Strains of Pseudomonas that produce antimicrobial metabolites and control soilborne plant diseases have often been isolated from
soils defined as disease-suppressive, i.e.,
soils, in which specific plant pathogens are present, but plants show no or reduced disease symptoms. Moreover, it is assumed that pseudomonads producing antimicrobial compounds such as
2,4-diacetylphloroglucinol (DAPG) or
phenazines (PHZ) contribute to the specific
disease resistance of suppressive
soils. However, pseudomonads producing antimicrobial metabolites are also present in
soils that are conducive to disease. Currently, it is still unknown whether and to which extent the abundance of antimicrobials-producing pseudomonads is related to the general
disease resistance of common agricultural
soils. Moreover, virtually nothing is known about the conditions under which pseudomonads express antimicrobial genes in agricultural field
soils. We present here results of the first side-by-side comparison of 10 representative Swiss agricultural
soils with a cereal-oriented cropping history for (i) the resistance against two soilborne pathogens, (ii) the abundance of Pseudomonas bacteria harboring genes involved in the biosynthesis of the antimicrobials DAPG, PHZ, and
pyrrolnitrin on roots of wheat, and (iii) the ability to support the expression of these genes on the roots. Our study revealed that the level of soil
disease resistance strongly depends on the type of pathogen, e.g.,
soils that are highly resistant to Gaeumannomyces tritici often are highly susceptible to Pythium ultimum and vice versa. There was no significant correlation between the
disease resistance of the
soils, the abundance of Pseudomonas bacteria carrying DAPG, PHZ, and
pyrrolnitrin biosynthetic genes, and the ability of the
soils to support the expression of the antimicrobial genes. Correlation analyses indicated that certain soil factors such as silt,
clay, and some macro- and
micronutrients influence both the abundance and the expression of the antimicrobial genes. Taken together, the results of this study suggests that pseudomonads producing DAPG, PHZ, or
pyrrolnitrin are present and abundant in Swiss agricultural
soils and that the
soils support the expression of the respective biosynthetic genes in these bacteria to various degrees. The precise role that these pseudomonads play in the general
disease resistance of the investigated agricultural
soils remains elusive.