Effect of cocksfoot mottle virus on resistant and susceptible cocksfoot cultivars grown alone or in combination with healthy or ryegrass mosaic virus-infected Italian ryegrass

Cocksfoot mottle virus (CfMV) was readily transmitted on cutting implements. However, after six harvests, only a quarter as many plants of the resistant cv. Cambria as of the susceptible cv. S.37 became infected. In addition to resistance to becoming infected, cv. Cambria also possessed some resistance once infected, and suffered less mortality than S.37. More CfMV-infected plants died when competing with healthy plants than when competing with other infected plants. Both CfMV and ryegrass mosaic virus (RMV) caused greater reductions in the yield of their respective hosts in mixed stands of cocksfoot and Italian ryegrass than in single species stands. CfMV reduced the proportion of cocksfoot in the mixture and RMV the proportion of ryegrass. However, CfMV-infected cv. Cambria competed better than CfMV-infected cv. S.37 with healthy or with RMV-infected ryegrass. It is concluded that grass cultivars selected for resistance to virus infection will slow down the rate of disease spread within the sward, and selecting for postinfection resistance will help maintain the aggressiveness and competitive ability of plants once they become infected.