Biological control of lettuce drop: the case study of streptomycetes

The application of biological control agents (BCAs) to manage plant pathogen epidemics is gaining more and more interest in agriculture. However, products available on the market are still few, in spite of the efforts of researchers to find new active microorganisms, and many factors hamper biological control efficacy. In fact, once applied, the BCAs often lack to colonize the substrates or to produce bioactive compounds, i.e. activate their mechanisms of action against pathogens. This might be due to the poor knowledge about their biology, interaction with the host and mode of application. Therefore, it is of major importance to study in depth how to deliver and to favor the antagonists to ensure the expression of their beneficial effects on pathogen control. In our research we studied root endophytic Streptomyces to manage lettuce drop caused by the soil borne fungus Sclerotinia sclerotiorum. Following dual culture assay of a large collection of streptomycetes, S. exfoliatus FT05W and S. cyaneus ZEA17I were chosen because they showed S. sclerotiorum mycelium growth inhibition of 75%, and were used for further experiments in growth chamber and in field. Our objectives were to assess the effects on the pathogen control of different doses and timings of the antagonist application. In growth chamber, when lettuce was sown the same day of pathogen and antagonist co-inoculation in the growing substrate, no reduction in disease incidence (DI) was observed. On the contrary, when a seven-day-interval was used between the substrate co-inoculation with the two microorganisms and lettuce sowing, the antagonists significantly reduced DI. Moreover, we observed a positive effect of antagonist inoculum dose on disease control. S. exfoliatus FT05W had an efficacy of 52% and 54%, while S. cyaneus ZEA17I of 37% and 44%, at lower and higher inoculum dose, respectively. In field experiment, lettuce seedlings were transplanted after antagonist inoculation and S. sclerotiorum was applied at two intervals: the same day seedlings were transplanted or one week later. However, no significant differences on DI were observed between the two treatments. In conclusion, the two streptomycetes showed excellent pathogen inhibition in vitro, and their efficacy was partially mirrored when applied in vivo. We observed that different timing of microbial inoculants application affected their performance. Therefore, it is of fundamental importance to thoroughly investigate the antagonist biology and the host-microorganism interactions to improve their activity as BCAs.