Viruses remain the least studied amongst all the plant disease-causing target organisms listed for mycorrhizae-mediated biocontrol. The general response of mycorrhizal plants to the presence of viral pathogens is as follows: (a) mycorrhizal plants apparently enhanced the rate of multiplication of viruses in some plants (Daft and Okusanya 1973; Nemec and Myhre 1984), (b) more leaf lesions were found on mycorrhizal plants than on nonmycorrhizal plants (Schonbeck 1978; Schonbeck and Dehne 1979), and (c) the number of AMF spores in the rhizosphere was reduced considerably (Jayaraman et al. 1995; Nemec and Myhre 1984). Enhanced viral multiplication and activity in mycorrhizal plants is speculated to be attributed to higher P levels compared to nonmycorrhizal plants. A similar effect was noted in nonmycorrhizal plants fertilized with P (Daft and Okasanya 1973; Shaul et al. 1999). Some workers found that host plants were more susceptible to AMF colonization following infection by a virus. For example, Schonbeck and Spengler (1979) reported that following the inoculation of mycorrhizal and nonmycorrhizal tobacco (Nicotiana glutinosa L.) with tobacco mosaic virus (TMV), mycorrhizal plants exhibited higher levels of AMF colonization. In contrast, mung bean yellow mosaic bigeminivirus reduced the AMF colonization and yield of mycorrhizal plants (Jayaraman et al. 1995), while lack of response to viral infection by a mycorrhizal host was also demonstrated (Takahashi et al. 1994). Early studies using electron microscopy revealed that mycorrhizae were not viral vectors because virus particles were absent in the AMF hyphae and around arbuscules, suggesting that AMF did not interact with viruses (Jabaji-Hare and Stobbs 1984). Thus, potential for the biocontrol of plant pathogenic viruses using mycorrhizae does not appear to be promising. However, it may be worthwhile to investigate the role of viruses in the reduction of mycorrhizal colonization and related host plant effects.
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