Race non-specific resistance to Bremia lactucae

Three ways of measuring race non-specific resistance to Bremia lactucae in lettuce were studied to determine the most important components of this form of resistance. Tests were conducted on leaf discs from 4–6 wk-old plants that were quantitatively inoculated and incubated under controlled conditions. Observations were made of disease intensity, latent period and production of conidia. Inoculum concentration markedly influenced disease intensity and latent period. Cv. Iceberg exhibited race non-specific resistance and this was characterised by a longer latent period, a lower disease intensity and reduced production of conidia compared to the susceptible control cv. Cobham Green. On older, intact plants, the differences in latent period and disease intensity between cultivars were even greater. Production of conidia was a particularly good criterion for discriminating levels of resistance.