Pycnidial fungi of the genus Ampelomyces are widespread intracellular mycoparasites of powdery mildew fungi worldwide. Their pycnidia are produced in hyphae, conidiophores and immature ascomata of their mycohosts. Thus, they suppress both the asexual and the sexual reproduction of the invaded powdery mildew mycelia, and then destroy them completely. Conidia of Ampelomyces are released from the intracellular pycnidia by the rupture of the pycnidial wall; conidia then germinate on the host plant surfaces, penetrate the intact hyphae of powdery mildew mycelia found in their vicinity and invade them internally growing from cell to cell through the septal pores of the mycohost. The early stage of mycoparasitism is apparently biotrophic, but the invaded cytoplasm then begins to die and a necrotrophic interaction results. Toxin production has not been detected in Ampelomyces, so it might act directly by invasion and destruction of the host cytoplasm. Experimental data showed that parasitized powdery mildew colonies can continue their growth, but their sporulation is stopped soon after Ampelomyces penetrated their mycelia. It is concluded that these mycoparasites represent a stress factor in the life cycle of their mycohosts but their role in the natural control of powdery mildew infections requires further investigations.
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