Microbial volatiles in soil contribute to the natural disease suppression of Pythium

• Although suppression of growth and germination of soil-borne pathogens by microbial volatiles (VOCs) has been documented, the role of VOCs in control of soil-borne diseases has not been shown. Based on their ability to diffuse through the soil matrix and contribution to fungistasis we hypothesize VOCs to be involved in natural disease suppression. • We assessed the role of soil volatiles in the suppression of Pythium by simultaneous performing volatile suppression tests and plant performance bioassays . GC/MS analysis and 16S pyrosequencing gave insight in the relationship between bacterial community composition and the release of suppressive volatiles. • Disinfested soils showed reduced production of Pythium suppressing VOCs which coincided with strong reduction in root biomass and increase of disease symptoms in bioassays. In the subsequent year, both VOC suppression and Pythium effects returned to the level of the untreated soils. Sequencing data combined with GC/MS profiles of soil VOCs identified coinciding changes in the occurrence of bacterial taxa and the presence of particular VOCs. This information provides a first indication on the identity of microbes and VOCs involved in suppression of Pythium. • Our data suggest that microbial VOCs form an important factor in the natural buffer against diseases caused by soil-borne pathogens.