The fungal plant pathogen <I>Sclerotinia sclerotiorum</I> (Lib.) de Bary is responsible for disease in a wide range of crops throughout the world. Sclerotinia disease, or cottony soft rot, in carrot (<I>Daucus carota</I> L.) crops is generally a post-harvest problem during storage. Research has, therefore, focused in this area. However, little attention has been paid to foliar and root crown damage caused by <I>S. sclerotiorum</I> during crop development, a problem prevalent in crops in the North East of Scotland. The conventional method for controlling <I>S. sclerotiorum</I> in the developing crop is fungicide application, which often gives only partial control. There is no recognised effective control method in organic systems. This study investigated a diverse array of potential control methods, involving combinations of cultural, biological, environmental and forecasting methods, all lying within the confines of organic regulations. A number of germination characteristics of the <I>S. sclerotiorum</I> sub-population in question were studied. Steam sterilisation of soil was then investigated as a possible replacement for methyl bromide fumigation. Steam was shown to have potential control properties regarding sclerotia of <I>S. sclerotiorum</I>, if combined with manipulation of soil matric potential. Commercially available biological control agents, soil amendments and conventional fungicides were compared in the search for the most effective control for Sclerotinia. The product found to be most effective in this group, and acceptable for use in organic systems, <I>Coniothyrium minitans,</I> was further investigated in combination with organically produced compost and readily soluble nitrogen fertiliser. <I>C. minitans</I> provided a level of control in both instances. The application of high levels of nitrogen fertiliser encouraged severe infections, as did excessive irrigation of crops. A number of methods and practices were unsuccessfully applied, or suggested by the studies, that can reduce the incidence of disease caused by <I>S. sclerotiorum</I> in carrot crops in North East Scotland, and are acceptable to both conventional and organic systems.

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.

Thesis (Ph. D., Plant Science)--University of Idaho, December 2008. / Major professor: Shree P. Singh. Includes bibliographical references. Also available online (PDF file) by subscription or by purchasing the individual file.

Thesis--University of Wisconsin. / eContent provider-neutral record in process. Description based on print version record. Bibliography: leaf 144.

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 93-97). Also available on the World Wide Web.

Thesis (Ph. D., Soil and Land Resources)--University of Idaho, May 2007. / Major professor: Guy R. Knudsen. Includes bibliographical references. Also available online (PDF file) by subscription or by purchasing the individual file.

Thèse (M.Sc.)--Université Laval, 2003. / Titre de l'écran-titre (visionné le 29 mars 2004). Bibliogr.

Sclerotinia stem rot caused by Sclerotinia sclerotiorum (Lib.) de Bary is an economic disease affecting canola (Brassica napus L). Since expression of sclerotinia stem rot symptoms shows much variability and the trait is quantitative in nature, reliable phenotypic evaluation methods for characterization are needed. The three major objectives of this dissertation were to: i) evaluate eight different inoculation methods to discriminate between S. sclerotiorum-resistant and susceptible B. napus germplasm; ii) develop breeding lines with resistance to multiple diseases, and; iii) to identify QTL associated with resistance to sclerotinia stem rot using association mapping (AM). The eight methods evaluated were the detached leaves, detached stems, petiole inoculation (PIT), straw-inoculation, stem-piercing with toothpick, mycelial spray (MSI), petal inoculation and oxalic acid assay. MSI and PIT can better discriminate between the isolates and germplasm. Breeding lines resistance to S. sclerotiorum, Leptosphaeria maculans, and Rhizoctonia solani were developed from a cross between two moderately sclerotinia stem rot resistant plant introductions (PI). F2 seedlings were screened for sclerotinia stem rot using PIT. Surviving plants were self pollinated and their progeny screened again. This process was repeated until the F6 generation. In addition, F5 seedlings were evaluated for their reaction to R. solani and F5 and F6 seedlings for their reaction to L. maculans. Eight lines were identified as moderately resistance to these three pathogens. The genomes of a group of 278 B. napus plant introductions were screened using Diversity Array Technology to detect QTL associated with resistance to sclerotinia stem rot. The population was classified into nine sub-populations and 32 significant markers each explaining between 1.5 and 4.6% of the variation were identified. Blastn search indicates that similar nucleotide sequences are distributed throughout the genomes of B. oleracea, B. rapa, and A. thaliana. Results of these studies suggest the PIT and MSI are reliable screening tools to evaluate materials for resistance to sclerotinia stem rot; materials identified as resistant to S. sclerotiorum were also moderately resistant against R. solani and L. maculans and could be valuable sources for canola improvement programs; and AM allowed us to identify QTL associated with resistance to sclerotinia stem rot.

