Use of non-pathogenic or hypovirulent fungal strains to protect plants against closely related fungal pathogens

Nonpathogenic (avirulent), or low virulent (hypovirulent) strains are capable of colonizing infection site niches on the plants' surfaces and protecting susceptible plants against their respective pathogens. Such phenomena have been demonstrated for a considerable number of plant pathogens. The modes of protection differ among the nonpathogenic strains, and one strain can protect by more than one mechanism. Competition for infection sites, or for nutrients (such as carbon, iron) as well as induction of the host plant resistance, have been demonstrated for several pathogens such as Rhizoctonia spp., Fusarium spp. and Pythium spp. Mycoparasitism was shown for Pythium spp. Transmission of double stranded RNA mycoviruses from hypovirulent strains to virulent strains renders the virulent strains hypovirulent. Chestnut trees infected with the chestnut blight pathogen, Cryphonectria (Endothia) parasitica, recovered after inoculation with transmissible hypovirulent strains. Nonpathogenic strains of various fungi are potential candidates for development of biocontrol preparations. Some strains are already used in Agriculture.