Involvement of clay type and pH in the mechanisms of soil suppressiveness to fusarium wilt of flax

To assess the influence of clay minerals and soil pH on the degree of soil suppressiveness, 25% (ww) kaolinite, Ca-montmorillonite and illite were added to a fusarium wilt-conducive soil (loam, pH 4.0) and 3 values of soil pH (pH 4.0, 5.0 and 7.0) were obtained by liming. The soil-clay mixtures were sown with wheat; after 6 and 30 weeks, respectively, of wheat growth, soil samples were taken. Their levels of suppressiveness to fusarium wilt of flax were assessed and several characteristics of the soil microflora were determined. After 6 weeks of wheat growth, the addition of either montmorillonite or illite significantly increased the degree of soil suppressiveness to fusarium wilts, but only at pH 7. Neither liming alone nor the addition of kaolinite, with or without lime, had an effect. After another 6 months of wheat growth, the level of suppressiveness had increased in all amended soils. The strongest effect was obtained when kaolinite, montmorillonite or illite was added and the soil pH was raised to 7. The level of soil suppressiveness was correlated with physicochemical characteristics (texture, pH, exchangeable Ca and Mg and EDTA-extractable iron), and biological characteristics (population densities of bacteria, fluorescent pseudomonads and autochtonous Fusarium oxysporum, and the induced respiration of the soil microflora after glucose supply). The results are discussed in relation to the common hypotheses related to factors and mechanisms involved in soil suppressiveness to fusarium wilts.