Alfalfa mosaic virus (alfalfa yellow spot)

As AMV is both seed and aphid transmitted, a range of control measures are required. Use of virus-free seed is an obvious initial requirement. Seed potatoes have been freed of virus by thermotherapy (Kaiser, 1984). Control of spread from overwintering hosts by spatial separation or by controlling the vector is the next major step. Insecticides are of limited use as the virus is transmitted non-persistently by aphids. However, combining insecticides with mineral oil (Reagan et al., 1979) or application of mulches between the rows (Kemp, 1978) gives sufficient insect control to limit infection. Various sources of resistance to AMV have been reported (see Hull, 1969) but these do not appear to be very effective. Transgenic protection has been reported (Fincham and Beachy, 1993; Brederode et al., 1995) and may be an effective future approach.

Two sorts of transgenic protection have been demonstrated. Expression of AMV coat protein (CP) gene in tobacco, tomato, pea, medicago and lucerne leads to a significant delay in symptoms and a reduction in virus accumulation (Fitchen and Beachy, 1993; Jayasena et al., 1997; Xu et al., 1998, 1999; Jayasena et al., 2001; Timmerman-Vaughan et al., 2001). Transformation of tobacco with mutated AMV replicase genes gave high levels of protection (Brederode et al., 1995). By analogy with other systems it is expected that CP-mediated protection would be effective against a wider range of strains than that afforded by the replicase.