Greater Fusarium wilt suppression after complex than after simple organic amendments as affected by soil pH, total carbon and ammonia-oxidizing bacteria

A field experiment was conducted to compare effects of four types of organic amendments on soil chemical, microbiological and disease suppression characteristics in an organic farm. The amendments were plant-derived fresh compost (C), steer-derived slurry (S), slurry plus dung (SD) and slurry, compost and dung (SCD). We hypothesized that amendments with more easily available carbon sources (S and C) might enhance Fusarium wilt of flax, while more complex amendments with lower carbon availability might suppress the disease. Fusarium wilt development was tested in bioassays with flax growing in Fusarium oxysporum f. sp. lini Snyder & Hansen inoculated soil samples from previously amended plots in a growth chamber, and area under the disease progress curves (AUDPCs) were determined per pot. Soil chemical and microbial analyses were conducted directly on soil samples taken from the field. Total DNA was extracted from soil and amplified with specific primers for eubacteria, Pseudomonas species, ammonia-oxidizing bacteria (AOB) and fungi. The respective microbial compositions were assessed by denaturing gradient gel electrophoresis, and species richness and diversity were calculated from the numbers and intensities of the amplicons on the gels. Fusarium wilt was significantly suppressed in soil from SCD treated plots and enhanced in that from C-treated plots. AUDPC values were negatively correlated with pH only. In stepwise and canonical discriminant analyses soil samples with low and high AUDPC values were distinguished based on pH, total carbon content, and diversity of AOB. It is suggested that AOB could be useful indicators for suppression of soil-borne pathogens.