The virulence factor in all known cases are biosynthesized by the pathogenic fungus. In this case of the symbiosis between "R. microsporus" and "B. rhizoxinica", the hosted bacteria population produces the causative agent of rice seedling blight. Toxin formation by the bacteria has been demonstrated in analogy with Koch's postulates through the discovery that rhizoxin-producing strains of "R. microsporus" contained symbionts. Removal of the symbionts from the host degraded rhizoxin production and the symbionts were then grown in pure culture. Lastly, the re-introduction of the bacteria grown in pure culture back into the host reestablished rhizoxin production.