III. Variation in plant viruses: Variation in ryegrass mosaic virus in relation to resistance

Some isolates of ryegrass mosaic virus are mild in all but a few plants, others are more severe. Infection with a severe isolate reduced the yield of a ryegrass population by 24% under field conditions and reduced survival following cold treatment by 85%. Field isolates from the short-lived Italian ryegrasses are generally severe while those from the long-lived late-flowering perennial ryegrasses are generally mild. This variation appears to be adaptive in origin since in the case of perennial ryegrass proneness to infection by severe isolates would probably lead to the elimination of both host and virus whereas Italian ryegrass can escape infection readily by copious production of seed.