English: A schematic diagram of the plant holobiont as a complex system, including beneficial plant-associated microbes—the symbiome—and the collective pathogenic microbes—the pathobiome. The beneficial and pathogenic microbes are in continuous antagonism and competition for space and nutrients. When pathogenic microbes prevail, they interfere with normal plant functions, leading to disease development. Co-operative interactions amongst pathogenic microbes may result in more severe disease and multiple infections. The stable, beneficial plant microbiome may be altered to facilitate the development of certain diseases, as observed in the olive knot disease.