Oak wilt, caused by the ascomycete fungus Bretziella fagacearum, is a lethal vascular disease affecting Fagaceae hosts, primarily oaks (Quercus spp.), in both forest and urban settings in the United States. B. fagacearum is vectored by sap beetles (Coleoptera: Nitidulidae) when spore-laden insects visit fresh wounds on healthy trees. The pathogen also spreads through functional root grafts. Oak wilt has been confirmed in 24 states in the U.S., with widespread occurrence in the Midwest and Texas. This dissertation investigates various aspects of oak wilt, including epidemiology, biology, diagnosis, complete genome analysis, and population genetics of B. fagacearum. The epidemiology of oak wilt was studied by inoculating mature northern red oaks (Q. rubra) at three field sites in northwest Michigan with a conidial suspension (2 x 106 conidia/ml) at 4-week intervals from March to November. Inoculated trees were evaluated to assess disease progression at 15-day intervals. Trees inoculated from March to September were succumbed to infection, with rapid disease progression in June and July. Inoculations in October and November did not result in infection. The progression of oak wilt was correlated with predicted sap flow rates, indicating that seasonal sap flow variations, influenced by sapwood development, affect disease incidence and progression. These findings provide valuable information on key risk periods for infection, which can be used to refine management guidelines to prevent tree injury. The biology of B. fagacearum was studied by monitoring sporulation on infected northern red oaks at 12 field sites in northwest Michigan, from March to November 2018-2020. Mycelial mats (sporulating structures) were categorized into eight developmental and morphological stages, with significant variation in colony-forming units among the stages. This visual rating system helps distinguish between infectious and non-infectious mats and assess the risk of above-ground infection, providing critical information for arborists and forestry professionals. The study identified two peaks of mycelial mat production, one in early summer (May and June) and another in early fall (August and September). B. fagacearum detection from diseased samples was enhanced through the optimization and validation of a TaqMan real-time PCR assay, offering greater specificity and sensitivity than traditional methods. A non-destructive sampling method for detecting B. fagacearum in red oaks and chestnuts was also developed and validated. Best practices for traditional, and non-destructive sampling were streamlined for culture-based detection method. The complete genome of B. fagacearum was sequenced and annotated, revealing a total length of 27,072,536 bp and 7,554 predicted proteins. This genome resource providing a crucial reference for population studies and insights into the molecular epidemiology and biology of the pathogen. Population genetic analyses using whole genome resequencing of 93 isolates from 15 states identified significant stratification, revealing four distinct genetic clusters corresponding to specific geographical regions: Upper Midwest, Mid-Atlantic and Southeast, Michigan, and Texas. The Upper Midwest served as a center of genetic diversity with potential gene flow among populations, while Texas isolates formed a highly distinct cluster, suggesting potentially limited gene flow and local adaptation. The first report of B. fagacearum infecting orchard-grown chestnut trees in Michigan underscores the need for constant vigilance and monitoring in chestnut orchards to promptly detect and manage potential infections. This dissertation also includes detailed methods for diagnosing oak wilt, B. fagacearum identification, storage and assays for pathogenicity trials in non-lignified seedlings and mature trees.
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