Organic transition effects on soilborne diseases of soybean and populations of pseudomonadaceae

During transition to organic production, various strategies can be implemented to enhance soil health, including the soil property of disease suppressiveness. We previously found increased levels of diseases caused by biotrophic pathogens associated with manure application, but manure also suppressed diseases caused by necrotrophic pathogens. In an extension of that study we evaluated soils from di erent cropping system and organic amendment treated plots using a bioassay of suppressiveness to two soilborne diseases, Rhizoctonia root rot and sudden death syndrome (SDS) of soybean [Glycine max (L.) Merr.]. Also, the soil population levels of Pseudomonadaceae were assessed for their suitability as an indicator of disease suppressiveness. Levels of soil suppression of both Rhizoctonia root rot and SDS were found to be associated with specific cropping systems, but the associations were not consistent from year to year. ­ ere were no associations between organic amendment treatments and disease suppression in the bioassay studies. However, there was an overall increase in soil suppressiveness to Rhizoctonia solani during the 3 yr of transition, regardless of cropping system or organic amendment treatments. In contrast, there was an overall decrease in suppressiveness to Fusarium virguliforme during the transition. ­ ere was no significant effect of cropping system or organic amendment treatments on the levels of Pseudomonadaceae, indicating that this was not a good predictor of general suppression in this study. While specific treatments were not found to consistently impact soil suppressiveness, the overall increase in suppressiveness to R. solani during the 3-yr transition did show the value of the transition process.