Botrytis cinerea
is a necrotrophic phytopathogen able to attack more than 200 different plant species causing strong yield losses worldwide. Many synthetic fungicides have been developed to control this disease, resulting in the rise of fungicide-resistance
B. cinerea
strains. The aim of this study was to identify
Streptomyces
strains showing antagonistic activity against
B. cinerea
to contribute to plant protection in an environmentally friendly way. We isolated 15 Actinomycete strains from 9 different Swiss soils. The culture filtrates of three isolates showing antifungal activity inhibited spore germination and delayed mycelial growth of
B. cinerea
. Infection experiments showed that
Arabidopsis thaliana
plants were more resistant to this pathogen after leaf treatment with the
Streptomyces
filtrates. Bioassay-guided isolation of the active compounds revealed the presence of germicidins A and B as well as of oligomycins A, B, and E. While germicidins were mostly inactive, oligomycin B reduced the mycelial growth of
B. cinerea
significantly. Moreover, all three oligomycins inhibited this fungus’ spore germination, suggesting that these molecules might contribute to the
Streptomyces
’s ability to protect plants against infection by the broad host-pathogen
Botrytis cinerea
.
IMPORTANCE
This study reports the isolation of new
Streptomyces
strains with strong plant-protective potential mediated by their production of specialized metabolites. Using the broad host range pathogenic fungus
Botrytis cinerea
, we demonstrate that the cell-free filtrate of selected
Streptomyces
isolates efficiently inhibits different developmental stages of the fungus, including mycelial growth and the epidemiologically relevant spore germination. Beyond
in vitro
experiments, the strains and their metabolites also efficiently protected plants against the disease caused by this pathogen. This work further identifies oligomycins as active compounds involved in the observed antifungal activity of the strains. This work shows that we can harness the natural ability of soil-borne microbes and of their metabolites to efficiently fight other microbes responsible for significant crop losses. This opens the way to the development of environmentally friendly health protection measures for crops of agronomical relevance, based on these newly isolated strains or their metabolic extracts containing oligomycins.