Ultrastructure of Fungal Plant Virus VectorsPolymyxa graminisin Soilborne Wheat Mosaic Virus-Infected Wheat andP. betaein Beet Necrotic Yellow Vein Virus-Infected Sugar Beet

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Abstract
The association of P. graminis with the viruses it transmits is thought to be internal; zoospores of Polymyxa do not acquire soil-borne wheat mosaic virus (SBWMV) in vitro and cystosori of Polymyxa in dry SBWMV-infected roots remain able to transmit SBWMV to wheat seedlings for a long period of time. The ultrastructure of various stages in the life cycles of P. graminis and P. betae was examined in SBWMV-infected wheat and beet necrotic yellow vein virus(BNYVV)-infected sugar beet roots, respectively. Although particles of both viruses were observed in close contact with the fungal vector in the host tissue, they were always outside the fungal structures. Owing to the density of the fungal cytoplasmic contents, virus particles were not seen within fungal plasmodia, zoosporangia or cystosori.