Seed-borne alfalfa mosaic virus infecting annual medics (Medicago spp.) in Western Australia

Infection with alfalfa mosaic virus (AMV) was widespread in introduction, evaluation and seed increase plots of cultivars and numbered selections of annual medics (Medicago spp.) in Western Australia; the virus was detected in plots of seven species. When seed stocks from the West Australian annual medic collection harvested in 1984–1986 were sown and seedlings tested, seed-borne AMV was found in all 12 cultivars and in 44/50 numbered selections, belonging to 10 species. Seed transmission rates to seedlings ranged from 0.3–74% and exceeded 5% in 33 seed lots. By contrast, when seedlings of four species grown from seed harvested in 1971–1978 were tested, no AMV was detected; the oldest infected seed stock found was from 1980. In commercial seed stocks of two cultivars released in 1987, the levels of seedling infection with AMV found were 0–0.2% for M. polymorpha cv. Santiago and 526% for M. murex cv. Zodiac. In commercial 1986 seed of M. polymorpha cvs Serena and Circle Valley, AMV was detected in 3/13 and 6/9 stocks respectively; transmission rates to seedlings in infected stocks were 0.1–0.7%. In a survey of 47 annual medic pastures in medium and low rainfall zones of the Western Australian wheat belt in 1987, the virus was detected in leaf samples from only three sites. When inoculated mechanically, AMV systemically infected 11 cultivars and 12 selections belonging to 13 species, but did not infect one selection each of M. aculeata and M. orbicularis. Infected plants in ten species developed only faint mosaics or were symptomlessly infected, but M. littoralis, M. polymorpha and M. tornata developed distinct mottling, reduction in leaf size and, in some instances, leaf deformation and dwarfing. In pot tests, AMV infection decreased herbage and root production (dry wts) of M. polymorpha cvs Serena and Circle Valley by about 30% and 50–60% respectively, but did not decrease herbage production in M. murexcv. Zodiac. In spaced plants growing outside, AMV decreased herbage, root (dry wts) and seed production of M. polymorpha cvs Circle Valley and Santiago by about 60%.