Abstract
Fourteen plant viruses transmitted by fungi have been recorded in several plants in Japan. Two orders of fungi, the Plasmodiphorales and the Chytridiales are vectors of barley yellow mosaic, wheat yellow mosaic, rice necrosis mosaic and soil-borne wheat mosaic viruses (Polymyxa graminis), beet necrotic yellow vein virus (P. betae) potato mop-top virus (Spongospora subterranea), tobacco necrosis, tobacco necrosis satellite, tobacco stunt, and lettuce big vein viruses (Olpidium brassicae), melon necrotic spot virus (O. cucurbitecearum) and pea stem necrosis virus (Olpidium spp.). Broad bean necrosis and Satsuma dwarf viruses may have a soil-inhabiting fungal vector, but no species has been comfirmed to be a vector hitherto. These fungi are obligate root parasites and their life cycles are described. The fungi infected their hosts through zoospores that carry the virus and introduce it into plants susceptible to the vector fungus. Plant viruses may be carried on the outer surface of the fungi, as in the case of tobacco necrosis, tobacco necrosis satellite and melon necrotic spot viruses, where specificity may be due to the adsorption of virus particles. Alternatively they may be carried inside the fungus, as in the case of barley yellow mosaic, wheat yellow mosaic, rice necrosis mosaic, soil-borne wheat mosaic, beet necrotic yellow vein, lettuce big vein, tobacco stunt and potato mop-top viruses, persisting for many years in viable resting sporangia, which can withstand total soil desiccation.
