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In this article we will discuss about the two examples of mycoviruses: 1. Mycoviruses of Mushrooms 2. Mycoviruses in Plant Patliogenic Fungi.
1. Mycoviruses of Mushrooms:
At least six viruses and VLP have been reported from the cultivated mushrooms, A.bisporus nearly from all countries where it is grown widely. The mycoviruses occur in a mixture of cells and are extremely hard to separate. So far it is unknown about what effects the individual myco-virus causes?
In some laboratory it could be demonstrated that the presence of viruses in sporophores of mushrooms resulted in reduction in crop yield, and decreased in mycelial growth on malt agar of cultures taken from the sporophores, whereas in some other it was disputed.
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In recent years, there has been many reports of normal yields and mycelial growth from virus infected mushroom crops with the suggestions that the mycoviruses are not pathogenic.
The possible reasons for the conflicting reports are:
(a) The presence of morphologically indistinguishable viruses of differing virion size in a single fungal cell,
(b) Existence of variants within one specific virus that contain additional (or fewer) dsRNA segments associated with pathogenicity. The number and size of dsRNA segments vary in different mushroom viruses. Totally, intracellular life of mycoviruses encourage the persistance of such variants. These combined presence resulted in disease, and
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(c) The development of tolerance against infection in present day genotypes than the former races of mushroom. This would have been through the suppression of virus replication and through the production of a mycotoxin (patulin) in several species of Penicillium and Aspergillus. Under certain conditions one or more mycoviruses may exist as DNA proviruses that can initiate rapid virus replication and the associated disease.
2. Mycoviruses in Plant Patliogenic Fungi:
Due to the presence of mycoviruses in pathogenic fungi, the virulence of pathogens gradually declines resulting in even death of fungi. Fungus isolates of take all of cereals (caused by Gaeumannomyces graminis) containing only one kind of VLPs were mostly more pathogenic than virus free isolate, whereas the isolates with both kinds of virus particles tended to be less^pathogenic than either of the other two classes.
In addition, a highly pathogenic isolate of G. graminis from wheat roots gradually lost the virulence over a period of 17 months in culture. In virulent isolates no viruses could be detected. After a few months, 35 nm virions and later on 26 nm virions were observed in increasing quantities resulting in gradual loss in pathogenicity of the fungus.
In Helminthosporium victoriae, the cause of Victoria blight of oat, two serologically unrelated mycoviruses designated as 190S and 145S from their sedimentation value occurred in hypo-virulent cultures. Hypo-virulence could be transferred to normal culture by hyphal anastomosis. From more than 40 plant pathogenic fungi, mycoviruses have been reported but no consistent correlation with hypo-virulence could be established.
Dodds (1980) has given a strong circumstantial evidence that the cytoplasmically transmissible hypo-virulence in chestnut blight fungus (Endothia parasitica) is correlated with the presence of dsRNA segments, and in at least one strain, with usual club shaped particles upto 300 nm long.
