sporulation was not significantly different between isolates onlesions of untreated or tebuconazole treated leaves. Larger leaf areas affected andadequate sporulation suggest that a less sensitive population would result in moredisease in tebuconazole treated fields.In conclusion, this study revealed the variability associated with the SouthAfrican 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 adaptto newly introduced, single gene resistance. For more durable resistance, higher levelsof 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 isolatesindicates a low risk for the development of fungicide resistance in the local R. secalispopulation. Other factors such as current crop rotation practices and the range offungicides being ~pplied also contribute to this low risk level. However, the status ofthese factors can change over time. The in vivo tebuconazole sensitivity study hasindicated 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 timelyintervals with view to successful barley cultivation in the future.

[发布机构] Stellenbosch University