Soil suppressiveness and functional diversity of the soil microflora in organic farming systems

Arable fields of 10 organic farms from different locations in The Netherlands were sampled in three subsequent years. The soil samples were analysed for disease suppressiveness against Rhizoctonia solani AG2.2IIIB in sugar beet, Streptomyces scabies in radish and Verticillium longisporum in oilseed rape. In addition, a variety of microbial, chemical and physical soil characteristics were assessed. All data were correlated by multiple regression and multivariate analyses with the objective to find correlations between soil suppressiveness and biotic or abiotic soil characteristics. Significant differences in soil suppressiveness were found between the fields for all three diseases. Multiple regression indicated a significant correlation between suppressiveness against Rhizoctonia and the number of antagonistic Lysobacter spp., as well as with % active fungi and bacterial diversity. Grass-clover stimulated Rhizoctonia suppression as well as the presence of antagonistic Lysobacter spp. (mainly L. antibioticus and L. gummosus) in clay soils. Streptomyces suppression correlated with the number of antagonistic Streptomyces spp., % of active fungi and bacterial population size. The presence of antagonistic Streptomyces spp. correlated with a high fungal/bacterial biomass ratio. Verticillium suppression was only measured in 2004 and 2005, due to the inconsistent suppressiveness along the years. Nevertheless, a significant correlation with pH, potential nitrogen mineralization and bacterial biomass was found. Bacterial and fungal PCR-denaturing gel electrophoresis fingerprinting of bacterial and fungal communities, in general, did not significantly correlate with disease suppression. Highly significant explanatory factors of the composition of the dominating bacterial and fungal populations were % lutum, pH, C/N quotient, biomass and growth rate of bacteria. Additionally, the % of organic matter and years of organic farming were explaining significantly the composition of the bacterial population.Thus, significant correlations between several soil characteristics and suppressiveness of different soil-borne pathogens were found. For two of the three pathogens, suppression correlated with biotic soil characteristics combined with the presence of specific bacterial antagonists. Probably the soil suppressiveness measured in the organic fields is a combined effect of general and specific disease suppression.