Phyllosphere Bacteria-Based Biocontrol Solution Against <i>Botrytis cinerea</i> and <i>Plasmopara viticola</i> in Grapevine

Plants harbor different microbial communities on and in their organs. The differences reflect not only the various environmental conditions to which plant parts are exposed, but also organ-specific expression profiles of genes, proteins, and metabolites. Consequently, the phyllosphere microbiota can differ strongly from root-associated communities. We hypothesized that the grapevine phyllosphere is a valuable source of biocontrol bacteria, whose adaptation to the aerial plant environment may enable them to reach their full protective potential against foliar pathogens. In previous work, we isolated phyllosphere bacteria and showed that many were very effective against such pathogens in vitro, inhibiting mycelial growth and spore development. Here, we investigated the biocontrol ability of these bacteria in leaf disc assays against two foliar pathogens, Botrytis cinerea (gray mold) and Plasmopara viticola (downy mildew). Our results show that 40 strains out of 46 affected at least one pathogen by altering spore physiology, reducing disease progression, and/or stimulating plant defenses. Among these strains, 27 affected both pathogens and 20 also stimulated plant defenses. Because bacterial consortia might perform better than single strains, we compared individual bacteria with associations of up to three strains. When combined, bacteria from the genera Bacillus, Cupriavidus and Herbaspirillum showed improved efficacy in vitro against B. cinerea and in whole-plant experiments against P. viticola, resulting in stronger protection than that obtained with individual strains, likely due to complementary effects on pathogen development and plant defense stimulation. These data suggest that phyllosphere bacteria could provide an effective and sustainable tool for managing foliar diseases.