Investigations were conducted to understand the epidemiology of Fusarium on containerized Douglas-fir seedlings. Types and importance of Fusarium inoculum sources, relationships between seedling infection and symptom production, amounts and types of diseases that occurred throughout typical growth cycles, and the importance of secondary pathogen spread were investigated. Levels of Fusarium on seed could not be used to accurately predict disease incidence within a seedlot. Inverse correlations existed between the amount of Fusarium and Trichoderma on seed. Higher levels of Fusarium were detected on bleach-treated seed, probably because of reductions of Trichoderma populations on treated seed. Fusarium commonly colonized seedling roots without causing disease symptoms. Investigations failed to show spread of Fusarium disease from one container to another. Factors affecting disease symptom expression and implications for control are discussed.
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