| Summary: | <p>Abstract</p> <p>Background</p> <p><it>Sclerotinia sclerotiorum </it>is a notorious plant fungal pathogen which spreads across the world. Hypovirulence is a phenomenon where the virulence of fungal pathogens is decreased, even lost, due to mycovirus infection. The potential of hypoviruses for biological control of the chestnut blight fungus (<it>Cryphonectria parasitica</it>) has attracted much interest, and has led to discovery of new hypovirulent strains in other fungi.</p> <p>Results</p> <p>A hypovirulent strain, strain XG36-1, was isolated from a typical lesion on the stem of rapeseed (<it>Brassica napus</it>) caused by <it>Sclerotinia sclerotiorum</it>. Strain XG36-1 grew on PDA very slowly (average 2.5 ± 0.1 mm/d) with sectoring, and developed abnormal colony morphology with few sclerotia. Unlike health strains (such as wildtype strain XG-13), it was unable to induce lesions on detached leaves of rapeseed. Sclerotia of strain XG36-1 produced apothecia rarely. A sexual progeny test showed that the phenotypes of all 104 sexual progeny were not different from wildtype strain XG-13 which shows normal phenotype of <it>S. sclerotiorum</it>, and protoplast regeneration tests showed that 25.5% of the regenerants of strain XG36-1 were recovered fully. Furthermore, the hypovirulence and its associated traits could be transmitted to XG36-1A34<sup><it>R</it></sup>, a hygromycin-resistance gene labelled sexual progeny of strain XG36-1, by hyphal anastomosis. Transmission electron microscope (TEM) observation showed that the cytoplasm of strain XG36-1 was destroyed and granulated; the membranes of nuclei and mitochondria were disintegrated; and mitochondrial cristae were cavitated. Viral particles (about 40 nm) in hyphae of strain XG36-1, but not in its sexual progeny and wildtype strain XG-13, could be observed with TEM, and several virus-like particles were uniquely enveloped by single layer membrane in the cells of strain XG36-1. Furthermore, the viral particles could be co-transmitted with the hypovirulence traits through hyphal anastomosis.</p> <p>Conclusion</p> <p>Hypovirulence and its associated traits of strain XG36-1 could be mediated by a fungal virus. Currently, we could not know the characteristic of this virus, but it likely represent a new type of mycovirus in <it>S. sclerotiorum</it>, and possibly in fungi.</p>
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