Irish perennial rye-grass plants infected with barley yellow dwarf virus (BYDV) showed inconspicuous leaf symptoms but were more stunted and yielded less total herbage, but more early spring herbage, than healthy plants. During a period of 2 years, infection did not decrease plant survival; infected plants produced more tillers and gave a higher ratio of vegetative to fertile tillers than healthy plants. BYDV caused a greater loss of yield in simulated rye-grass swards cut twice a year than in those cut four times. Swards in which half the plants were infected gave yields intermediate between those of healthy and infected swards. Infected plants tended to dominate healthy ones in swards cut four times, but in those cut twice a year the initial status quo was maintained. In swards containing rye-grass and white clover, the clover yielded more for most of the year when with BYDV-infected than with healthy rye-grass, but this trend was reversed in spring.
The different effects of barley yellow dwarf virus on rye-grass plants and of cocksfoot streak virus on cocksfoot seem reflected by the different effects of the viruses on the competition between infected grass, healthy grass and white clover plants, when these are grown in different combinations.