Evidence for the involvement of the soil microbiota in the exclusion of Fusarium from coniferous forest soils

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Abstract
Fusarium was recovered from only 1 of 14 Pacific Northwest [USA] coniferous forest soils but from all 7 nursery soils tested. Assay using high concentrations of F. oxysporum macroconidia added to soil determined that forest soils stimulate macroconidial germination (.hivin.x = 12%) often followed by germ-tube lysis or the formation of stunted chlamydospores, while nursery soils support little macroconidial germination (.hivin.x = 1.5%). Treatment of forest soils with aerated steam (minimum of 45.degree. C for 30 min), radiation or propylene oxide reduced the amount of germination in forest soils, suggesting that the forest soil microbiota is involved in the stimulation of macroconidial germination. Bacteria isolated from F. oxysporum hyphae placed in forest, field, or nursery soils on nylon screens did not significantly influence germ-tube lysis when combined with macroconidia in a nutrient broth. Chemical, physical and nutritional properties of forest soils likely affect the capacity of the forest soil microbiota to influence Fusarium macroconidial germination and germ-tube lysis.