Rhizoplane-associated compost bacteria as indicators of compost-mediated disease suppression

Summary

Compost application has been shown to suppress numerous plant diseases. However, suppressiveness varies among composts, and the underlying mechanisms—likely driven by compost microorganisms—remain unknown. To identify bacteria potentially involved in compost-mediated disease suppression, we screened 37 composts for their suppressive activity against the widespread soilborne pathogen Globisporangium ultimum (formerlyPythiumultimum)incress. Additionally,we analysed the bacterial communities in both the composts and the rhizoplane of healthy cress plants grown in compost-amended peat substrate.
While the bacterial community structure of the composts themselves was not associated with disease-suppressive activity, the cress rhizoplane communities showed a significant association (R² = 6.9%, Pseudo-F= 2.6, p < 0.001). Moreover, when focusing only on the compost-derived bacterial amplicon sequence variants (ASVs) in the rhizoplane (excluding those likely originating from the peat substrate), we found that both their number and cumulative relative abundance in the rhizoplane positively correlated with disease-suppressive activity (rho = 0.44, p = 0.006; rho = 0.37, p = 0.024). Further, by comparing the nine most suppressive and the nine least suppressive composts, we identified 137 compost-associated and 242 rhizoplane-associated bacterial ASVs enriched in suppressive composts and in the rhizoplane of cress plants grown in them, respectively. Of these, 23 bacteria were enriched in composts and rhizoplanes. Through an isolation effort from compost and rhizoplane samples, we obtained matching isolates for 45 of the rhizoplane-associated bacterial indicators and six of the compost-associated ones.
These findings suggest that rhizoplane communities and their associated compost bacteria are promising indicators of compost-mediated disease suppression. To further explore their role, the available compost and rhizoplane isolates are currently being screened for disease-suppressive activity in-vitro and in-planta.