Graduate Thesis Or Dissertation
 

Biology and control of Fusarium spp. on Douglas-fir (Pseudotsuga menziesii)

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  • Fusarium spp. are among the most important pathogens of Douglas-fir (Pseudotsuga menziesii) seedlings in bareroot nurseries. Currently the most effective disease control method is soil fumigation with methyl bromide. As methyl bromide is phased out, however, other disease management strategies may become more important. We explored two alternative approaches to management of Fusarium diseases in Douglas-fir seedlings. The first approach was to transfer certain components of conifer forest soil, where Fusarium usually does not occur, to nursery soil, where Fusarium is often abundant. Specifically, we tested two hypotheses: (1) conifer forest soil contains a greater proportion of bacteria antagonistic in vitro to Fusarium than does nursery soil, and (2) pine needles, humic acid, ectomycorrhizal fungi, or bacteria obtained from forest soil will protect Douglas-fir seedlings against disease caused by soilborne Fusarium spp. if added to nursery soil. We did not find conclusive evidence to support either hypothesis. The second approach was to investigate the potential for biological control of seedborne Fusarium. Previous workers have shown, and we have confirmed in our studies, that Fusarium present on a small percentage of Douglas-fir seeds before cold stratification may spread during stratification; we recovered Fusarium from nearly all stratified seeds, representing three seedlots, plated on three culture media. When these seeds were planted in pasteurized soil, however, seedborne Fusarium did not cause significant disease in seedlings. Biological control of seedborne Fusarium during stratification may be important both to reduce the contribution of seedborne Fusarium to nursery soil and to protect germinants from soilborne Fusarium. We hypothesized that Pseudomonas chlororaphis isolate RD31-3A, a fluorescent pseudomonad previously shown to control Fusarium on Douglas-fir, would reduce the spread of Fusarium during stratification and protect seedlings from damping-off when seeds were planted into soil infested with pathogenic F. oxysporum. High populations of RD31-3A were recovered from seeds after stratification, and bacterial treatment reduced the spread of Fusarium without affecting seed germination or subsequent seedling dry weights. RD31-3A did not, however, protect seedlings from soilborne F. oxysporum. Nevertheless, seed treatment with bacteria during stratification may be an efficient way to deliver biological control agents to conifer seeds.
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