An algorithm was developed for assessing disease risk and improving fungicide timing for control of Sclerotinia blight of peanut, caused by Sclerotinia minor. A 5-day index (FDI) of disease risk was calculated daily by multiplying indices of moisture, soil temperature, vine growth and canopy density and summing the values for the previous 5 days. Spray thresholds of FDI 16, 24, 32, 40, 48 were compared to a 60, 90, 120 DAP (days after planting) schedule and the standard demand program. Field trials in 1994 indicated that fluazinam (0.58 kg a.i./ha) applied at an FDI of 32 performed similarly to the demand program and was more efficient than the DAP schedule. However, the original FDI 32 algorithm triggered sprays 13 days subsequent to disease onset in 1995, indicating the need for improved vine growth and temperature parameters as well as DAP-dependent FDI thresholds. Results from 1996 and 1997 demonstrated that algorithms with new vine growth and temperature parameters coupled with DAP-dependent thresholds performed as well or better than the original FDI 32 algorithm, demand program, or DAP schedule. Protection intervals of 7 and 14 days improved the performance of iprodione (1.12 kg a.i./ha) while fluazinam provided protection for up to 21 days when applied according to the original FDI 32 algorithm. Planting date was evaluated for its effect on disease and fungicide use patterns. Late planting (20-28 May) delayed disease onset and reduced early season disease incidence three of the four years tested. When averaged across planting dates, the original FDI 32 algorithm performed as well or better than the demand program in 1994 and 1995, as did algorithms utilizing new vine growth and temperature parameters with DAP-dependent thresholds in 1996 and 1997. Chemicals for altering plant architecture were compared to defoliation by corn earworm and leaf spot for suppression of Sclerotinia blight. Chlorimuron (8.8 g a.i./ha) and withholding fungicide for leaf spot control demonstrated the most significant disease suppression and yield improvement. Results show the importance of fungicide timing and plant growth and canopy architecture modification for control of Sclerotinia blight of peanut. / Ph. D.

This work encompasses studies on the development of biocontrol strategies to manage the disease lettuce drop, caused by the fungi Sclerotinia sclerotiorum and S. minor, using the mycoparasitic fungus Coniothyrium minitans, and to better understand interactions during mycoparasitism at the biochemical level. Results from field experiments revealed that two applications of C. minitans at manufacturer recommended rates significantly reduced the incidence of lettuce drop caused by S. sclerotiorum but not by S. minor. Applications of other biocontrol products tested did not significantly reduce disease incidence caused by either pathogen. Sclerotium population studies revealed that soil populations of S. sclerotiorum in lettuce production fields ranged from 0.08 to 2.9 sclerotia/100g of soil and were generally aggregated in their distribution. Continued field studies revealed that there was no significant effect of irrigation (sprinkler vs furrow) on either the impact of sclerotium density or the efficacy of C. minitans. Studies on the evaluation of different application rates of Contans against S. minor revealed that two applications of Contans at 5 X manufactures recommended rates significantly reduced the disease incidence. Examination of sclerotial exudates of Sclerotinia spp. revealed that crude exudates from both Sclerotinia spp. stimulated C. minitans spore germination and the stimulation was due to compounds within the polar fraction. Studies on the role of lectin-carbohydrate binding during fungal-mycoparasite interactions revealed that many plant lectins as well as crude proteins extracted from sclerotia of either Sclerotinia spp. induced agglutination of C. minitans spores in vitro. Spore germination of C. minitans stimulated by sclerotial exudates of either Sclerotinia spp. was significantly inhibited in the presence of the lectin Con A but not other plant lectins. In vitro studies on the directional growth of C. minitans preceding mycoparasitism revealed the involvement of G proteins for optimal response of C. minitans toward Sclerotinia stimulus.