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P. graminis is a parasite but is not considered to be a pathogen: its presence in roots of infected plants does not generally produce any overt external symptoms, except sometimes a light yellow-brown discoloration of the cortex under high inoculum potential. It does not appear to disturb the development of the host plant under natural conditions.
In contrast, P. graminis-transmitted viruses induce several severe and consequential diseases on various crops. Major symptoms induced by these viruses are the following. Oat mosaic virus induced a mottling on oat in the first flush of growth in the spring, and a stunting late in the season (Hebert and Panizo, 1975). Barley yellow mosaic virus causes mosaic and stunting in barley, sometimes with the leaves showing complete yellowing with necrotic patches (Inouye and Saito, 1975). Wheat spindle streak mosaic virus causes a mosaic disease of wheat characterized by chlorotic to necrotic spindle-shaped streaks in the leaves, slight stunting, reduced tillering and grain loss (Slykhuis, 1976). Soilborne wheat mosaic virus causes light-green to yellow mosaic and stunting on wheat and barley (Brakke, 1971). Rice necrosis mosaic virus causes mosaic symptoms in rice characterized by spindle shaped yellow flecks and streaks visible on the lower leaves, and necrotic fleck lesions on the basal portions of stems and sheaths in some plants (Inouye and Fujii, 1977). Rice stripe necrosis virus causes bright yellow stripes along leaves, necrosis of the leaves or of the whole plants, dwarfing, crinkling and reduced tillering on rice (Fauquet et al., 1988). Peanut clump furovirus and Indian peanut clump virus cause stunting in groundnut. Younger leaves show chlorotic lesions (mottling, ringspot) whereas all the mature leaves are dark green (Thouvenel and Fauquet, 1981a). In wheat and barley, Indian peanut clump virus was shown to induced symptoms of stunting, chlorosis and necrosis (Delfosse et al., 1999). In sugarcane, peanut clump furovirus induces red leaf mottle symptoms (Baudin and Chatenet, 1988).
Despite some chemicals having been found to be effective against cereal viruses (McKinney et al., 1957; Slykhuis, 1970; Thouvenel et al., 1988), chemical measures are currently not acceptable for economic and ecological reasons (Friedt and Ordon, 1995; Delfosse et al., 1996). Therefore breeding for resistance is currently the best option for protection against cereal viruses transmitted by P. graminis in temperate areas (Adams et al., 1993; Friedt and Ordon, 1995). Numerous studies have been performed or are in progress to screen for resistance and to characterize the resistant genes involved (see, for example, Larsen et al., 1985; Huth, 1989; Himmel et al., 1991; Chen et al., 1992; Giunchedi et al., 1992; Chen and Ruan, 1993; Myers et al., 1993; Bauer and Graner, 1995; Rumjaun et al., 1996). Resistance is often directed against the virus (Kucharek et al., 1974, Larsen et al., 1985; Adams et al., 1987; Himmel et al., 1991), although some wheat cultivars were found to be less susceptible to P. graminis development (Lapierre et al., 1985; Adams and Jacquier, 1994). However, this control method is not applicable to all the viruses transmitted by P. graminis: there was no resistance in nearly 9000 A. hypogaea germplasm lines tested to infection by Indian peanut clump virus under field conditions (Delfosse et al., 1996).
Cultural practices such as the date of sowing (Gates, 1975; Slykhuis, 1975; Huth, 1984; Delfosse et al., 1996), the choice of a suitable crop rotation (Thouvenel et al., 1988; Maraite and Legrève, 1994; Delfosse et al., 1996) and trap-cropping (Delfosse et al., 1997b) are among the methods available for the management of virus diseases transmitted by P. graminis. These methods aim to limit the multiplication of the inoculum potential of P. graminis in soil, or to reduce the infection potential of P. graminis resting spores at the moment of sowing a crop species susceptible to a P. graminis transmitted virus, with the object of limiting infection by the virus.
P. graminis does not appear to cause any crop loss (Barr, 1979), but it is economically important as the vector of numerous plant viruses worldwide (Adams, 1990; Maraite, 1991). Several of these viruses cause major widespread diseases on barley, wheat and groundnut in Africa, Asia, Europe and North America (Reddy et al., 1988; Adams, 1990). For example, in wheat, yield losses arising from soilborne wheat mosaic virus (Campbell et al., 1975; Chen, 1993; Bonnefoy et al., 1994) and wheat yellow mosaic virus (Bonnefoy et al., 1994) have been estimated at 10-70%, whilst wheat spindle streak mosaic virus caused 3-87% yield loss (Cunfer et al., 1988; Miller et al., 1992); losses in barley of between 15 and 100% resulting from barley yellow mosaic virus and barley yellow mosaic virus complex have been recorded (Maroquin et al., 1982; Huth, 1984; Plumb et al., 1986; Friedt et al., 1990; Froidmont et al., 1993); 66-78% yield loss in oats due to oat mosaic virus were reported (Catherall and Boulton, 1979); rice stripe necrosis virus caused 14-100% yield losses (Fauquet et al., 1988); peanut clump furovirus and Indian peanut clump virus resulted in losses of 40-70% and often the production of pods of no commercial value (Germani and Dhéry, 1973; Reddy et al., 1983).
Recently, because of the specificity of their infection process, obligate endoparasites such as species of Polymyxa and Olpidium have been regarded as interesting mediators for the introduction of foreign genes into vascular plants (Zhang et al., 1994). If plants can be regenerated from root tissue it may be possible to use Polymyxa as a carrier of useful genes for plant transformation.