The Incidence of Ryegrass Mosaic Virus in Crops of Ryegrass grown for Seed in some Southern Counties of England

Abstract

SUMMARYItalian ryegrass, cultivars Aberystwyth S22 and perennial ryegrass, CV.S321, grown for the production of seed in the A.D.A.S. S.E. Region and Wiltshire were surveyed for ryegrass mosaic virus (RMV). Visual leaf symptoms were used to assess infection; the diagnosis was checked using the electron microscope.RMV infection was widespread in both grasses and about 30 per cent of the fields carried an infection level greater than 10 per cent (based on number of plants infected/examined). Higher levels of infection were encountered in crops of Italian ryegrass than in perennial ryegrass. Out of 69 fields of Italian ryegrass, only 12 showed no infection whilst nine fields showed a level of infection greater than 30 per cent. Twenty‐eight of the 61 fields of perennial ryegrass showed no infection and the highest level of infection found was 26 per cent.All of the Italian and most of the perennial ryegrass crops were in their first harvest year. Since Italian ryegrass crops were occasionally encountered with more than 70 per cent of the plants infected, it is obvious that RMV can spread very quickly.In perennial ryegrass fields an increase in the amount of N fertilizer applied in the spring led to an increase in RMV; an increase from 100 to 200 kg N/ha led to an increase in the level of RMV from 9 to 37 per cent.By the first harvest year, fields sown in the spring of the previous year (usually under a cereal crop) were generally much more heavily infected than those sown in the autumn of the same year.