Comparative studies on genetic variability and fungicide resistance in Tapesia yallundae

[摘要] ENGLISH ABSTRACT:Eyespot is an important disease of spring wheat (Triticum aestivum L.). Four speciesof Ramulispora are associated with this disease, of which Tapesia yallundae and T.acuformis. are common. This thesis investigates the broader subjects of geneticvariability, reproductive dynamics and fungicide resistance in Tapesia yallundae.Each of the chapters treats specific but related topics. T. yallundae, which is the onlyspecies thus far reported from South Africa, has been associated with yield losses ofup to 50%. To enable the implementation of more accurate and effective controlmeasures, understanding the dynamics of reproduction and the genetics of thepathogen is of utmost importance. Of the many plant disease control measures suchas cultural practices, sanitation, biological control, etc., fungicide application is themost commonly resorted to measure in eyespot control. This thesis investigates thebroader subjects of genetic variability, reproductive dynamics and fungicideresistance of Tapesia yallzll7dae.Fungicide application, however, is not without problems. The pathogen canbuild up resistance to fungicides. The most commonly used fungicides in eyespotcontrol include the benzimidazole carbendazim, triazoles such as flusilazole,tebuconazole, propiconazole, bromuconazole, flutriafol, fenbuconazole, triademinol,and the imidazole, prochloraz. Cases of resistance to the groups listed above havebeen reported. Frequent monitoring for resistance is thus crucial to prevent wastageof fungicide and unnecessary impregnantation of the environment with potentiallyineffective chemicals. In chapter 2 of this thesis 300 isolates of T. yallundae from 15fields were evaluated for resistance against carbendazim, flusilazole, tebuconazole,propiconazole, bromuconazole, flutriafol and fenbuconazole. These results indicatedthat to some triazoles, such as fenbuconazole, a high level of resistance was alreadypresent in field populations.In a sexually reproducing fungus such as T. yallundae, knowledge pertainingto its ability to pass resistance factors to offspring is equally important. Matingstudies were, therefore, also conducted with parental strains that showed signs oftriazole resistance. Three generations were subsequently tested for resistance to fivetriazoles, namely flusilazole, tebuconazole, propiconazole, bromuconazole andflutriafol. Results of this study showed variable sensitivity in progeny, whichindicated quantitative inheritance of resistance to triazoles. Although the sexual stage has not yet been observed in the field in South Africa, this knowledge lays thefoundation for the long-term understanding of the population dynamics of the fungus.The ability of a heterothallic ascomycete population to reproduce sexually isdependent on the availability of its two mating types, MATI-I and MATI-2, theirdistribution, and female fertility amongst other factors. In the UK. the teleomorph iscommonly observed in the field, which is in contrast to the situation in South Africa,where it has only been induced in the laboratory. A comparative study between theSouth African and the UK. populations was therefore undertaken. Isolatesrepresentative of the two populations were mated with tester strains as both spermrecipients and as sperm donors. This allowed the percentage of hermaphrodites to bedetermined. No difference in terms of female fertility was observed between theSouth African and the UK. populations, with both populations showing low effectivepopulation numbers. These data suggested, therefore, that the teleomorph would alsooccur more frequently in South Africa if the climate was more indusive to itsdevelopment.The overall results of this study indicated that eyes pot could still be controlledby means of fungicide application in South Africa. Although a shift in sensitivity wasobserved towards fenbuconazole and flusilazole, no resistance was detected towardscarbendazim. The latter might be due to the absen<.:eof the sexual stage in the field,coupled by the monocyclic nature of the pathogen and sensible fungicide regimes.The absence of T. acujormis makes the disease situation less complicated in terms offungicide application and management. Continuous surveys will have to beconducted, however, to monitor this situation in future.

[发布机构] Stellenbosch University