Compost applications vary in their plant growth promotion and plant disease suppression, likely due to differences in
physico-chemical and biological parameters. Our hypothesis was that bacteria are important for plant growth promotion
and disease suppression of composts and, therefore, composts having these traits would contain similar sets of indicative
bacterial taxa. Seventeen composts prepared from five different commercial providers and different starting materials were
classified accordingly with bioassays using cress plants and the pathogen Pythium ultimum. Using a metabarcoding
approach, bacterial communities were assessed in bulk composts and cress rhizoplanes. Six and nine composts showed
significant disease suppression or growth promotion, respectively, but these traits did not correlate. Growth promotion
correlated positively with nitrate content of composts, whereas disease suppression correlated negatively with factors
representing compost age. Growth promotion and disease suppression explained significant portions of variation in
bacterial community structures, i.e. 11.5% and 14.7%, respectively. Among the sequence variants (SVs) associated with
growth promotion, Microvirga, Acinetobacter, Streptomyces, Bradyrhizobium and Bacillus were highly promising, while in
suppressive composts, Ureibacillus, Thermogutta and Sphingopyxis were most promising. Associated SVs represent the basis for developing prediction tools for growth promotion and disease suppression, a highly desired goal for targeted compost
production and application.