While the control of soil-borne phytopathogenic fungi becomes increasingly difficult without using chemicals, concern over the intensive use of pesticides in agriculture is driving more environmentally sound crop protection managements. Among these approaches, the use of compost to suppress fungal diseases could have great potential. In this study, a multidisciplinary approach has been applied to characterize microbiota composition of two on-farm composts and assess their suppress and biostimulant activities. The on-farm composting system used in this study was able to produce two composts characterized by an antagonistic microbiota community able to suppress plant pathogens and biostimulate plant growth. Our results suggest a potential role for Nocardiopsis and Pseudomonas genera in suppression, while Flavobacterium and Streptomyces genera seem to be potentially involved in plant biostimulation. In conclusion, this study combines different techniques to characterize composts, giving a unique overview on the microbial communities and their role in suppressiveness, helping to unravel their complexity.

Green waste composts are obtained from agricultural production chains; their suppressive properties are increasingly being developed as a promising biological control option in the management of soil-borne phytopathogens. The wide variety of microbes harbored in the compost ecological niches may regulate suppressive functions through not yet fully known underlying mechanisms. This study investigates alpha- and beta-diversity of the compost microbial communities, as indicators of the biological features. Our green composts displayed a differential pattern of suppressiveness over the two assayed pathosystems. Fungal and bacterial densities, as well as catabolic and enzyme functionalities did not correlate with the compost control efficacy on cress disease. Differences in the suppressive potential of composts can be better predicted by the variations in the community levels of physiological profiles indicating that functional alpha-diversity is more predictive than that which is calculated on terminal restriction fragments length polymorphisms (T-RFLPs) targeting the 16S rRNA gene. However, beta-diversity described by nMDS analysis of the Bray–Curtis dissimilarity allowed for separating compost samples into distinct functionally meaningful clusters and indicated that suppressiveness could be regulated by selected groups of microorganisms as major deterministic mechanisms. This study contributes to individuating new suitable characterization procedures applicable to the suppressive green compost chain.

In the present study, 133 bacterial isolates from 11 composted aromatic plant wastes were selected for their ability to inhibit the mycelial growth of the soil-borne phytopathogenic fungi Sclerotinia minor and Rhizoctonia solani. Successively, a subset of 35 from them were further characterized for their ability to control, in vivo, rocket damping-off caused by the two fungi. Moreover, the isolates were characterized for morphology of the colonies, Gram reaction, siderophore production, P-solubilization and for the presence of antimicrobial lipopeptide genes in the genome. The screening for the in vitro antagonisms showed a mycelial growth reduction ranging between 31.7% and 56.1% for R. solani and 34.4% and 59.4% for S. minor. All the isolates were not able to produce siderophores and some of them were able to solubilize P. The isolates contained two or more of the five lipoproteins coding genes investigated in this study. The most promising isolates were identified at species level by 16S-rRNA partial gene sequence analysis and were grouped in two main clusters related to Bacillus subtilis and Bacillus amyloliquefaciens reference strains. Results indicated that Bacillus isolates from compost are good candidates for application in the biocontrol of cultivated plants.