Abstract Mortality of Douglas-fir seedlings caused by Fusarium oxysporum in nursery beds was lessened by soil fumigation and increased by applications of urea fertilizer. Losses were directly related to amount of urea applied (0 to 112 kg N/ha), regardless of timing (before or up to 11 weeks after sowing seed). Mortality rates were temporarily lower in plots inoculated at or shortly after sowing time with basidiospores of the ectomycorrhizal fungus Laccaria laccata than in noninoculated plots. This treatment did not influence subsequent mycorrhizal formation. A factorial experiment showed that growth of seedlings during the first season was increased by presowing addition of 179 kg/ha of P in superphosphate but not by up to 168 kg/ha N from urea or up to 134 kg/ha K from potassium sulfate. Root rot losses were least where nitrogenous fertilizer was withheld during the first growing season. Infected seedlings were stunted but many remained alive. With biweekly applications of nitrogenous fertilizer in the second season, previously stunted seedlings grew nearly as large as seedlings that had remained healthy during the first-year period of susceptibility. These findings imply that (a) further research on inoculation with mycorrhizal fungi should focus upon establishment of the fungal symbiont; (b) through soil fumigation and fertility management, losses of Douglas-fir seedlings in nurseries to Fusarium root rot can be reduced to acceptable levels while quality of the final seedling crop is maintained. Forest Sci. 21:390-399.
Sinclair et al. (Mon,) studied this question.
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