The general aim of this research was to analyse the genetic diversity in Sclerotinia species. More specifically the aims of this research were: to separate the three species of Sclerotinia, S. sclerotiorum (Lib.) de Bary, S. minor Jagger and S. trifoliorum Erikss.; to determine the breeding mechanism in S. minor and S. sclerotiorum; to test S. minor for the possibility of causing head rot of sunflower; to examine isolate of S. sclerotiorum from sunflower for aggressiveness and to see if this correlates with underlying genetic diversity. Sclerotinia species were separated using a variety of morphological and molecular criteria. S. minor, S. sclerotiorum and S. trifoliorum were analyzed on characters including host, sclerotial diameters, ascospore morphism, breeding type and RFLP analysis. Cloned DNA fragments from S. sclerotiorum were used as probes, these were compared with a cloned rDNA probe from Neurospora. These probes enabled clear separation of the Sclerotinia species. Sclerotial diameters appear to be good criteria for separating S. minor from S. sclerotiorum and S. trifoliorum. Host species may be sufficient criteria for separating S. sclerotiorum and S. trifoliorum for the plant pathologist in the field, however it was inadequate to accurately separate all isolates. S. minor and S. sclerotiorum were found to be homothallic ascomycetes. Apothecia were raised from all eight ascospores of a single tetrad from four isolates of S. minor and from an isolate of S. sclerotiorum indicating that inbreeding may be the predominant breeding type mechanism of S. minor. Ascospores from asci of S. minor and S. sclerotiorum were predominantly monomorphic, but rare examples of ascospore dimorphism similar to S. trifoliorum were found. Ascospores of S. minor were shown to be capable of causing head rot of sunflower (Helianthus annuus L.) when inoculated onto the floral face during anthesis. This is the first record of the carpogenic germination of S. minor in Australia and demonstration of infectivity of the ascospores on sunflower. Isolates of S. sclerotiorum differ significantly in aggressiveness on sunflower. One hundred and twenty isolates were collected from head and basal stem rots of sunflower in two locations in south east Queensland. The inoculation of sunflower stems with mycelial plugs and the measurement of lesion development were found to be reliable and revealed differences in lesion lengths produced by the different isolates. The time of assessment after inoculation was found to be of significance. Assessment two days after inoculation was more reliable than after three days or the linear rate of lesion development. The significant differences between isolates indicated that the pathogenicity testing method would also be good for virulence testing. The significant differences between the isolates however, was not consistent with repetition and division of the isolates into groups with different aggressive levels was therefore not possible. Differences in aggressiveness were more indicative of a continuous variation in pathogenicity rather than discrete aggressive groups. The number of significantly different isolates was most associated with the statistical test employed. The different multiple comparison procedures used made more difference in interpretation of aggressiveness than the data itself. Isolate aggressiveness did not correspond to the location of collection. Isolates collected from both head and basal stem rots were capable of causing stem infections therefore no specificity for mode of reproduction or infection was found. S. sclerotiorum attacking sunflower in Queensland and New South Wales was found to belong to one large population. Hierarchical sampling only detected one example of a plant lesion infected by more than one genotype. Thus most diseased plants are the result of a single infection only. Population substructuring could not be detected using 11 single copy Restriction Fragment Length Polymorphism (RFLP) loci, suggesting gene flow occurs within the Australian population. Mycelial Compatibility Groups (MCGs), Random Amplified Polymorphic DNAs (RAPDs) single and multicopy RFLPs analysis indicated differences amongst the genotypes identified by each criteria. From 120 isolates the number of genotypes ranged from 13 to 24 depending on the marker used. However there were many similarities in the assemblages of isolates within each genotype. Genotypic diversity could not be correlated with aggressiveness. Initial mode of infection could not be correlated with genotypic differences. Genotypes were also not specific to geographic locations around Australia. However, genotypes identified in Australia were unique from genotypes identified in Canada and United States. Temporal studies also indicated a single population as genetic uniformity was maintained between years. Frequent recovery of the same genotypes around Australia suggests a clonal population structure. The Australian S. sclerotiorum population attacking sunflower appears to have a large asexual component most likely due to the sclerotial production and homothallic sexual reproduction. Gametic disequilibrium was found for all the populations. However, clonal correction of the samples meant that the majority of populations were not at gametic disequilibrium, indicating random associations among loci. Therefore genetic exchange and recombination would appear to be a component of the reproductive cycle of S. sclerotiorum in Australia.
| Identifer | oai:union.ndltd.org:ADTP/253789 |
| Creators | Ekins, Merrick Grindon |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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