THE SIGNIFICANCE OF VIRUS DISEASES FOR THE PRODUCTIVITY OF GRASSLAND

The effect of cocksfoot streak virus (CSV) on Dactylis glomerata L. and barley yellowdwarf virus (BYDV) on Lolium perenne L. was investigated under both single-plant and simulated-sward conditions. The effect of cocksfoot mottle (CFMV) was examined on single plants. CFMV was lethal, and in conditions of inter-plant competition CSV was semi-lethal and BYDV was non-lethal. When the virus was lethal, a considerable lag-phase was observed between mortality and complete compensation in yield. When the virus was non-lethal, plant growth was restricted, either horizontally (CSV) or vertically (BYDV). In mixed, healthy and infected plant populations healthy individuals compensated only for reductions in lateral growth.
CSV infection transformed plants of a pasture variety into 'hay' types and BYDV infection transformed plants of a hay variety into 'pasture' types. CSV increased the plant's aggressiveness in hay conditions, but reduced its ability to withstand frequent defoliation. Conversely, BYDV increased the plant's aggressiveness in pasture conditions, but reduced it in hay conditions. Compensation from healthy plants occurred only in CSV-infected swards subjected to frequent defoliation. Some possible reasons for the absence of compensation in other treatments are discussed.