Plant diseases cause major losses in agriculture and are currently being eradicated with conventional or organic pesticides. Agriculture needs to become more productive as well as more sustainable to face the challenges of a growing population, climate change and other environmental problems. Protecting crops from diseases in an environmentally friendly way is a big challenge. The phyllosphere is the surface of the plant exposed to the atmosphere and it is occupied by microorganisms. The microorganims in the phyllosphere are known to interact with the plant they are inhabiting. It has been shown before that a healthy microbial population on the phyllosphere, a.k.a the phyllosphere microbiome, could help prevent or treat plant diseases and promote plant growth. Furthermore, previous research showed that a greater variety of bacteria in the phyllosphere increases plant productivity. This research aims to better understand the phyllosphere microbiome. This understanding will be used to treat and adjust the phyllosphere microbiome and thereby improve crop production and protection. Mixtures of bacteria that could have a positive effect on the plant, "plant probiotics", will be formulated. These plant probiotics will originate from isolated and cultured phyllosphere bacteria as well as from fermented compost extracts. Finally the effect of these plant probiotics on the phyllosphere microbiome and on plants will be studied.