The control of plant diseases using microorganisms has been the subject of intensive research over recent decades, leading to the identification of numerous beneficial microorganisms and the marketing of a growing number of microorganism-based biocontrol products. To assess whether these products are actually being used, a survey was carried out among French farmers and farm advisors. The survey revealed that biocontrol microorganisms are still poorly used against plant diseases, the main reason being that their efficacy is often considered variable and sometimes unsatisfactory. This instability in protective efficacy may be due to their non-optimal use in the field. Indeed, numerous biotic and abiotic factors can modulate the efficacy of biocontrol agents, which can make their practical use more complex than that of chemicals. It is therefore essential to manage this complexity to optimize their application in the field, and to combine them properly with other protective tools as part of integrated pest management (IPM) strategies. To this end, a database has been set up, bringing together all the information available in the scientific literature on the efficacy factors of biocontrol microorganisms registered in Europe against plant diseases. This database will be used to establish practical guidelines to help farmers choose the most suitable biocontrol products for specific situations, and to obtain indications on the most effective way of using them.