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These symptoms often result in serious economic losses. However, abnormal symptoms are occasionally observed in bag cultures, such as the growth of white or fluffy mycelia on the surface of substrate, inadequate or imperfect substrate browning, and malformations of the fruiting body. Complete browning of the exterior surface of the substrate is an important marker that normal fruiting bodies will develop in the following stage of cultivation. In a bag culture, the shiitake mycelium is fully grown in the substrate until brown pigment is produced outside and the substrate becomes stiff. Currently in Japan, about 75,016 t (82% of all shiitake) are produced indoors annually using bag cultures with a sawdust-based substrate. In the 1970s, shiitake cultivation was performed by inoculating mycelium spawn on oak logs however, this labor-intensive method was gradually replaced by indoor cultivation using sawdust substrate supplemented with rice bran. In the USA, dsRNAs have also been observed in shiitake strains, but these appeared to be latent. However, unlike La France disease of the white button mushroom, mycoviruses have not been associated with shiitake diseases because these mycoviruses have commonly been found in healthy fruiting bodies. In the 1970s, viruses that infect the cultivated mushroom Lentinula edodes, or shiitake, were extensively studied in Japan, and three morphologically distinct viruses were detected by electron microscopy. The results suggest that LeV represents a novel family of mycoviruses, found thus far only among the basidiomycetes. As suggested from the genome sequence, AFM revealed that the structure of LeV was linear unencapsidated dsRNA. The clade was placed apart from the Totiviridae and Chrysoviridae families. Based on phylogenetic analysis of the putative RdRp sequence, LeV grouped into a clade with dsRNA found in the basidiomycetes Phlebiopsis gigantea and Helicobasidium mompa. Similarity with coat protein of mycoviruses was not found within the whole sequence. There was a 62-bp intergenic region between ORF1 and RdRp. In addition, a region coding for a NUDIX domain was present in ORF1. ORF1 coded for a hypothetical protein and ORF2 for a putative RdRp, respectively. The genome encoded two open reading frames (ORFs). The 11,282-bp genome of LeV was obtained. Because no virion particles associated with the dsRNA were observed by electron microscopic observation, atomic force microscopy (AFM) observation was chosen for achieving molecular imaging of the virus. Based on the deduced RNA-dependent RNA polymerase (RdRp) sequence, phylogenetic analysis of LeV was conducted. MethodsĪ cDNA library was constructed from a dsRNA purified from the fruiting body of L. In this study, the dsRNA genome of a mycovirus recently found in a shiitake commercial strain was sequenced and its molecular structure was characterized. In the 1970s, mycoviruses were identified that infected the edible mushroom Lentinula edodes (shiitake), but they were not regarded as causal agents for mushroom diseases.