Aeromonas media in compost amendments contributes to suppression of Pythium ultimum in cress

Soil-borne diseases such as damping-off caused by Pythium sp. are responsible for high yield losses in organic vegetable production and are difficult to control. Compost amendments have been shown to improve in many cases survival and growth of plants in soils infested with soil-borne diseases. Yet, not all composts are disease suppressive and suppressiveness of composts can be pathogen-specific. Cress (Lepidium sativum L.) was grown in a standard peat substrate amended either with coco fiber (conducive control) or with composts differing in their suppressiveness towards Pythium ultimum. Bacteria were isolated from the rhizoplane and the most abundant species identified by MALDI-TOF MS. The bacterial composition in the rhizoplane of plants grown in non-suppressive substrates and in the suppressive compost was essentially different. Aeromonas media was the main species isolated from the rhizoplane of plants grown in a suppressive compost, whereas Enterobacter cloacae was the dominating species in the less suppressive compost and in the conducive control. A. media was then added to all substrates to evaluate its role in disease suppression. Addition of A. media improved suppressiveness against P. ultimum in all substrates. The effect was most pronounced in the substrate amended with the compost with low suppressiveness, resulting in levels of suppressiveness comparable to the highly suppressive compost. We conclude that presence of A. media in composts can contribute to disease suppression and might provide a useful marker for qualitative analysis of composts.